A furnace is meant to produce adequate heat for your house. But it is frustrating to see it not heating. Besides, the unit will keep running to give you the desired temperature and end up overheating. So, when a furnace does not provide enough heat, check and fix the problem. 

The primary reasons your furnace is not heating include dirty air filters, thermostat issues, blocked vents, leaking ductwork, and insufficient gas. Potential fixes are clearing the air filters and vents, sealing the duct leaks, setting the thermostat, and ensuring enough gas. 

There are many more reasons, and this article will share all the major and minor problems behind a furnace not heating up. If you cannot understand the reason behind the problem, consider calling an HVAC expert. 

Why is your furnace not heating?

The biggest problem is identifying the exact problem with your furnace and the method of troubleshooting it. 

To find it out, you must observe the behavior of the furnace and ask yourself certain questions, for example:

• When did the furnace stop heating?
• Is there any leakage?
• When was the last time you fired up the furnace?
• What is the heat source?
• What type of thermostat do you use?
• What kind of heating system do you have?

Asking these questions will help you identify the main issue because the problem can be as simple as the thermostat setting or as major as a faulty blower motor.

The problem could also be in the gas regulator or a bad flame sensor. 

These are the safety measures that prevent your furnace from working properly. 

Now, let’s get into the reasons without further delay. 

1. Thermostat issues

Thermostats might get turned off, or settings can get flipped by mistake, for which your furnace will not heat up. 

Or, your thermostat may have understandably turned off for a reset cycle.

Even the slightest change can make your thermostat work differently, for which the furnace may not work properly. 

The thermostat is connected to the furnace to regulate the unit’s heating. 

These thermostat issues can make your furnace stop heating based on the temperature, status, and room size. 

Another reason is dead batteries. 

A furnace will only start heating when the thermostat sends signals to the unit after you set a certain temperature. 

If the thermostat has a low battery, it won’t let your furnace heat your room. 

Wire problems also do the same thing. 

The white wire helps in heating the room. 

If the wire is frayed or loose, the thermostat won’t be able to signal your furnace to heat your room. 

Make sure that all the wires in the thermostat are tight. 

If they are loose, tighten them with screws or ask for an expert’s help. 

Change the batteries from time to time. 

If your thermostat is set to COOL, change it and set it to AUTO or ON.

Raise the temperature to 5 degrees more than the room temperature. 

If the thermostat is dirty, open the cover and dust around the coils and contact plate with a soft paintbrush. 

3. Loose or worn out belt or faulty blower motor

You will hear a hissing or squealing sound from your furnace sometimes.

The reason is a loose belt, especially if the furnace is very old. 

These belts can wear and tear and release a burning smell from your unit. 

You must turn off your furnace and contact an HVAC professional. 

Sometimes, the belt might be fine, but the blower motor might have been worn out due to the daily wear and tear. 

The motor bearings may also get locked up. 

The motor needs to be replaced. 

Other signs of a faulty blower motor are high energy bills, overheating, or weird noises. 

You cannot fix the problem yourself. You need an HVAC expert to handle the matter. 

4. Blocked blower

The blower could accumulate dirt and debris if your furnace is quite old. 

If the blower is blocked due to dirt accumulation, it cannot move properly.

As a result, the hot air cannot circulate through the ducts to your house efficiently. 

If you do not fix the problem, it can also damage your heat exchanger after some time. 

To resolve the situation:

• Turn off the furnace.
• Remove the unit’s screw to lift the outer panel. 
• Unscrew the panel covering the blower motor and disconnect the wires. 
• Remove the shelf’s bolts on which the control panel sits and lift out the panel. 
• Unscrew the motor and fan to remove them. 
• Use a handheld vacuum with a brush attachment to clean the wheel fins.
• Once you have cleaned, put everything back in place and turn on the furnace to check. 

If you cannot resolve it yourself, call an HVAC expert. 

5. The blower capacitor is not working

A blower capacitor works like a battery to help the furnace’s blower operate power. 

When the capacitor fails to work, the blower will stop working and fail to spread the hot air. Additionally, it will make loud noises. 

Due to a failed blower capacitor, the furnace will not be able to produce enough heat. 

You will have to replace the capacitor with the help of an HVAC expert. 

