A furnace is useful in winter as it releases warm air into the house. The air treated inside the furnace should be odorless or pleasant. But if the smell is weird, it indicates something is wrong. 

A furnace smells due to electrical issues, overheated motor, cracked heat exchangers, dirty air filters, sewer traps, dirty ductwork, etc. Hire an HVAC team for electrical issues, overheated motors, and cracked heat exchangers. Clean the filters, traps, and ductwork every few months. 

Not all types of smell are hazardous. Some furnace smells might be weird, but they are not dangerous. This article will highlight and explain the reasons behind the furnace’s weird smell and how to troubleshoot the problem. 

A furnace releases a chemical smell.

Furnaces emitting a chemical smell can be due to several reasons, like improperly vented appliances, combustion byproducts, electrical issues, or ignition and blower motor issues. 

1. Improperly vented appliances

Whenever a furnace starts, it will emit a chemical smell. 

The reason is an improperly vented system. 

When you turn on the furnace, it casts fumes that smell like chemicals.

Improper venting will lead to the accumulation of carbon monoxide in your room. 

So, while turning on the furnace, you must ensure enough ventilation so the furnace fumes can easily escape your house. 

You can do this by installing a window or door in the furnace or having some special vent. 

Make sure to talk to a technician for the furnace repair if you find any strange odor and are confused about it. 

2. Combustion byproducts

A furnace smelling like a chemical suggests a problem in the combustion process. 

When furnaces burn by propane or natural gas, they produce water vapor, carbon dioxide, and other gasses. 

These are combustion byproducts. 

These byproducts will cause a chemical scent. 

If you are uncomfortable and worried about the smell, turn off your unit and call an HVAC team to look into the matter. 

To avoid dangers, schedule regular annual maintenance of your unit. 

3. Electrical issues 

The furnace wiring can overload and release a chemical or burning smell. 

If the furnace motor gets overloaded, it will produce a chemical odor, which can also lead to a fire hazard.

Turn off your unit and hire an HVAC team to look into the matter. 

4. Cracked heat exchanger 

A cracked heat exchanger will release toxic carbon monoxide, which smells like a chemical. 

A heat exchanger can crack over time due to poor maintenance, rust, or erosion.

When this happens, you and your family will be at risk. 

Putting a carbon monoxide (CO) detector in your furnace room will let you know whether the furnace is releasing gas. 

When you get the smell and detect the CO release, turn off your furnace, open the doors and windows to let the gas out, and call a professional to replace the heat exchanger. 

5. Ignition problem and blower issues 

When a furnace does not ignite well, it will lead to a gas leak, therefore, the chemical smell. 

Another reason is a faulty blower motor. When the blower is at fault, the air cannot distribute properly. 

As a result, the furnace will work harder to give you the desired temperature and overheat, creating a chemical-like smell. 

Turn off the furnace and call an HVAC expert to handle the matter. 

The furnace releases a rotten egg smell.

A furnace releasing a rotten egg smell is quite problematic. Here are some common reasons:

6. Cracked components

When your heat exchanger cracks, there will be a hole that releases a putrid smell similar to rotten eggs or formaldehyde. 

It would be best to call an HVAC to replace the component as soon as possible. Otherwise, the problem can lead to a fire outbreak. 

7. Dampness

Dampness settles over your unit if your furnace is near a damp location. 

When you light up the furnace, the unit becomes hot, the moisture from the dampness evaporates, and creates a rotten egg smell. 

But there is no harm in the smell because it will fade if the furnace burns for a long time. 

8. Excessive dust

Dust builds up inside the furnace when it remains dormant in the summer. 

When you turn it on in the winter, this accumulated dust will burn and release a rotten smell. 

But it should fade after your furnace runs for some time. 

To prevent it, clean your furnace before the winter strikes your region. 

9. Gas leaks

The natural gas the furnace uses is laced with sulfur which can give you a rotten egg smell. 

Adding sulfur to the gas helps alert house owners during gas leaks. 

Call an HVAC expert to fix the matter if you encounter this smell. 

10. Contaminated air intake

Most of the recent furnaces take in the air outside your house. 

So, any other smell close to the furnace’s intake system will make the furnace the same odor. 

For example, if your furnace smells of rotten eggs or sewer, the furnace will take in the smell. 

So, when your furnace releases the smell, it is due to the rotten smell or sewer opening near your house. 

There is no harm in it. 

As the rotten smell from the air disappears, your furnace will stop producing the rotten egg. 

11. Mold

Mold can sometimes smell similar to the rotten egg smell. 

If your furnace or the ducts suffers dampness, they will grow mold over time. 

It occurs when the humidity inside your house is too high. 

When you turn on the furnace, your unit will spread a rotten egg smell. 

But over time, the smell will vanish. 

Mold can be dangerous for your health, especially if you suffer from lung disease or asthma. 

The oil furnaces are more prone to releasing the rotten egg smell due to mold.

Call an HVAC expert to get rid of the mold. 

12. Vent contamination

Some prevalent vent contamination is due to pet hair and urine odor, which can release a rotten egg smell. 

Look around the vent openings for pet excretion and take care of the mess to resolve the smell problem. 

The furnace releases a burning smell.

Be it a gas furnace or an electric furnace, a burning smell from the unit is stressful. 

Sometimes, the smell may be normal in certain situations. 

Still, in other cases, it indicates a serious issue that requires immediate addressing. 

Here are some common reasons:

13. Dirty air filter

Dirty air filters are a major reason behind a burning smell. 

Dust, fur, hair, lint, and other dirt can block the filter pores. 

When the air handler forces the air through the filter, this dirt and debris will become hot enough to burn, which releases a burning smell. 

Changing the filters every 3 to 4 months can prevent the burning smell. 

Thick filters take more time to become dirty than thin ones. 

So, instead of waiting for 3 to 4 months, check your filter every month and change it whenever required.

14. Incomplete combustion 

The burner flame must be consistent in a gas furnace. 

A burner needs fresh air to maintain the combustion procedure. 

The process further needs a consistent supply of gas.

Suppose the gas supply valve, burner, pilot, ignition, or airflow creates any problem. 

In that case, it will lead to an incomplete combustion process, leading to soot and ash.

When the burner ignites, this soot and ash will burn and make the furnace release a burning smell. 

You may call an HVAC to look into the components causing incomplete combustion and fix them soon. 

15. Dirty combustion chamber

When you do not use the furnace for a very long time, like 6 months or more, the combustion chamber will have a layer of dust particles. 

The dirt will be mostly in the burner and the heat exchanger. 

When you turn the unit, it will begin its first heating cycle, and the dirt particles will start burning, releasing a burning smell. 

The smell will fade after some time of the unit’s running. 

16. Dirty ductwork

When the furnace is not in use, along with the unit, the ducts will collect dirt and debris over time. 

The dust will become hot and burn whenever you turn on the unit, releasing a burning smell. 

It should fade after some time. If not, turn off the unit and call an HVAC.  

17. Electrical malfunctioning

Gunpowder or burning sulfur smell will be due to the overheated components inside the furnace, which may start burning over time. 

This situation is an emergency. 

Turn off your furnace and contact the HVAC immediately to address the issue. 

Do not turn on the unit until the components get repaired. 

The furnace releases a burning plastic smell.

The furnace releasing a plastic smell is quite weird. 

Let’s look at some common reasons:

18. An overheated circuit board and wires or plastic inside the unit

The circuit board and wires of the furnace will have plastic insulation. 

Overheating can melt this plastic insulation, therefore causing a burning plastic smell. 

Whenever you get a plastic smell, please turn off your furnace and check the reason behind it. 

If it is due to overheating, you must call an HVAC to fix the insulation. 

19. Plastic items stuck inside the furnace, duct, or vent

Plastic items can reach your house’s vent, ductwork, and furnace. 

Whenever you turn on the furnace, these plastic items will become hot enough to start burning, thus the melted plastic smell. 

Turn off the unit and clear the plastic items from the vent, duct, or furnace. Call an HVAC if needed. 

Keep such items away from your vents, furnaces, and ducts. 

20. Furnace protective finish

A specific furnace protective finish is applied to some areas of the furnace to prevent rust and erosion.  

The coating may sometimes come in contact with the heat, and the gas will be released by the furnace, resulting in a burning plastic smell. 

The coating will release the smell for the first few times after turning on the furnace. 

You will need a technician if the smell does not disappear permanently after some days. 

21. Burned filters

A clogged filter can sometimes release a plastic-burning smell when too much dirt accumulates in the filter. 

Due to the dirty filter, the blower motor will work too much to release the heated air, overheat it, and release a burning plastic smell.

22. Faulty heat exchanger 

The heat exchanger is a metal shield between the blower and the combustion chamber. 

It helps distribute the hot air from the furnace to the house through the ductwork. 

If the heat exchanger breaks, it will release a plastic-burning smell. 

Sometimes, the smell may also resemble chemicals or rotten eggs. 

With a cracked heat exchanger, your whole heating system will overheat, which can lead to the melting of the plastic insulation of the wires and fire hazards. 

Turn off the unit and call an HVAC to replace the heat exchanger. 

23. Loose furnace parts

A furnace contains several components, each of which has individual responsibilities. 