You can check whether the capacitor is the one creating issues with the following steps:

• Turn off the furnace (and gas supply if it is a gas furnace) and remove the blower to get the capacitor. 
• Remove the wires and the capacitor with a screwdriver having an insulated handle across the terminals. 
• For dual capacitors, touch the screwdriver from the FAN to the common terminal and then from the HERM to the common terminal. 
• Join each terminal to the multimeter’s probes to read the levels. 
• Compare the reading with the value written on the capacitor’s side. 
• If the reading does not match, you must buy a new capacitor to get it replaced by an HVAC expert. 

6. Pilot light problems

In the older furnaces, you will have a pilot light which should remain lit whenever your furnace is on. 

If any dirt and debris clog the pilot orifice, it will prevent the light from lighting up. 

The pilot light indicates the gas flow to the furnace. 

If the light does not ignite, the gas flow will stop, and your furnace will no longer ignite to heat your room. 

Sometimes the light won’t be lit up, so the gas does not flow to your furnace. 

To fix the problem, hire an HVAC expert.

7. Low pilot light flame

Sometimes, the pilot light’s flame setting becomes too low, and the furnace cannot receive enough gas to heat up. 

You need to hire an HVAC expert to fix the light flame. 

If you want to try it yourself due to some experience you gather, try the following:

• Find the screw near the pilot light. It will be written ‘pilot adjust.’ The screw will be next to this writing.
• Remove the screw and poke in your screwdriver. 
• Light the pilot light.
• Turn the screwdriver until the light increases its flame size. 

8. Dirty flame sensor

The flame sensor in the furnace senses the flame to fire up the furnace. 

If the sensor is dirty or faulty, it cannot sense the flame to fire up the furnace. 

Even if it fires up the furnace burners, the unit cannot heat up properly. 

So, it would be best to clean it properly. 

To fix the problem:

• Unscrew the front panel of your furnace and close the gas valve. 
• Find the small metal rod with a porcelain base on the right side of your furnace.
• Use a nut driver, loosen the rod, and bring it out for cleaning.
• For cleaning, you can use rough paper, light sandpaper, a dollar bill, or emery cloth (3M Emery Cloth, 03008, 3 ⅔ in. x 9 in. 6 sheets per pack). 
• Put it back in its place and tighten the screws. 
• You need to replace the sensor if you find any missing pieces or cracks. 

9. Check for the flame 

If your furnace’s flame is out, an HVAC team will need a new one replaced. 

Since the flame is the heating element, your furnace won’t heat up if the flame is out. 

If the flames are up, but the blower motor is not blowing warm up, and the furnace is not heating, consider turning off the unit for a minute and then turning it on again.

Restarting the unit is an easy fix to start the flames and allow the furnace to heat. 

10. Fuel and ignition problems

If your furnace is on but does not produce enough heat in your room, it could be a problem related to the fuel supply or ignition system. 

These are two of the main things, among others, that help in heating the furnace. 

If you suspect these problems, consider checking the following things:

• Check the gas valve. It should remain on for enough gas flow. Open it if you find it closed. 
• Check the gas tank to ensure enough gas is left if you have a gas furnace that uses liquid propane or oil as the heating fuel. Call your gas supply to refill the tank if there isn’t. 
• If you use natural gas, call the provider for any interruption. 
• If the furnace has an electronic ignition system, the metal strip of the igniter switch might be dirty. Gently clean it off, but since the component is fragile, call an HVAC for help. 

11. Dirty air filters

When you turn on the furnace, it takes in the cold air to heat it and spread throughout your house. 

While taking in the cold air, the air gets filtered through the filter, and all the dirt and air particles get stuck. 

Over time, the filter will become dirty and block the warm air from spreading. 

So, the furnace will no longer heat your room. 

You need to check the filters once a month and change them every 3 to 4 months. 

Open the furnace and look for the filter inside. Try checking it through to estimate the dirt level. 

Remove the filter from its slot and replace it with a new filter. 

If your filter is washable, remove the filter and wash it. 

Let it dry with a paper towel and put it back in place. 

Avoid using any harsh chemicals. 

The frequency of changing filters depends on the size:

• 1 to 2-inch filters – Replace them every 1 to 3 months
• 3 to 4-inch filters – Replace every 6 to 9 months
• 5 to 6-inch filters – Replace every 9 to 12 months

12. Leaky ducts

The ductwork runs through your house to supply hot air from the furnace to all the rooms. 

The ducts must be sealed enough so that the hot air does not lose through any leaks. 