Together they make and emit warm air throughout your house. 

A dislodged component can melt the plastic wire insulation and release a burning plastic smell. 

However, you may not have this problem with your new system. 

The old ones are more prone to the burning plastic smell. 

You need to call an expert to fix the loose parts and rule out the problem quickly. 

You can also tighten the loose parts if you have the proper tools. 

Furnace releasing sewer smell.

When your furnace releases a sewer gas smell, the problem is with your sewer lines, not the furnace. 

24. Dry sewer trap or sewer leak

A dry sewer trap will release a bad sewer smell. 

You must put some water down the basement floor drain to fix the problem. 

If the smell does not fade, there might be a sewer leak. 

You need an expert to repair the leakage. 

The furnace releases a musty smell. 

A musty smell is probably due to mold and mildew inside the unit or ducts. 

25. Dirty ductwork

The furnace uses the ducts to release the air throughout your house. 

Over time, moisture and dirt can gather inside your duct and encourage mold and mildew growth. 

Whenever you turn on the furnace, it will release a musty smell. 

The air released from the duct will have mold spores. 

If inhaled, you will soon suffer from allergies. 

The air is dangerous for people with lung disease and asthma. 

Call an HVAC team to clear the ducts. 

26. Dampness 

Furnace dampness will release a musty smell. 

When a furnace is near a damp or humid location, the dampness will settle inside and around the furnace. 

So, when you turn on the unit, you get a stale smell. 

Over time, the dampness can lead to mold and mildew, releasing a musty smell. 

If the smell dissipates after some time, then there is nothing to worry about here. 

But, if the smell remains, it is probably due to mold buildup. 

Ensure to keep your furnace away from damp areas, like the roof or gutter water falling over your unit. 

Call an HVAC for mold cleaning. 

Furnace releasing fuel oil, smoke, or exhaust smell 

When you smell oil or exhaust from your furnace, you must turn off the unit and go for a quick inspection for flames or smoke. 

Here are some common reasons:

27. Delayed ignition

If the ignition gets delayed, the oil remaining inside the furnace will create a dense fog. It will release an oil or exhaust smell.

When they ignite suddenly after some time, the unburned oil will light up at once and cause a hazardous fire. 

If the unlit furnace smells like oil or exhaust, do not light up the furnace and ask for an expert’s help. 

28. Recently filled the tank

When you have recently filled up your gas tank for your gas furnace, it is common for the furnace to release the oil smell. 

It should dissipate over time. 

29. Initial furnace use

When you turn the furnace on after a long summer break, the furnace might release the oil or exhaust smell. 

It is common and shall fade after some time. 

30. Furnace vents

Sometimes your furnace will smell like car exhaust, suggesting that the furnace venting is at fault. 

The furnace uses a vent pipe to remove the gas created during combustion. 

If you have a high-efficiency furnace, the pipe will be PVC and must be free of obstructions. 

The gas cannot release outside if the venting pipe has any obstacle, like snow, ice, or bird nests. 

As a result, you get a car exhaust odor and encounter carbon monoxide poisoning. 

Keep a carbon monoxide detector to get alert messages about gas leakage. 

Turn off your unit, open the doors and window, and contact an HVAC. 

31. Clogged filters, cracked heat exchangers, and incomplete combustion

Dirty and clogged air filters, cracked heat exchangers, and incomplete combustion processes can also lead to oil or exhaust odors. 

Excessive old furnace filters sometimes smell like motor oil. 

Change the air filters every 3 to 4 months. 

If replacing the filter does not solve the problem, it could be a significant oil leakage. 

It would be best to call an HVAC. 

Incomplete combustion is due to problems in the gas supply valve, burner, pilot, ignition, and airflow. 

Hire an HVAC to deal with these problems.

Also check:

• Furnace Only Works After Reset: 9 Problem+Fix
• 15 Reasons Your Furnace Won’t Turn On (+Fix)
• 14 Reasons Your Furnace Won’t Turn Off (+Fix)
• 16 Reasons Your Furnace Is Not Heating (+Fix)
• 12 Reasons Your Furnace Is Blowing Cold Air (+Fix)
• 10 Reasons Why You Shouldn’t Run A Furnace Without A Filter
• 9 Reasons Your Furnace Is Short Cycling (+Fix)
• 11 Reasons Your Furnace Won’t Stay Lit (+Fix)
• 9 Reasons Why Your Furnace Shuts Off After 30 Seconds

Final thoughts

A furnace could release several smells, like chemicals, burning, rotten eggs, oil or exhaust, mustiness, etc. 

The smells can help you identify the ongoing problem in your furnace. 

Sometimes, one problem can lead to several smells. 

For example, cracked or faulty heat exchangers can release chemical smell, burning smell, and sometimes fuel oil smell. 

Similarly, dirty and clogged filters could release a motor oil smell or burning smell. 

So, you need to inspect the real cause and then approach troubleshooting. 

If you are in a dilemma, turn off the unit and call a local HVAC professional to look into the matter. 

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Reference: Wikipedia.

I know how stressful it would be if the furnace caused issues, especially during the winter, by not providing the warmth you crave. If your furnace is shutting off after 30 seconds, and you want to know the possible reasons, you are at the right place. 

When you notice your furnace shutting off after 30 seconds, inspect components like a thermostat, flame sensor, pressure switch, induced motor, and control board. You can fix it by replacing the faulty parts. Other possible causes are airflow issues and choosing the wrong size of the furnace.

In the article, let’s discuss the possible reasons for your furnace shutting off after 30 seconds. So, keep reading. 

Why is my furnace shutting off after 30 seconds?

A furnace is a basic requirement during winter.

It is important to check the working condition of the furnace before winter arrives and troubleshoot it as soon as possible.

Is your furnace turning on and switching off immediately after 30 seconds?

It may be due to any of the causes mentioned below. 

Start by inspecting the root cause, and you can fix the issue by following the steps listed below.

1. Thermostat faults

The thermostat is an integral part of the furnace.

It is used to signal the furnace when the temperature in the house falls below the fixed temperature. 

It is usually mounted on the house wall and will detect the temperature up to the limited surrounding but not throughout the home. 

It is common to have multiple thermostats installed in a house to set the temperature in each area based on the heating and cooling requirements of the particular area. 

It acts as the furnace’s brain to turn it on and off when the temperature falls in a particular area. 

If the thermostats are faulty, a furnace may shut off and on immediately. 

Another issue arises in the location of the thermostat.

Placing the thermostat near the heat source may cause a malfunction in the functioning of the furnace. 

Also, ensure you have placed the furnace in the central area where you and your family spend the most time. 

If the thermostats are faulty, replace them with new ones.

Consider seeking help from a professional to replace the thermostat.

2. Flame sensor issues

The main role of a flame sensor is to monitor whether the flame or the gas is being produced.

You can locate the flame sensor in the burner’s front panel, and it looks like a rod that protrudes out in the same place where flame arises out of the burner. 

Flame sensors are positioned in front of the ignitors.

For example, if the flame sensor is placed on the right side, you can find the ignitor on the left side. 

Burners in the furnace produce flame, and the rod-shaped flame sensor detects the flame to signal the furnace’s control panel to convey the system’s proper functioning. 

When the flame is not produced in the system, the flame sensor will stop sending the electrical signal to the control panel, indicating the improper functioning of the system. 

With this indication, a control panel will switch off the furnace for a few minutes or seconds, leaving the gas valve open. 

By doing this, a flame sensor prevents the gas leakage from causing any danger. 

If the flame sensor is faulty, your furnace will turn off after a few seconds. 

Check if the flame sensor is accumulated with dirt and debris; it won’t detect the flame causing the issue. 

So, cleaning the flame sensor may fix the fault if it is not faulty. 

If you find the rod in the flame sensor broken, you must replace it.

You can hire a professional to replace the flame sensor in your furnace.

3. Airflow issues

Airflow in the furnace is essential to keep a balanced temperature in each area by transferring the heat through air ducts. 

If one area of your house is warmer than another, it is likely due to airflow issues.

It will also cause the issue of the furnace switching off after a few seconds. 

If there is low airflow, the heat exchanger will start to heat.

When it gets heated, the high-limit switch in the furnace will be tripped, switching off the furnace to prevent hazards. 

There are certain causes for the low airflow; let’s discuss the causes. 

Dirty air filters

Air filters play an important role in pulling the air out of the heat exchanger.

If the air filter is accumulated with excess dirt and debris, it will restrict enough airflow, causing poor air circulation through the system. 

To prevent this issue, replace the air filter once every six months.

Or you can also clean the air filters with the assistance of a professional. 

Leaking air duct

Air ducts are responsible for transferring the air from the heating system to the house.

As air ducts are far from sight in the ceilings and wall interiors, issues in the air ducts are hard to examine. 

If there are cracks and damages in the air duct, the air will start leaking through it.

Resulting in a low airflow issue.

It is best to consult a professional to solve the issue. 

Dampers issues

Dampers are connected to the air ducts.

Doors in the dampers open and close to allow airflow inside the air duct. 

If the dampers are stuck closed, there will be no air circulation in the air duct. 

You can control the dampers with the thermostats.

If there are electrical issues, consult a professional to repair them. 

Damaged air handles

Air handles act as a medium to facilitate airflow in the system for ventilation and heat exchange. 

If the air handles are damaged automatically, it reduces the airflow in the system, leading to an imbalance in temperature in the house. 

To fix this issue, you must repair or replace the air handles in your furnace.

It is best to hire a professional to do the job.

4. Pressure switch fault

The pressure switch is crucial in the furnace system as it detects pressure. 

When you turn on the furnace, the pressure switch opens, and inspect the pressure. 

A pressure switch closes itself when it detects the negative pressure in the furnace, letting the exhaust expel out of the furnace without any risks. 

 As a pressure switch is attached to the electrical circuits, the furnace starts to work when it closes. 

If the pressure switch sensor shows no negative pressure, it shuts off the furnace, preventing exhaust gas leakage. 

So it acts as a safeguarding feature by ensuring safety. 

Here are the common causes of the pressure switch fault:

Damaged hose

A pressure switch is connected to the two hoses.

Check if they are tightly attached.

If the hosses are damaged or loosely attached, a pressure switch will not close. 

If the pressure switch is not closing, a furnace will not operate. 

In such a case, replace the hoses to fix the issue. 

Debris accumulation in the switch

Check if dirt and debris are accumulated.

If the switch is dirty, clean it thoroughly to fix the issue.

Consider seeking help from a professional to clean the switches. 

Damaged switch

Another important cause is when a switch or part of the switch is broken.

If that’s the case, you must replace the switch.

Because it is hard to repair the switch, consider hiring a professional to replace it efficiently.

5. Induced motor faults

The inducer motor plays an important role in bringing air into the burner and expelling the air from the flue.

The function of the induced motor is important for the pressure switch to function as it helps to bring enough airflow to enable the pressure switch to open and close effectively. 

It also helps to activate the ignition process in the furnace. 

The effective function of the induced motor is crucial for the functioning of the pressure switch.

Another important function of the induced monitor is to help the furnace expel excess gas in the heat exchanger, as it will be tuned a minute before the burner.

This function of the induced motor is to ensure that the heat exchanger doesn’t become sooty and prevents dirt accumulation. 

The induced motor is the actively functioning part of the furnace, as it keeps functioning until it is switched off.

It also helps to blow air circulation throughout the system, increasing the furnace’s efficiency. 

If the induced motor is faulty, the furnace’s efficiency will be affected as it may get dirty without the induced motor functioning well. 

You can understand that the induced motor is faulty by hearing the unusual buzzing noise in the furnace system. 

Your furnace will stop working completely if the induced motor becomes faulty. 

In such a case, you must replace the induced motor.

Consider seeking help from professionals to replace the induced motors.

6. Control board faults

The control board of the furnace monitors and regulates all the system functions.

A furnace will not start if the control board is faulty. 

You can locate it inside the system near the blower motor. 

It receives the signal from the thermostats, a pressure switch, and an induced motor to decide whether to turn it on or off the furnace. 

Another important function of the control board is you can use it to diagnose the issue in your furnace. 

A series of light codes indicate certain issues in your furnace.

You can refer to your user manual to understand the codes and their meaning. 

If your control board is faulty, replace it with the guidance of a professional.

As you can’t repair the control board, replacing it is the only solution to fix the issue.

7. Blocked heat gates

Heat gates are located at the house’s interior, and the hot air from the furnace is transferred to the interior of your house through these heat gates. 

If the heat gate is blocked by dirt and debris particles, the hot air will be stuck inside the heat gates.

Leading to the overheating of the system. 

If the overheating occurs, the furnace will shut down and turn off after a few seconds if you again turn it on. 

To fix this issue, you must clean the heat gates regularly.

Inspect if there is a blockage in the heat gates and remove them for the efficient functioning of the furnace.

8. Blocked exhaust vent

The exhaust vent expels poisonous gases from the furnace to the outer environment. 

When the furnace is turned on, it begins to burn the gases.

As a result of burning gases, water vapor, carbon dioxide, and heat are produced. 

In addition, other gases like carbon monoxide, nitrogen, and sulfur oxide will also be produced.

These are poisonous gases, and they are extremely harmful to human beings. 

If nests, dirt, or debris particles block the exhaust vent, these poisonous gases will be released inside your home.

And also, to some extent, the furnace will stop working and shut off immediately. 

To fix this issue, you must clear the blocks in the exhaust vent.

9. Wrong size of the furnace

People should avoid buying a too-big or too small-sized furnace for their house. 

If your furnace turns off after a few seconds, it may be due to the wrong size. 

If your furnace is large, it is necessary; there will be an imbalance in the temperature in the different areas of the house.

One place would feel cool and the other one hot. 

As it turns on and off constantly, the utility bill will increase, and the function of a furnace will not be efficient.

And it also reduces the lifespan of the furnace. 

Buying an undersized furnace causes issues like not being able to maintain the desired temperature for the longest time.

It also increases utility bills. And it also reduces the lifespan of the furnace due to constant turning on and off status. 

To fix this issue, measure your house and buy a furnace that will suit your house better.

Also check:

• Furnace Only Works After Reset: 9 Problem+Fix
• 15 Reasons Your Furnace Won’t Turn On (+Fix)
• 14 Reasons Your Furnace Won’t Turn Off (+Fix)
• 16 Reasons Your Furnace Is Not Heating (+Fix)
• 12 Reasons Your Furnace Is Blowing Cold Air (+Fix)
• 10 Reasons Why You Shouldn’t Run A Furnace Without A Filter
• 9 Reasons Your Furnace Is Short Cycling (+Fix)
• 11 Reasons Your Furnace Won’t Stay Lit (+Fix)

Final thoughts

The furnace is a safety device to keep your home warm and your family’s health in check during winter. If it shuts off after 30 seconds, it may be due to certain faults in your furnace. 

In the article, I have mentioned 9 potential reasons why your furnace is shutting off after a few seconds. Some of the common causes are faulty thermostats, faulty flame sensors, low airflow in the furnace, faulty pressure switches, and control board faults. 

One or two parts may become faulty in your furnace. You must figure out the cause and fix the issue by following the steps mentioned in the article. 

Consider referring to the user manual to understand the internal parts of your furnace, as the functioning of the furnace may differ based on its brand and model.

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Reference: Wikipedia.

Most houses have furnaces to create and distribute warm air in the cold winters. But if the unit malfunctions when needed, it sends shivers down our spine. So, it is important to call an HVAC team to check the unit before winter strikes your region. 

The furnace won’t stay lit due to a malfunctioning thermostat, dirty filters, burners, flame sensors, and a defective ignitor. Possible fixes involve fixing the thermostat, replacing the filters, and cleaning the burners and flame sensors. For defective ignitors, call an HVAC. 

There are more, and this guide will explore all the reasons and how to solve them. It is not advisable to troubleshoot all furnace problems yourself, especially electrical issues. Hire a professional HVAC team to look into the matter. 

What are the different ignition systems in a furnace?

There are three main types of ignition systems in the furnace:

Standing pilot light

The gas valve knot on the furnace will have a standing pilot light system, where it will be written OFF/ ON/ PILOT. 

This furnace system also contains a thermocouple, which is a heat-sensing tool. 

The thermocouple confirms the gas traveling to the burners when the pilot light signals it. 

Intermittent pilot ignitor

The intermittent pilot ignitor is an electronic ignition system. 

The intermittent pilot ignitor uses the pilot flame like the standing pilot light. 

The difference between the intermittent and standing pilot is that the pilot flame in the intermittent ignitor lights only when the thermostat signals the heat to turn on. 

That’s how it works:

• The electronic control board signals the ignitor. 
• The ignitor lights up the pilot flame.
• The pilot flame further turns on the gas. 
• After the burning cycle, the burner and the pilot flame will deactivate. 

Hot surface ignition

This ignition system contains a hot surface ignitor or HSI. 

The HSI has a fork-like attachment, a plastic base, and 2 wires. 

When the thermostat signals the heat to start, the fork attachment will warm up and signal the gas valve to open and ignite the gas. 

Why won’t the furnace be lit?

When you turn on the furnace, but it does not light up, it could signify numerous problems. 

The internal parts in the furnace differ based on the furnace type, and each part has its functions. 

So, inspect all of them to resolve the problem. 

Here are some of the common reasons behind a furnace not lighting up:

1. Malfunctioning thermostat 

Thermostat issues are quite common and easy to fix. 

Most house owners are unaware of it. 

If your furnace has to ignite, you should set the thermostat to HEAT. 

If the setting is not set to heat, your furnace will turn on but won’t ignite. 

Another reason behind a furnace not igniting is faulty thermostat connections or sensors. 

Solution 

If your thermostat setting is the problem, change the setting to HEAT. 

If the problem is in the connections or sensors, get professional help to fix the matter. 

2. Broken or malfunctioned thermocouple

As I mentioned earlier, furnaces with a standing light pilot light have a thermocouple that confirms the gas flow to the burners when the pilot light signals it. 

It is a sensor that detects the working of the pilot light. 

If the pilot light is off, the thermocouple will turn off the gas valve to prevent accidents.