Generally, ductwork can lose up to 40% of air. An extra 20% of the heat will escape if there is a leakage. 

If 60% of heat escapes through the leaks, your house will receive insufficient warm air from your furnace, no matter how hard your furnace is working. 

A visual inspection is required. 

Walk along the duct system of your house to look for signs of leaks, cracks, dings, or holes. 

Turn the furnace on before you walk along the ductwork to understand from which place the air is leaking. 

Call an HVAC expert to seal the ducts.

13. Blocked or closed air vents

Your air vents can get blocked due to some objects or furniture. 

Sometimes, people turn off the vents for rarely used rooms to save energy bills.

The air vents allow the air to distribute throughout your house. 

When the furnace heats the air, the blower will spread the air through the ductwork to your house. 

The air is pushed out to all the air vents found in the building to spread all the rooms. 

Any blockage or closing will not let the air escape. 

As a result, your furnace won’t heat your room properly. 

Additionally, the vents will forcefully try to release the air and end up with constant wear and tear.

Also, if the vents of some rooms are open, the blocked air will enter these rooms and make them exceptionally hot. 

Take a walk through your house, especially in areas where the vents are present, and check if they are closed or blocked. 

Open up the closed vents and remove the objects blocking them. 

14. Wrong furnace size

Furnaces are available in various sizes. 

Before you buy one for your house, you need to know the measurement of your house. 

The right size depends on the house size. For example, a 1,200 square footage will require a 36,000 to 72,000 BTU furnace, and a 1,500 square footage will need a 45,000 to 90,000 BTU furnace. 

The furnace size also varies based on the lowest temperature of your zone. 

For 1,200 square footage, you can get a 36,000 BTU furnace if the temperature is average. 

But if the temperature is much lower than tolerable, you need a 70,000 BTU furnace. 

That is how the furnace size varies. 

If you choose a smaller furnace, it will struggle to give you the desired temperature but only in vain. 

As a result, your furnace won’t be able to heat your room enough.

So, you must choose the right furnace size. Here is a small guide to help you:

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

15. Poor insulation

Sufficient insulation in the house will let your furnace work properly and give you enough heat. 

But with poor insulation, your house will receive all the cold outside air. 

As a result, the heat provided by your furnace won’t be enough to heat your room. 

All the furnaces use a recommended amount of heat level. 

If your house has poor insulation, you may have to use it higher than the recommended level, which can increase the energy bills. 

Besides, your furnace has to work harder than normal without stopping. 

Also, Drafty windows are another reason behind the furnace’s not heating enough. 

Make sure your house has adequate insulation. 

If you have drafty doors and windows or they have cracks, consider sealing them with caulk or weather stripping. 

If you already have weather stripping, they might have been worn out. Replace them with new ones.

16. Maintain your furnace regularly

Many people have furnaces since they are important, especially during the winter. 

But unfortunately, they do not touch or observe its behavior post-installation.

As long as it provides heat, the house owners forget its existence. 

So, not attending regular maintenance and servicing will raise issues, not heating being one of them.

Furnace maintenance ensures the unit’s efficiency and smooth functioning. 

For proper maintenance of your furnace:

• Keep the air filters clean and free of debris and dirt. Check them monthly and change or clean them every 3 to 4 months. 
• Test the carbon monoxide detectors for proper working
• Make sure the furnace does not collect any dirt and debris inside. 
• Ensure that the flame sensor is clean and does not have any cracks. 

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 failing to heat will have multiple reasons. 

Some are simple fixes, and some need an HVAC expert. 

For example, cleaning the air filters and flame sensors and fixing the thermostat issues are simple problems. 

Others will require the HVAC’s attention. 

Knowing the reason will let you know whether you must call for help and prevent you from delaying the troubleshooting.

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Reference: Wikipedia.

As a beginner, you may think a furnace can constantly run until you turn it off. But that is not the case. A furnace runs in cycles, just like air conditioners. But if the furnace is constantly running, there might be some issues which we will explain in this article.

The reasons your furnace is constantly running are the wrong thermostat setting, a damaged controller, a dirty filter, leaky ducts, a damaged compressor contactor, or an undersized furnace. Correct the thermostat, clean the filters, get the right size, and replace the blower motor, compressor, and ducts.