If the thermocouple is faulty, it will turn off the gas valve at the wrong time and won’t let the burner ignite. 

Solution 

Whether the thermocouple requires replacement or not will be an expert’s decision. 

So, call a local HVAC technician to fix the problem. 

3. Defective ignitor or pilot light

If your furnace suddenly stops igniting, the problem might be in the ignitor or pilot light. 

These two are the systems that come before the ignition process goes out. 

First, you need to check whether these two parts are working. 

Please pay attention to the behavior of your furnace when it tries to ignite. 

Suppose the furnace makes a clicking noise but does not ignite after the combustion cycle. 

In that case, the problem is in the ignition system.

It would be best to replace the ignitor. 

It could be because the ignitor is old and has worn out, or the part is incompatible with your furnace. 

A power surge will also affect the ignitor’s performance. 

If the furnace ignition starts but fires only for a few minutes and then shuts off, the problem is in the standing pilot light. 

Solution 

Furnace issues related to power and gas supply will need the expert’s attention. 

Call an HVAC professional to replace the ignitor or the pilot light. 

4. Dirty or broken flame sensor

A flame or ignitor sensor is a metal rod that senses the flame to ignite the furnace burners. 

It turns off the gas burners to avoid gas buildup.

If the sensor is broken or dirty, it won’t sense the flame, stop the burners from igniting, and won’t light up the furnace. 

Solution 

You need to clean or replace the sensor yourself. 

You may also hire a professional if you are uncomfortable doing it yourself. 

The steps to clean or replace the sensor are as follows:

• Turn off the furnace at the breaker box.
• Open the access panel and locate the flame sensor.
• The sensor will be a metal rod with a porcelain mounting end. 
• Remove the screws and gently slide out the sensor. 
• Use light grit sandpaper or emery cloth to clean the sensor, or slowly scrape the dirt with a knife.
• If there are any cracks, replace the sensor with a new one. 
• Once cleaned, put it back in place as it was before. 
• Close the access panel and turn on the unit. 

Take professional help if you hesitate to do it yourself. 

5. Defective draft inducer motor

The furnace’s draft inducer motor helps the unit pull the surrounding air and blow it out of the vent or chimney. 

It ensures the vents are open for the poisonous carbon monoxide to escape from the house instead of building up inside the unit. 

It creates a humming noise when the thermostat signals for heat and before you hear the swoosh sound of the burners igniting. 

The inducer motor works just before the burner starts and helps to clear the combustion gasses from the previous cycle. 

When the draft motor is fine, the pressure switch will close. 

The switch measures the airflow volume through the heat exchanger. 

If the draft motor malfunctions, it will open the pressure switch and prevent the furnace from lighting up. 

Solution

All the furnaces do not have draft motors. 

Call an HVAC expert to repair your furnace with a defective inducer motor. 

Most draft inducer motor designs are difficult to repair. 

However, these motors won’t need frequent replacement if done by experts. 

So, call an expert. 

6. Gas line problem

Gas line problem is one of the major reasons behind a furnace not lighting up. 

When the gas line in which the furnace draws the fuel might get blocked or is undersized compared to the furnace’s needs. 

If the gas valve or supply gets interrupted, the furnace will not receive enough gas to light up. 

Therefore, it will leave your house cold and chilly. 

Another problem with the gas line is a bigger gas line. 

You can confirm it by the gas pressure gauge. 

If the gas line is bigger, the gas pressure will exceed normal. 

As a result, your furnace may not be able to handle excessive gas flow through the furnace. 

Besides, excessive flow can damage the unit’s internal parts and stop your unit from lighting. 

On the contrary, smaller gas lines will restrict adequate gas flow and stop your furnace from lighting up. 

Solution 

The solution in both cases is to contact the technician to check the furnace’s gas source and check for any interruptions. 

In most cases, it will be a blockage. 

The gas line must have at least 6 inches of clearance for adequate functioning. 

Also, call them to let them have a look at the gas line. 

If the line is smaller, it can be fixed by adding an extra natural gas piping or an adjustable regulator. 

Bigger lines require replacement.

7. Dirty gas burner

Dirt and debris can gather around your HVAC system and stop them from burning properly. 

The flames will not completely burn and light up your furnace if dirt and debris have accumulated in the burners. 

Dirty burners fail to bring in oxygen required by the furnace for ignition. 

A clogged burner will create a high amount of carbon monoxide in your system, which is quite hazardous. 

Solution 

You can clean the burners by blowing compressed air through each of the burners and wiping them outside with a brush or rag. 

If you feel uncomfortable, call a professional to clean them. 

8. Dirty air filters

Adequate airflow needs to get inside the furnace so the unit can warm it up to distribute around the house. 

The furnace burns by a mixture of air and natural gas. 

If the air inside the system does not flow properly, it won’t produce enough gas or air mixture. 

As a result, your heating system will not ignite. 

In most cases, the air can flow due to the dirty air filters. 

The air sucked in by the furnace is filtered by this air filter. 

So, the dirt particles are trapped in the filter, and fresh air enters your system. 

Over time, the filter will become very dirty and restrict the airflow. 

Solution 

You must change the air filters every 3 to 4 months. 

Sometimes, the frequency of changing the filters depends on the filter thickness. 

So, it would be best to change the filters whenever you see them dirty. 

Check the filter’s condition every 3 to 4 weeks to determine the changing time. 

9. Failed high-limit switch 

The high limit switch, also called the limit switch, determines when the furnace blower turns on or off. 

It will also control the hot air blowing through the vents to your house and prevent the heat exchanger from overheating. 

If the fan limit does not work, it won’t let your furnace light up. 

Reasons behind a faulty limit switch could be:

• Dirty switch sensor
• Clogged air filters
• Overheated blower motor
• Other faulty components like thermostats, contractors, gas valves, or circuit board 

Solution 

Ensure annual furnace maintenance before you start using it in the winter. 

The cleaning during the maintenance should make sure that the fan limit works as it is supposed to. 

Fixing the fan limit is beyond your limit. So, hire an HVAC team to fix it. 

10. Tripped breaker

Modern gas furnaces do not run on natural gas completely. 

The electronic ignition systems are replaced with standing pilot lights, which makes the furnace more efficient. 

When the breaker connected to the furnace has tripped, it won’t let the electronic ignition ignite the burner. 

Solution 

You may reset the furnace or the breaker. 

To reset the breaker:

• Make sure that the switches and plugs are off. 
• Open the breaker box and check the tripped switch of the furnace. 
• If the breaker switch is between ON and OFF, turn it to OFF, wait for some time, and then turn it ON. 
• If the breaker is in the OFF position, push it to the ON position. 

Some furnaces have reset buttons. 

If your unit has one, press it for a few seconds until you hear a clicking sound.

Turn off the furnace at the breaker box before resetting the unit. 

Once you hear the clicking sound, leave the button and turn on the furnace. 

The unit will start as soon as it receives a signal from the thermostat. 

11. Old furnace

A furnace generally lasts 15 to 20 years. Over time, it will become old and exhausting. 

If your furnace has been in your since your grandparents, it is time to buy a new one. 

Once the system ages, it will become less efficient, increase energy bills, and create issues in the components. 

Due to the old components inside the furnace, the unit will work harder to give you the desired temperature and may overheat. 

An overheated furnace is a ticking bomb. 

Solution

There is no negotiation with an old furnace replacement. 

You must replace it with a new one. 

Replacing an old furnace will reduce energy bills, increase efficiency, and ensure you do not have a ticking bomb inside your house. 

Also check:

• Furnace Only Works After Reset: 9 Problem+Fix
• 15 Reasons Your Furnace Won’t Turn On (+Fix)
• 14 Reasons Your Furnace Won’t Turn Off (+Fix)
• 16 Reasons Your Furnace Is Not Heating (+Fix)

Final thoughts

A furnace may not light up for several reasons. 

You may troubleshoot some problems yourself, like dirty air filters, defective and dirty flame sensors, tripped breakers, dirty burners, and malfunctioning thermostats.

If you cannot do it yourself, call for help.

But other problems, like the gas lines, failed high limit switches, defective draft motor inducers, defective ignitors, pilot light systems, and broken thermocouples, will require HVAC’s attention. 

If your furnace is quite old, you must change it and buy a new one. 

Since the recent furnaces are highly efficient, they will reduce energy bills. 

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Reference: Wikipedia.

A furnace constantly turning on and off signifies short cycling. When a furnace causes short cycling, it prevents your home from getting comfortable, warm air in the winter. 

The primary reasons your furnace is short cycling include a bad flame sensor, overheating, dirty filter, oversized furnace, blocked heat grates, blower malfunctioning, defective fan switch limit, and thermostat issues. To fix the issue, check the obvious problems; if not, consult an HVAC specialist.

You can easily solve some short-cycling reasons, but the rest will need an HVAC expert. This guide will share the signs of the short-cycling, the reasons behind it, and how to resolve the problem. 

What is a furnace short-cycling?

A furnace is responsible for releasing warm air throughout the house. 

When you turn the unit on, it will begin a heating cycle to suck in the surrounding air, warm it up, and release it around the rooms. 

The time the heating cycle runs depends on the temperature set in the thermostat. 

Unless and until the unit gives you the desired temperature, it will keep running. 

Generally, one heating cycle runs for quite a long time. 

You will hear a clicking sound in your thermostat when the device sends a signal to the furnace for starting and shutting down. 

You will also hear the soft sound of the airflow when warm air blows into your living room, the burner ignition, and the blower motor running. 

Furnace short-cycling is just as the name suggests – short heating cycles. 

The heating cycle should run until it reaches the desired temperature. 

But it runs in short cycles and shuts down after some minutes of running without reaching the desired temperature. 

A good way to tell that your furnace is short-cycling is when you observe that the heater turns on and off several times without producing any warm air. 

Signs of furnace short-cycling

• A cold house is one sign as the unit shuts down quickly without warming your house. However, a cold house can also mean poor insulation, undersized furnaces, or faulty and outdated windows. 
• The furnace heating cycle lengths become shorter, less than five minutes. 
• Your furnace will start frequent cycles, like 5 to 8 times per hour. 
• Your room and thermostat temperatures won’t match each other. 

Why is your furnace short-cycling?

Multiple reasons could be responsible for a furnace short-cycling. 

Some house owners can easily correct the problems with a little guide. 

In contrast, the other reasons will need an HVAC’s immediate attention. 

Let’s learn the reasons in detail and how to solve them:

1. Oversized furnace

Short-cycling time: Mostly 3 to 7 minutes 

An oversize furnace is one of the major reasons behind short cycling quickly. 

If your furnace is short-cycling too much and your energy bills are quite high, consider checking the unit size. 

Since the unit will be bigger than your house size, it will quickly finish the heating cycle to warm up the air and turn it off. 

Besides that, it will unevenly distribute the air throughout your house. 

Solution 

Unfortunately, you have nothing to fix here except replacing your furnace with a new one based on your house size. 

Different house sizes require different furnace sizes.  

• 1,200 square feet house – 36,000 to 72,000 BTU
• 1,500 square feet house – 45,000 to 90,000 BTU
• 1,800 square feet house – 54,000 to 108,000 BTU
• 2,100 square feet house – 63,000 to 126,000 BTU
• 2,400 square feet house – 72,000 to 144,000 BTU

The size will also differ based on your living zone. Hotter zones require small furnace sizes, even if the house is big. 

2. The thermostat was mounted in the wrong place.

Short-cycling time: Whenever the house does not heat up properly.

The thermostat’s location can affect the furnace unit’s heating capacity. 

A thermostat present near the furnace can heat faster than normal. 

The furnace emits warm air to reach the desired temperature that you set in the thermostat. 

A thermostat present receiving direct sunlight will also heat up faster and affect the temperature reading.

So, when the thermostat heats up faster than normal, it will signal the furnace to stop running the cycle. 

Therefore, your furnace will short cycle and shut down without warming up your house.

Solution 

Keep your thermostat in the central location of your house. 

It will stop the device from heating fast and keep your furnace from short-cycling. 

A local HVAC can also help you with the furnace and thermostat location and suggest the best place for them. 

3. Dead thermostat batteries

Short-cycling time: Whenever the unit gets the signal to shut down the heating cycle. 

When your thermostat battery is low or dead, it sends the wrong signal to the furnace. 

It will create trouble in the accurate reading and affect the cycle length. 

For example, the thermostat may signal the unit to shut down frequently without letting the unit warm up the house completely. 

Solution

Change the batteries to resolve the issue, especially if it has been 10 years. 

While buying, ensure the battery suits your thermostat’s make and model. 

Some thermostats are wired directly to the HVAC system. 

4. Dirty furnace filters

Short-cycling time: 2 to 5 minutes 

A filter restricts the airflow from the furnace, leading to short cycling. 

When the air fails to go out of the system, it can build up inside the furnace and overheat the unit, causing short cycling. 

Besides affecting the heating cycles, dirty air filters will prevent your house from getting warm air during the winter. 

A broken air filter will also have the same effect. 

Solution

One simple solution is to change the filters every 3 o 4 months. 

The changing frequency depends on the filter thickness. For example:

• 1 to 2-inch thick – Change every 1 to 3 months
• 2 to 4-inch thick – Change every 3 to 6 months
• 5 to 6-inch thick – Change every 9 to 12 months

To change the filter:

• Turn off your unit.
• Remove the access panel of the furnace. 
• Unscrew and take the filter out. 
• Replace it if the filter is clogged or damaged. 
• Wash, dry, and put it back if the filter is reusable. 
• Please ensure the arrows remain in the same direction as before. 
• Close the access panel and turn on the unit. 

5. Clogged exhaust flue or vent

Short-cycling time: 1 to 5 minutes

Random materials around your house can make their way to the flue. 

The deadly carbon monoxide is dispersed through this exhaust flue. 

If the flue gets blocked, the carbon monoxide won’t be able to escape and start spreading around your house. 

The furnace will shut down frequently when the exhaust flue clogs, preventing carbon monoxide from spreading. 

Leaves, birds’ or animals’ nests, bee hives, and other things can block the exhaust vent. 

Sometimes, house owners put screens on the vents to prevent pests from entering, blocking the exhaust.

Solution 

Fortunately, removing the blockage prevents short cycling. 

It would be best to call an HVAC for a clogged exhaust flue. 

Call a pest specialist if the blockage is due to any animal nest or bee hive. 

6. Blocked heat grates

Short-cycling time: 3 to 10 minutes.

When the air flows from your furnace, the heat grates or HVAC registers will allow the warm air to leave the system and distribute in all the rooms in your house. 

The grates have dampers that allow you to control warm air distribution in the rooms.

So, when the heat grates get blocked or remain closed, it prevents the system from releasing adequate heat and air to warm your house. 

As a result, the warm air will build up inside the furnace, overheat, and cause short-cycling without letting the air distribute evenly. 

Solution 

Check the registers for any blockages. 

Sometimes, debris can collect in the heat grates. 

Removing the dirt and debris can quickly solve the issue. 

Otherwise, you may need HVAC’s help. 

Regular maintenance can prevent the problem. 

Make sure that at least 75% of the heat grates are open. 

If any furniture is blocking, remove the item to some other place. 

7. Blower motor

Short-cycling time: 3 to 5 minutes.

The blower receives warm air to distribute throughout the house. 

When it does not work correctly, the system cannot spread the air properly and fail to reach the desired temperature. 

As a result, the warm air will stay inside the unit and make the system shut off frequently due to overheating. 

The heat exchanger in the furnace cannot heat the air when the blower motor is off. 

Solution 

When you suspect a short cycle, go to the vents to feel the airflow. 

If the flow is weak or there is no air, the problem is in the blower. 

You need an expert’s help to fix the blower motor.

8. Bad flame sensor

Short cycle time: Few seconds after the burner fires up. 

A dirty or faulty flame sensor will not let your furnace light up. 

Instead, the flame sensor will go out immediately after lighting up for once. 

The flame sensor lights up your furnace when they sense the flame. 

The sensor cannot sense the flame completely if it is dirty or faulty. 

Therefore, it will shut off the natural gas valve to prevent the pumping of raw gas into your house for safety purposes. 

A bad flame sensor will shut off the flame immediately after lighting up, resulting in a short furnace cycle. 

The ignition sensors will also crack due to rust or corrosion. 

The furnace produces water during the heating cycle, especially the high-efficiency furnaces with 2 heat exchangers, leading to corrosion. 

Solution 

Replacing the flame sensor is the best way to encounter the problem and prevent further short cycling. 

Doing so will require proper tools and techniques. 

It would be best to leave it to the HVAC experts. 

To clean the flame sensor yourself, follow the following steps:

• Turn the furnace power at the breaker box. 
• Remove the furnace’s access panel and locate the sensor. 
• It will be a bent metal rod with a white porcelain base at the mounting end. 
• Remove the screws and gently take out the sensor rod. 
• If the tip of the rod is black or dirty, rub it with steel wood or scrape it lightly with a knife. You can also use an emery cloth (3M Emery cloth, 03008, 3⅔ in. x 9 in.) to clean the rod. 
• If any part of the sensor is cracked or broken, replace it with an HVAC’s help. 
• Once cleaned, put the sensor back and screw it properly. 
• Put back the furnace access panel and turn on the unit. 

 9. Defective fan limit switch

Short-cycling time: Whenever the unit overheats. 

If the switch is faulty, there would not be assumed timing.

The fan limit switch of the furnace lets the unit blower know when to turn it on and off. 

It controls the hot air movement. 

The fan limit switch in the furnace is a safety switch that triggers whenever it senses something wrong in the unit. 

The switch will shut down the unit frequently if there is any overheating, leading to short cycles. 

Besides that, if the switch is faulty, it may send the wrong signal to the unit and frequently turn the unit on or off, leading to short cycling. 

Solution 

Fan limit switches are sealed devices. 

So, do not repair them, but replace them. 

The cost can range between $100 and $200 without the labor charges. 

The price will increase based on the labor charges.

Why is it important to fix the short cycling in the furnace?

Furnace starting properly is one of the most important things for furnace operation. 