There are many other reasons behind a furnace not stopping, and this guide will explore all the possible reasons and solutions. You can fix some of them, but most require HVAC experts’ attention.

How does the furnace run?

It is quite common for a beginner to think that furnaces constantly run to heat the room. But that is not the thing. 

A furnace will run in cycles. It turns on and off every 15 minutes. 

Whenever you turn the furnace on, it will try to give you the desired temperature you set in your thermostat. 

Once done, it will turn off and again turn on when the room cools down. 

It works the same way as the air conditioners. 

If your house has enough insulation, the furnace will run 2 or 3 times per hour to give you the desired temperature. 

Why shouldn’t the furnace run constantly?

The furnace must not run continuously. 

The heating cycle’s length varies based on factors like the outside temperature, the temperature set in the thermostat, and house insulation. 

It should turn on and off every 10 to 15 minutes. 

If your unit is constantly running to warm up your house, your furnace has some issues, and you must call an HVAC expert to repair it. 

A furnace not turning off may seem irritating, but it can be dangerous too. Constant running of the unit causes overheating and increased energy bills. 

Additionally, overheating can lead to an explosion. 

So, let’s get to the reasons behind a furnace not stopping.

1. Wrong thermostat setting

The right setting for the furnace in the thermostat is AUTO. 

It will make the blower run only when the heat is on and shut off when your room has reached the desired temperature. 

If the thermostat is set to the ON position, the blower fan will run even if the cycle is complete. 

As a result, this will overheat your unit, cause fast wear and tear, and increase the energy bills. 

How to fix it?

You need to transfer the thermostat switch from ON to AUTO. 

It is one of the easiest fixes for a furnace not stopping. 

If changing the setting does not solve the problem, you must check for other issues and contact an HVAC expert to deal with them. 

2. Malfunctioning thermostat

An old thermostat or a thermostat with bad wire faults will fail to signal the furnace to stop. 

Also, a bad thermostat will not send the right signals about both stopping or turning on the unit.

As a result, your heater will stay on without shutting off between the cycles.

What to do?

Instead of trying to fix the problem yourself, replace the unit or repair the faults by hiring an HVAC expert. 

However, you can confirm the problem yourself. 

Open the thermostat cover and find loose or frayed wires. 

3. Damaged primary controller

A primary controller inside the furnace is an electronic relay that controls the burners in response to the room temperature. 

The controller responds to the signal from your thermostat and the limit control to handle the starting of the furnace, running the cycles, and shutting it off. 

Generally, a primary controller:

• Reacts in the existence or lack of flame.
• Manages the furnace startup and guarantees the flame’s pressure before turning on the ignitor, gas valves, and burners.
• Controls the furnace’s shut-off when the thermostat signals that the desired temperature has been achieved or when the limit control opens.

If the controller is damaged, it cannot send signals to the thermostat about reaching the desired temperature, making the unit run constantly. 

How to fix it?

Modern furnaces have primary microprocessor controllers more than stack switches. 

So, the HVAC experts should check and fix the primary controllers. 

They have better knowledge about the controllers of modern times. 

4. Dirty air filters

A dirty air filter is a problem in all types of furnaces. 

When the heater does not turn off, expect a dirty air filter because the warm air struggles to pass through the filter and let your room receive the desired temperature. 

When the furnace starts, the cold air is sucked from your house into the heating chamber through this filter. 

The filter cleans the air before getting into the heating system.

As a result, the debris and air particles stay stuck in the filter. 

When the filter gets dirty, little air passes to the heating chamber. 

You receive a small amount of warm air. 

When you do not receive the desired temperature, the furnace will struggle for the preferred temperature and continuously run without ending the cycle. 

It will also increase your energy bills and overheat your unit. 

How to fix it?

Depending on the furnace type and when you changed the last filter, you must replace or clean the filter. 

Replace the filthy fiberglass, pleated, and electrostatic air filters. 

Wash or clean the permanent electrostatic and high-efficiency pleated air filters.

Change or clean the filters every 3 to 4 months and keep checking them once a month. 

If new filters get dirty too fast, call an HVAC expert to look into the matter. 

5. Faulty blower fan motor

A blower fan motor in the furnace helps pull the cold air from your house to the heater to transfer into warm air and send it to your house. 

The blower motor runs in sync with the heating cycle of your furnace. 

If the blower fan motor is at fault, your furnace will continue running. 

Furnace blowing cold air is another sign of identifying the problem. 