When your furnace keeps turning on and off, it greatly affects the unit’s efficiency and reduces lifespan. 

Over time, it can lead to system breakdown and long-term damage. 

Following are some reasons why you must fix a short-cycling furnace:

1. Maintains safety 

A furnace that keeps short cycling may have a bigger and more serious impact if kept untreated. 

For example, overheating is a very serious problem. 

If you do not fix the short cycling due to overheating, it can lead to fire hazards. 

Fixing the problem will keep you safe from such dangers. 

2. Maintains comfort

If the furnace constantly turns on and off, the indoor temperature will be either too hot or too cold. 

If these temperature changes keep happening for a long time, it will affect your health and encourage poor sleep.

Fixing the problem will stop this uncomfortable situation and keep you and your family healthy. 

3. Prevents system breakdown

Furnace short-cycling can deteriorate the furnace components due to the extra strain during multiple startups. 

It can eventually result in a much bigger problem in the future. 

Repairing that problem will be too complicated and expensive. 

So, instead of letting the problem stay and causing expensive damages, fix the problem in the first place with little effort. 

4. Increases the energy efficiency

Whenever the furnace starts, it uses more energy than when it is running. 

So during the short cycling, the energy bills increase too much whenever the unit starts up unnecessarily. 

It will lower energy efficiency and increase energy consumption and energy bills.

Also check:

• Furnace Only Works After Reset: 9 Problem+Fix
• 15 Reasons Your Furnace Won’t Turn On (+Fix)
• 14 Reasons Your Furnace Won’t Turn Off (+Fix)
• 16 Reasons Your Furnace Is Not Heating (+Fix)
• 10 Reasons Why You Shouldn’t Run A Furnace Without A Filter

Final thoughts

Constant short cycling of the furnace will cause an early breakdown of the heating system. 

Change the air filters every 3-4 months, remove the obstacles blocking the heat grates, and clear the exhaust vents and flame sensors. 

If you can’t do it yourself, call an HVAC team.

Replace the thermostat batteries when they are dead and place them at some central position in your house, where it can signal the furnace correctly. 

Consult an HVAC team to fix the blower motor, defective fan switch, and cracked flame sensors.

If your furnace is oversized, get the right size for your house. 

Do not let the problem stay for long. Otherwise, you might have to face severe issues in the future. 

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Reference: Wikipedia.

House owners forget about the furnace filters until their unit starts behaving abnormally. So, it is important to change the dirty filters regularly. But does the furnace need a filter? What happens if I run the furnace without a filter? Let’s find out.

Running a furnace without a filter can lead to poor indoor air quality, high energy bills, allergies, mold, contaminated ducts, fire hazards, and the shutdown of the entire system. So, it is recommended to regularly change/clean your furnace’s filter and avoid running the system without it.

So, filters are important, and you must change or clean them every 3 to 4 months. This article will explain the risks behind running a furnace without a filter in detail, how often to change the filter, and how to install it correctly. 

Can I run a furnace without a filter?

Furnaces are an important part of the house, especially during the winter. 

Without the furnace, you won’t be warm and comfortable in the winter. 

The air filters in the furnace are an essential part of the furnace. 

The filters will filter the air the unit sucks in from your surrounding atmosphere. 

After the air is treated, it will send out warm fresh air around your room. 

So, filters play an important role in your furnace. 

Running a furnace without a filter for extended periods can cause several issues like poor air quality, allergies, reduced airflow, and even a fire hazard. 

How long can you run a furnace without a filter?

You may be able to run a furnace without a filter, but it should be temporary for a short period. 

Whether you can run a furnace without a filter depends on what the experienced HVAC team suggests. 

Some house owners have said running a furnace without a filter for one night is fine. 

At most, you may run a furnace without filters for 2 days. 

However, it is better not to run.

Every time you run the furnace without filters, you risk its efficiency and damage its potential to work properly. 

Filters are present in the furnace for a good reason, and running a unit with one will be dangerous and harmful. 

If you do not prefer running a furnace without a filter, even for a single day, wait for one more day until you clean or buy a new filter the next day. 

Why is a filter important in a furnace?

Your house will always be filled with microscopic stuff like hairs, dead skin cells, dust, dirty air particles, etc. 

You cannot do anything about it unless you have an air purifier. 

Your furnace should blow warm air around you to make you comfortable. 

When you start the furnace, it gathers and sucks the air from the surrounding environment through the intake vents. 

The filter traps all these microscopic stuff and stops them from entering your furnace with the air. 

The air gets filtered with the help of the air filter and releases clean warm air. 

It keeps your unit efficient for longer periods.

When a furnace runs without a filter, dirt and debris enters your furnace, gather around the other components, and reduce their lifespan and efficiency. 

Why should you not run your furnace without a filter?

Running a furnace without a filter can lead to several issues. 

Here are the risks of not running a furnace without a filter and why you should not run them without a filter:

1. Poor air quality

First, pet hair, bacteria, and other harmful particles wander in the air, and ultimately gravity allows them to settle on the floor. 

When the furnace sucks in the air to treat it, it will irritate these particles and mix them up with the air again. 

As a result, these particles will travel inside the furnace. 

The furnace’s heat may destroy some particles, but the rest will pass inside the furnace through the vents if you do not have a filter. 

When this air releases into the surrounding air, it deteriorates indoor air quality. 

So, it is no longer healthy for you or your family.

They are now airborne and will deteriorate the health of the family members. 

Your lungs cannot tolerate dust particles and pet hair, and you will have severe breathing issues and allergies. 

It can be more dangerous for people with lung diseases, asthma, and COPD (Chronic Obstructive Pulmonary Disease). 

Since the air filters will filter the air by trapping these air particles, you must have a filter in your furnace.

2. Increased illness in the house

Along with breathing and lung issues, you may also have several other illnesses due to running a furnace without filters. 

When the dirt and air particles mix with the breathing air, your body constantly fights them, reducing your immune system. 

A reduced immune system means you will easily become susceptible to all the general sicknesses and viruses.

You will suffer the most during winter. 

The combination of a reduced immune system and plenty of bacteria around you will give you a perfect storm of illnesses.

So, make sure that your furnace is running with a filter. 

3. Risk of fire hazards

The small dust particles could be flammable. 

Therefore, your unit will catch fire when these particles slowly accumulate inside a furnace without a filter. 

A fire can damage your furnace beyond repair. 

But if the fire spreads into your house, it will also endanger you and your family. 

The filters can trap these flammable particles, prevent them from entering the furnace, and stop this dangerous fire. 

So, remember to add a filter to your furnace. 

4. Contaminated ductwork and mold growth

When you run a furnace without a filter, dirt particles and air toxins travel through the ductwork to reach your furnace. 

The debris and other toxic particles can accumulate inside the ductwork because they get trapped by the ribbed aluminum tubes. 

This accumulation of dirt particles will gather moisture inside the ductwork, creating a great environment for mold growth. 

Over time, these mold spores will start traveling through the ducts to the furnace and circulate your house through the air. 

It can be extremely dangerous for you and your family, especially if people with lung disease, asthma, and allergies. 

So, do not run your furnace without a filter. 

5. Reduced airflow  

Running a furnace without a filter will reduce the airflow over time. 

Without a filter, the dust particles gather inside the furnace around the other areas, like, the air ducts, vent covers, blower motors, heat exchangers, etc. 

Since these are the major components behind heating and releasing the air, they won’t be able to push the air out of the unit due to the dust cakes, reducing airflow. 

Fixing these parts is quite difficult and expensive. 

But changing the filters is easy. So, install filters in your furnace. 

6. Short cycling and overheating

When the furnace turns on and off repeatedly due to overheating, it is called short cycling. 

When the furnace does not reach its target temperature, it will start back up after cooling down. 

Repeated turning on and off will wear out your furnace faster and reduce its efficiency and lifespan. 

Sometimes, it can also blow off the motor and compressor. 

The unit and the furnace components accumulated with dust will need to struggle to spread the warm air out, and the unit will overheat.

7. Reduced efficiency

As I said earlier, without a filter, the other parts, like the evaporator coils, blower fan motors, ducts, and vents, will get covered with dust particles. 

As a result, they lose their efficiency and struggle to give you the desired temperature. 

Hard work will overheat the unit, release less airflow, and increase the energy bills. 

Over time, these parts will stop working.

The HVAC system will be taxed much more than normal, mostly during winter. 

It is the time when the furnace works overtime to give you the desired temperature. 

For example, the evaporator coil is a winding aluminum tube and part of the air conditioner. 

It helps in removing the heat from the air. 

If you do not have a filter in your furnace, this tube will play the role of the filter due to its metallic fins and tubes. 

As a result, the coil will be caked with dust and reduced efficiency. 

8. High energy bills

As I mentioned earlier, the furnace parts, for example, will have motor and compressor issues due to the lack of air filters.

The dirt will accumulate in the motor and compressor and create malfunctions due to straining. 

So, the parts will need to work harder, and the furnace will keep running to reach the desired temperature, increasing energy bills.

9. Damaged furnace

Only a fire will not damage your furnace. 

Other things can also damage your furnace due to the absence of a filter inside the furnace. 

The air filters trap these particles and keep them from accumulating inside furnace components. 