Common reasons are wrong thermostat setting, faulty thermostat wire, fan limit switch set to ‘manual override,’ or a faulty fan limit switch wire.

How to fix it?

You can correct the blower fan motor fault by correcting the thermostat or fan limit switch setting. 

You need an HVAC expert to fix the problem for other electrical faults like the wires. You might also have to replace the blower motor.

6. Leaky ductwork

A leaky ductwork can affect your financial and health status. 

According to the University of Florida Institute of Food and Agricultural Sciences, ductworks lose around 40% of heating energy. 

If the ducts leak, your system will lose an additional 20% of heating energy, up to 60%. 

It is because the ductwork is present in areas like the attic, garage, or basement. 

So, if your ducts have leaks or holes, they will lose a huge amount of heat through those holes. 

So, your heater will work hard, constantly run to give you the desired temperature, and won’t turn off. 

The ducts can leak over time due to ongoing wear and tear. 

A common sign behind identifying the problem is noticing temperature fluctuations between the rooms. Warm air does not reach all the rooms equally. 

How to fix it?

You can seal the leaky ducts immediately so that they can recover the lost hot air supply and spread it throughout your house. 

However, sealing is the job of professionals. 

They can also suggest whether sealing will help or it requires replacement. 

7. Damaged compressor contactor

A compressor contactor is a small device that controls power flow to different parts of the HVAC system.

Based on the heater’s power requirements, the compressor contactor will provide enough power supply and shut it off when not in need. 

If the compressor contactor works incorrectly, it will keep sending power even when the furnace doesn’t need it and refrain the unit from turning off. 

How to fix it?

You need to professionally replace the faulty compressor contactor by calling an HVAC expert. 

Fixing it yourself may be risky, especially if you do not know about it. 

Besides, you may do something wrong, further increasing expenses. 

8. Stuck buttons and switches

Before calling the HVAC experts, consider checking all the buttons and switches of the furnace for once. 

A furnace contains switches that deal with heating. 

Sometimes, these switches pressed downwards are the common reason behind a furnace not stopping. 

Sometimes, the switches themselves are at fault. 

What should I do?

The good news here is you can fix the problem yourself. 

Turn off the furnace to shut off the electricity running to the unit and gas. 

If the switches are pressed down, push them upwards and check if the furnace works properly like before.

If the buttons and switches malfunction despite staying in position, you need an HVAC expert to replace them.

9. Relay is stuck

If the furnace is not stopping, it could be because some element inside the unit is stuck. 

You can create a vibration to see if you can shift the stuck components like a mechanical switch or a relay on the circuit board. 

How to fix it?

To create a vibration:

• Turn on the furnace.
• Give your furnace a jolt, or smack it lightly. It will stop the blower. 

If there has been a stuck component, it will get unstuck with the jolt or smacking. 

But if the furnace problem still needs to be resolved, you need to hire an HVAC expert to fix the problem. 

10. The gas valve stuck open

Sometimes, you will unplug your furnace or turn it off, but the unit will keep running. 

It could be because the gas valve may be stuck open.

If the valve is stuck mechanically, it will burn the natural gas and keep your furnace running even after staying off. 

How to fix it?

• Find the gas stop near the furnace. 
• Turn off the gas. 
• Contact your HVAC expert to fix the problem. 

11. Relay contacts are pitted

There might have been some problem with the relay. 

If you find sparks inside the unit contacts, they may be pitted and arc welded. 

In such a malfunction, the contacts will stop opening and keep the furnace running. 

What should I do?

Since the problem is electrical, you cannot tackle it yourself. You need an expert’s help to fix the problem. 

The expert will also check your system and fix everything correctly. 

12. Wrong furnace size

The size of the furnace depends on the size of your house. 

You must find out the square footage of your house to find out how many BTUs you need in your furnace. 

For example, a 1,500 square footage house will require a 45,000 BTU furnace. 

It may increase or decrease based on the weather conditions. 

The BTU will increase if the outside temperature in winter is low and decrease if the temperature is average.

If you have an undersized furnace, it cannot warm all your rooms properly. 

So, it will constantly run to give you the desired temperature.

What should I do?

You may have to replace your furnace and buy a new one that suits your house. 

Make sure to avoid getting an oversized one. 

That will quickly warm your house and constantly turn on and off, reducing your furnace’s efficiency and lifespan. 

A short guide about furnace size:

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

The size can increase or decrease based on the outside temperature of your zone.

13. Electrical problems

If you have looked through all the above reasons and still failed to troubleshoot the problem, the electrical sensors might be the culprit. 

Though the problem is rare, one should not ignore it. 

The problem arises from the electrical sensors that improperly register the heat or timing. 

When this happens, the furnace cannot pick up the signals and warnings of upcoming problems or settings. 

The furnace will therefore keep running and end up overheating and exploding. 

You cannot fix it yourself. 

The best thing here would be to call a maintenance specialist to fix the problem. 

14. Furnace lifespan and repair cost

If your furnace is maintained properly, you can expect it to run for at least 15 to 20 years. 

An electric furnace has a longer lifespan, around 20 to 30 years. 

Some furnaces also last for 40 years, but it is unexpecting. 

For a better idea, check the warranty and lifespan online. 

The cost to repair a furnace varies depending on the damage type, damage level, and region. 

The average cost to repair an HVAC will cost around $50 to $100. 

The total cost, including the labor and installing new parts, may cost around $300 to $450. 

Also check:

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

Final thoughts

There are a handful of reasons behind a furnace not stopping, and you must find out the real cause and fix it soon to prevent it from worsening. 

Before you call an HVAC expert, consider finding out the real cause yourself. 

Though most issues will need an expert’s attention, you can solve some for yourself, like correcting the thermostat settings, changing filters, and fixing the relay switches. 

But problems like broken blower motors, leaking ductwork, and other things will need to be fixed by an expert. 

No matter what the problem is, you can at least figure out the symptoms and the possible problem with the help of this guide to inform your technician. 

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Reference: Wikipedia.

The best thing about furnaces is you receive warm and comfortable air even during harsh winter. But the worst thing is the unit fails to start sometimes. This article will explore the various reasons behind a furnace not turning on. 

A furnace does not start due to tripped breakers, thermostat problems, clogged filters and condensates, dirty flame sensors, or safety switches. Potential fixes include checking the breakers, cleaning the furnace filters and ignition, and emptying the condensate drains. 

Even if you need an expert’s help fixing the furnace, knowing how to solve some problems can save time and money. Since there are multiple reasons, cross-check the reasons to confirm it and then approach troubleshooting. Now let’s get to the reasons right away.

1. Tripped breaker

When the breaker connected to the furnace trips, your furnace will not turn on. 

It occurs when the same breaker powers too many heavy-duty appliances and ultimately trips off due to overloading. 

Overloading can lead to short circuits. 

Tripping off the breaker means it is protecting itself from severe short circuits. 

Sometimes, it could be due to some ground fault. 

How to fix it?

When a breaker trips, the handler will remain in the center between the ON and OFF switches. 

You need to reset the breaker. 

To resetting it:

• Make sure that the switches and plugs in the room are turned off. 
• Open the breaker box and find the tripped breaker switch. It will be easy to find because the electricians mark the breaker for easy finding. 
• Suppose the breaker is between the ON and OFF positions. First, move it to the OFF position, wait for some minutes, and then turn it ON. 
• If the breaker is already in the OFF position, push it straight to the ON position. 

These steps should solve the problem. 

But, if the switch doesn’t turn ON or re-trips after resetting, do not reset it again. 

Please turn it off and call for the HVAC team’s help. 

Check for damaged wires, switches, or fixtures. 

2. Thermostat problem

Thermostat problems are common.

It could be due to wrong thermostat settings, wire issues, or dead batteries. 

Setting the thermostat to OFF or COOL won’t let the unit start.

Some thermostats have low battery indicator settings that inform you about low batteries.

Other reasons could be dust buildup, dead fuse, time error, or incorrect repair. 

How to fix it?

The solutions will differ based on the exact problem. 

For the wrong thermostat setting, set the thermostat to HEAT and not OFF or COOL. 

Make sure the temperature is at least 5°F above the room temperature. 

If the thermostat is dusty, open the device and blow the dust away. 

Use a soft brush to clean it, but be careful of the wires. 

Replace the dead batteries. Use batteries that suit your thermostat model. 

Call an expert if there are any electrical wire faults, like loose or frayed wires. 

Replace a dead or blown-up fuse, and make sure that the fuse matches the thermostat’s make and model. 

Sometimes, incorrect times and dates are the culprits. 