Without a filter, the dust particles will accumulate in other furnace components, causing malfunction and ultimate breakdown. 

You will sometimes see the furnace not working or hear weird sounds from the belt. 

Sometimes, most of the parts fail together. 

If nothing serious is yet to happen, you are fortunate enough to have time to correct this problem by cleaning the furnace and installing a filter. 

If the problem persists, you will require an expensive repair or replacement of the whole unit. 

The furnace will always choose an excessively cold day to malfunction due to heavy usage. 

You must pay extra during an emergency repair, especially if the timing is outside the normal working days or hours. 

10. Water leakage

The condensate drain will clog when you run a furnace without a filter. 

The drain helps to drain the condensation and extra humidity removed by the air conditioner from your house. 

When you do not put a filter, the air particles will accumulate in this pan. 

As a result, when the moisture does not get a place in the condensate drain, it will come back to your unit and start leaking around it. 

Adding the filter will trap these particles and stop them from accumulating in the condensate drain. 

Therefore, the removed moisture will go to the condensate drain to be drained outside your house. 

How often should I change the furnace filters?

Generally, you can change the furnace filters every 3 to 4 months. 

But, if you are too busy for that, change them at least once per year to encourage the furnace’s smooth functioning. 

The frequency of the filter change depends on the filter size. 

The thinner the size, the more frequent change it requires. 

If the filter is 1 inch thick, you should replace it every 1 to 2 months. 

You should replace filters measuring 2 to 3 inches in thickness every 3 to 6 months. 

If the filters are 5 to 6 inches, replace them every 9 to 12 months. 

You can look at the filter size and estimate a good time for a replacement. 

Some filters do not need to be changed but only cleaned. 

Some filters are washable. You must wash them with water, dry them, and put them back in place. 

If you do not change the filter for a long time, your furnace will lose its proficiency. 

Over time, the safety switch will get activated, and the system will frequently shut off. 

Signs of a dirty air filter

One of the common signs behind a dirty air filter is no heat from the furnace, but the blower keeps working. 

The furnace will work harder but won’t give you the desired temperature. As a result, the energy bills will keep increasing. 

Sometimes, your furnace may give you the desired temperature after a long cycle. 

The energy bills will still increase, indicating that a dirty filter has made the furnace work harder than normal. 

Another sign is decreased performance. 

Your furnace will not work up to your expectations. 

It may struggle to provide warm air, blow cold air, overheat easily, leak water, or produce weird noises. 

A dirty air filter could be the problem.

You will also receive hot and cold spots in your house because the unit struggles to distribute the warm air to every room. 

When a filter is dirty, your furnace circulates allergens and dust from the dirty filter. 

You might experience frequent headaches, allergies, and breathing issues. 

What does it mean if you have a black furnace filter?

It is common for the filter to turn gray. 

It is a sign that your filter has been kept the same for a long time, and dust and debris have settled on it. 

But if the filter is black, something is seriously wrong with it. 

It could be an incomplete combustion cycle, for which the unit has produced toxic carbon monoxide and stained the filter. 

You need an HVAC expert to handle the matter. 

Other causes could be mold or excessive amounts of dirt accumulation. 

Candles and fireplaces could be the reason behind mold as they produce soot, hence staining the filter with black color. 

When you see this, turn off the unit and call your local HVAC to look into the matter.

How to change furnace filters?

• Shut off your furnace at the breaker box. 
• The filter will be inside the furnace or return air vents. Remove the service panel or check the furnace manual for guidance. 
• The old filter’s frame will show you the filter size. The size varies based on the type of furnace. 
• Use the furnace manual for the filter’s type, size, and thickness. 
• Remove the old filter and dispose of it. 
• Slide the filter in its position and ensure the arrows on the frame point are in the same direction as the previous one.
• If the filter is reusable, wash it off and place it back. 
• Close the service panel and turn on the furnace to check.

A short fix

Suppose your furnace filter has become very dirty and requires immediate replacement, but the right filter size for your furnace is unavailable.

In that case, either run the furnace without the filter or let the filter stay for a few more days. 

If you are concerned about the consequences of running the furnace without a filter or letting the dirty filter stay, you can try a short fix. 

You can buy 1-2 oversized filters and cut them to fit your furnace. 

Use duct tape to keep them together and install them in the filter slot.

Running a unit with this kind of filter is better than nothing. 

When you get the filter suiting your furnace, install it immediately.

Also check:

• Furnace Only Works After Reset: 9 Problem+Fix
• 15 Reasons Your Furnace Won’t Turn On (+Fix)
• 14 Reasons Your Furnace Won’t Turn Off (+Fix)
• 16 Reasons Your Furnace Is Not Heating (+Fix)

Final thoughts

It is very unsafe to run a furnace without a filter.

It can lead to various issues, like reduced airflow, low efficiency of the furnace, high energy bills, leakage, fire hazards, poor air quality, etc. 

Some suggest running a furnace without a filter, but that is temporary. You may run it for only 1-2 days. 

But, I do not suggest taking the risk, even for a day. You reduce the unit’s lifespan and performance whenever you turn on the furnace without a filter. 

Getting a new filter for your furnace is better when you find it very dirty. 

Gray filters, no warm air, high energy bills, and hot and cold spots in the house are common signs of a dirty filter.

Depending on the thickness, change the filters every 3 to 12 months. 

FAQs

Should I change the filter more often in winter?

During the winter, the furnace runs over time. So, there might be more chances of dirt and soot buildup. 

So, you might have to change the filters frequently in winter. 

Experts suggest changing them once in winter or every 2-3 months. 

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Reference: Wikipedia.

When winter arrives, all we care about is an efficient furnace to receive warm and comfortable air inside the house. But it is stressful to see the furnace blowing cold air. Today, we shall explore the reasons behind a furnace blowing cold air. 

The major reasons your furnace blows cold air are incorrect thermostat settings, dirty filters, clogged condensate lines, insufficient gas, leaky ducts, and dirty flame sensor. To fix it, clean the filters, condensate lines, and flame sensors, check for gas supply, and repair the ducts if needed.

There is more in the list, and this article will explain all the possible reasons behind a furnace blowing cold air. Always hire a professional if you cannot understand or fix the problem yourself. 

Why does a furnace blow cold air?

A furnace is an integral part of the house, especially during winter. 

It helps produce warm air throughout your house in the winter to keep you comfortable. 

But, if the furnace produces cold air, it does not serve its purpose. 

It is because the furnace might be facing some issues. 

The problems can begin with the wrong thermostat setting and end with air leaks and everything in between them. 

We have enlisted all the possible reasons behind a furnace blowing cold air and the ways to troubleshoot each of them:

1. The furnace is yet to warm up

When everything is fine in your furnace but still gives you an icy blast of cold air, it will build up a sense of panic.

If you are a beginner in using a furnace for the first time, you should know that the furnace will take some time to warm up the air. 

Once you turn on the furnace, wait for some minutes to let the unit warm up the air slowly. 

After turning the furnace on, the unit will start its heating cycle after 10 to 15 minutes to give you the desired warm temperature.

2. Thermostat issues

A thermostat problem is considered one of the major problems behind a furnace blowing cold air.

A thermostat regulates the functioning of the furnace and signals the unit when to run and stop the warm air. 

So, if the setting is wrong in the thermostat, your furnace might blow cold air instead of warm air. 

The blower will keep running even if the heating is off, therefore, making a chilly condition inside your house. 

Another problem with the thermostat is the wire issues. 

The newly installed thermostats might have incorrect or incompatible wires. 

As a result, the thermostat will send wrong signals to the furnace, for which the unit may blow cold air.

Solution 

You must set the thermostat to HEAT to receive warm air throughout your house. 

Another setting should be done for the fan. Please set it to AUTO instead of ON. 

Keeping it ON means the fan constantly runs, with or without heat. 

If the thermostat is not working and jumping the values, consider replacing it. 

But if everything is correct, your furnace should start blowing warm air after a few seconds of the correct settings. 

It would be best to have HVAC guidance to fix the wire issues. 

3. The pilot light is off

The old gas furnaces have a pilot light that should light up whenever your unit runs.

The pilot light must remain on when you turn on the furnace. 

It ensures that the furnace is supplied with gas. 

The heating process will not start if the pilot light is off. 

Otherwise, your furnace will receive cold air if the pilot does not light up. 

A pilot light may go out due to a strong wind, a blocked pilot orifice, or a defective pilot system. 

First, determine whether the pilot light is in good condition and can function well. 

If the pilot light is a standing pilot, it must have a small flame. 

Check if you can see the flame. 

You will also hear a spark when the pilot lights or see a hot furnace ignitor glow. 

Solution 

Here are the steps to relight the pilot light:

• Turn off the furnace power at the breaker box. 
• Find the pilot light. Turn off the gas valve and wait until the gas escapes the pilot light. 
• Turn the gas valve to PILOT. Press and hold the reset button near the gas valve. 
• While holding the reset button, press the ignitor to lighten the gas from the pilot. 
• You can also use a BBQ lighter or a long match to light the pilot. Continue holding the reset button for at least 20 to 30 seconds. 
• Turn back the gas valve and power your furnace. 

Call an HVAC expert to help you if these steps do not work. 

4. Defective flame sensor

The furnace flame or ignition sensor is a thin, bent metal rod over the porcelain base burner assembly in front of the pilot light. 

When the furnace is on, gas comes out from the gas valve into the combustion chamber, and the pilot lights the burners. 

The flame sensor confirms that the flames are burning after the gas valve opens. 

If the sensor cannot detect the flames when the gas turns on, it will shut down the system to save your house from getting loaded with toxic gas. 

The flame sensor can accumulate dirt, soot, and carbon deposits over time from the flames. 

A dirty or faulty ignition sensor cannot properly detect the flames. 

It will shut down the furnace, but cold air will blow from the vents because the fan will remain on. 

The air becomes cold because the heat exchanger heats up until the flame sensor turns off the burners. 

Solution

You can test the flame once by turning off the unit for 5-10 minutes and then starting it on. 

If the problem remains, the sensor is either dirty or faulty. 

Here are the steps to clean or replace the dirty or faulty flame sensors:

• Turn off the furnace power at the circuit breaker. 
• Remove the access panel of the unit, and find the flame sensor. 
• It will look like a bent metal rod with white porcelain at the mounting end.
• Remove the screws and slide out the rod gently. 
• If the sensor’s tip is black, rub the metal part with steel wood or emery cloth. You can also use a knife to scrape the dirt. 
• If the porcelain part has cracks, replace the sensor. 
• After cleaning, put the sensor back in its place and screw it tightly. Put back the access panel.
• Turn on your furnace. 

If you still face the same issue, call an HVAC expert. 

5. Cracked heat exchanger

A furnace blowing cold air could be due to a cracked heat exchanger because it will fail the ignition. 

A cracked heat exchanger can also overheat your unit, trigger the main limit switch, and shut down your furnace. 

But the fan will be on until the whole system shuts down, and your furnace will blow cold air. 

Solution 

When you have a cracked heat exchanger, your house will face carbon monoxide poisoning.

You must immediately turn off your furnace and hire an HVAC expert to replace the heat exchanger. 

6. Overheating

Reasons behind overheating are dirty filters, mechanical failure, or old units. 

Overheating will make your unit shut down the cycle. The fans will remain on and release cold air into your house. 

Restart your furnace to see if the problem goes away. 

If the unit still overheats, find the reason behind overheating. 

When a unit overheats, the high-limit switch gets activated. 

It shuts off the system to prevent a fire, carbon monoxide leak, or explosion. 

Cold air will blow from the vents until the entire system shuts down.

A bad exchanger will fail the ignition, and the furnace will not release the warm air. 

But since the fan will be on, the unit may release cold air. 

The air will struggle to pass through the dirty air filter. 

As a result, your furnace will overheat and turn off. 

But the unit will blow cold air because the fan will remain on and won’t turn off until the entire system shuts down.  

Sometimes, the reason behind a furnace blowing cold air is old age. 

Over time, the unit will lose efficiency, work harder to provide warm air, and overheat. 

Solution 

You need an HVAC expert to fix the heat exchanger. 

Change the filters every 3 to 4 months, or clean them every 1-2 weeks. 

A furnace lasts for 15 to 20 years. If your unit is old enough, replace the old unit with a new one. 

7. Low gas supply

A gas furnace will need a pre-defined gas pressure to run properly and circulate warm air. 

A gas furnace will not be able to operate if the gas pressure is low due to insufficient gas or other interruption. 

Other reasons include closed gas valves or gas pipe leaks. 

Without adequate gas, the furnace will not give you warm air and will blow out cold air whenever you turn it on. 

Solution 

You need to turn off the unit and gas valve and call an HVAC technician to check for the problem. 

If your furnace uses propane or natural gas, call your gas provider to refill your empty tank or to check out for any service interruption.

If it is not the gas supply, ensure the gas valve is open when your furnace is running. 

8. A faulty circuit control board

The furnace’s control board is the unit’s brain as it passes instructions to its various parts for proper functioning. 

When the control board receives the heating cycle signal from the thermostat, it will send a signal to the furnace blower and furnace burner. 

If the board is at fault, the blower may turn on, but not the furnace burners. 

So, if your furnace is blowing cold air, it could be due to the faulty circuit control board. 

There could be many reasons behind the failure, with lack of maintenance being common. 

You must check for leaking water because water dripping on the control board can corrupt and disable the furnace parts. 

Solution 

It is an electrical problem. 

You might need a professional technician to fix the control board.

9. Blocked air filters

The air filters in the furnace are behind the return air vent to trap the dirt and debris of the air particles and filter the air. 

Filtering the air will keep the air supply clean and healthy. 

Since the filters filter the air, they can become blocked due to the excessive accumulation of dirt and debris after a few months. 

Once there is a blockage, air cannot pass through the furnace and give your house the desired temperature. 

So, your furnace will struggle to give you a warm temperature and overheat. 

Overheating will turn off the furnace cycle to prevent fire.  

The burners will be off, but the fans will run and produce cold air. 

Solution 

The only solution to this problem is to clear the filters every 1-2 weeks or replace them every 3 to 4 months. 

Ensure the filters installed are always clean and fresh at the beginning of every heating system. 

10. Leaky air ducts

Ducts run through your house to give you hot air from the furnace throughout your house. 

The ducts must be sealed properly so that no air can escape from openings. 

If the ducts are leaking, the hot air will escape from the leaks, and the air you receive will feel cold when it reaches the vents. 

Leaks are difficult to spot. 

However, check it by walking along the ductwork. 

Solution 

It is not something you can fix. You can only confirm the problem and inform your local HVAC experts. 

The experts will check for leaks and damage levels and seal or replace the ducts. 

11. Clogged condensate lines

The high-efficiency furnaces have two heat exchangers, for which they condensate more and produce water. 

This water drains into the PVC pipe called the condensate lines.

Condensate lines clogging over time with dirt and debris are natural. 

Water can accumulate around the unit if the condensate gets clogged. 

As a safety measure, the furnace will shut down. 

But since the fan will be on from the thermostat setting, the furnace will continue circulating air throughout your house. But it will be cold air.

You can confirm the problem if your furnace has a damp feeling or it is leaking water. 

Solution 

You must unclog the condensate lines so your furnace can blow hot air again. 

Here are the steps to unclog the condensate lines:

• Turn off the furnace power at the circuit breaker. 
• Find out the condensate pan of your furnace and dry up the water in the pan with a wet or dry vacuum. 
• Remove the pan to wash it with dish soap and water. Replace it if it is broken. 
• Follow the condensate line from the pan until its ends. The line will end outside the house.
• Use a wet or dry vacuum to suck out all the dirt and debris from the end of the line. 
• After cleaning, turn on your furnace to check. 

If you cannot fix it properly, call an HVAC expert.

12. Clogged coil

The evaporator coil is a part of the cooling system. 

Your furnace will have an evaporator coil inside it if you have a central air conditioner and the furnace. 

Heat is absorbed from the surrounding air by the refrigerant in the HVAC’s evaporator coil,

Though the coil is a part of the cooling system, treated air passes through the coil due to its location to enter the duct. 

The air will struggle to pass the coil if it is dirty or clogged. 

Too much clogging will block the warm airflow and make your furnace release cold air.

Solution 

To clean the evaporator coil:

• Remove the blower compartment panel and then the panel for the evaporator. 
• Use soft towels to hand-clean the coil.
• Take a soft brush to clean the coil fins. 
• Now, clean the blower compartment and assembly. 
• Put everything back in place as it was before. 

If you feel uncomfortable, call an HVAC expert to clean the coils. 

Regular maintenance can prevent problems in the future

Some people forget about caring for the furnace once they install it and receive enough warmth. 

But, you must care for the unit and perform regular maintenance and annual servicing. 

Preventive care can reduce these problems and help you discover the issue before the unit shuts down. 

• Replace the air filters regularly, for example, every 90 days or whenever they are dirty. 
• Check the venting systems and accessory units before winter arrives. 
• Confirm the security of the pipes, humidifiers, electronic air cleaners, and other things. 
• Check for leakages and fix them immediately. 
• Clean all the interior and exterior units. The blower wheel, fan motor, blades, and burners must be clean and free of rust and debris. 
• Check for the electrical wiring and controls. 

Also check:

• Furnace Only Works After Reset: 9 Problem+Fix
• 15 Reasons Your Furnace Won’t Turn On (+Fix)
• 14 Reasons Your Furnace Won’t Turn Off (+Fix)

Final thoughts

A furnace is meant to produce warm air in your house to make you comfortable in the winter. 

But a furnace blowing cold air indicates a problem in the unit. 

There could be several reasons behind it. 

You can fix some simple issues, for example, wrong thermostat settings, dirty air filters, flame sensors, condensate lines, and evaporator coils. 

Others, like cracked heat exchangers, damaged ductwork, faulty control board, thermostat wire issues, and issues in the pilot light, will need an HVAC expert’s attention. 

Knowing the exact problem lets you know whether you can fix the problem or whether an HVAC is necessary and prevent you from delaying the troubleshooting. 

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Reference: Wikipedia.