Ensure that the thermostat has the correct times, dates, and timer. 

If you still cannot figure it out, contact an HVAC expert. 

3. Clogged furnace filter

Over time, the furnace filters accumulate dust and debris, compromising the furnace’s efficiency. 

Check the filter monthly and replace it every 3-4 months for smooth functioning. 

A clogged furnace will make a humming noise, indicating dirty air filters. 

Ignoring will make the furnace will stop working after some weeks of humming.

If a new filter gets easily clogged, the possible reasons could be:

• Mold and soot
• Excessive carbon monoxide
• Damaged vents 

How to fix it?

If the filter is dirty, remove it, clean it, and then put it back in the furnace. 

Sometimes a clogged filter means it has worn out over time and requires replacement. 

Open the furnace cover and replace it. 

Make sure the filter is installed in such a way that airflow can circulate properly. 

If the culprit is carbon monoxide, mold, or damaged vents, you may require an HVAC expert’s help to fix the problem. 

4. Dirty or faulty furnace flame sensor 

The ignition or flame sensor inside the furnace is an essential part of the furnace that senses the flame and fires the furnace. 

If the flame sensor is dirty or faulty, it won’t sense the flame, and the furnace will not turn on. 

The furnace will click, but it won’t start or shut down immediately after turning it on. 

How to fix it?

Remove and clean the dirty sensor and then put it back in place. 

If the flame sensor is at fault, replace it. 

Here are the steps to clean or replace the flame sensor:

• Turn off the main power at the breaker. 
• Close the gas valve. 
• Unscrew the sensor and pull it out of the furnace.
• Check the sensor properly to see whether it is dirty or damaged. 
• See the porcelain base with the metal rod and loosen the screws to remove it.
• Use a clean rod with steel wool or a knife and remove the buildup. 
• You can also use an emery cloth to clean the dirt, for example, a 3M Emery cloth, 03008, 3 ⅔ in x 9 in, 6 sheets per pack. 
• If the sensor is damaged, cracked, or has missing pieces, replace it. 
• Reinstall the sensor, switch on the furnace, open the gas valve, and fire the furnace. 

If the problem persists, you need an HVAC expert. 

If the furnace works after cleaning, you do not have to replace it. Only do it if the furnace remains off after cleaning. 

5. The furnace gas supply interrupted

The unit will not turn on if the gas line does not send gas to the furnace. 

The new furnace models have an automatic combination gas valve to control the gas supply and pressure regulator. 

So, issues with the gas valve mean an interruption in the gas supply. 

The gas valve will open and close for the gas flow to the furnace burners and pilot light. 

What should I do?

Check the gas flow by turning on other appliances that run on gas. 

If the appliances work, the interruption is due to some furnace problems. 

You need to figure out that first. 

Some possible checkings you must do are:

Gas line and gas valve

Check the gas valve of the furnace to ensure it is open. 

A closed valve will indicate a problem in the gas line. 

Call a professional if the valve is closed. 

When the other gas appliances run properly, the problem is not with the gas valve or gas line. 

But if none of the gas appliances are running, check the main gas valve controlling the gas flow and make sure it is turned on. 

The gas valve will be beside the furnace and look like a box with a disk or a lever with the thumb size. 

It will sit over the gas line around 6 feet from the unit. 

If the lever is parallel to the gas pipe, the valve is open and closed if the level is perpendicular. 

Check the street valve

If the gas valve is open, check the street valve. 

Sometimes, the utility technicians close the valve after the gas line maintenance. 

If both valves are open, contact the natural gas provider from your locality to look into the matter.

Check the pilot light.

Most of the furnaces will have a pilot light that lights up whenever the gas flows to the furnace. 

If the light remains off, it indicates the gas is not flowing, and there is some problem. 

Here too, the HVAC expert must handle the matter. 

If you see any signs of leakage or bad smell, immediately turn off the gas supply valve and contact the supply company. 

6. The pilot light is not working

As I mentioned earlier, the pilot light indicates the gas flow to the furnace. 

If the pilot light does not light up, your furnace will not ignite and heat your room. 

It requires the HVAC’s attention. They will fix the light and reignite or replace the pilot light if needed.

7. Bad blower motor

The blower motor in the furnace circulates air from the unit to the vents and your house.

If the motor fails to work well, the whole heating system of your house gets compromised. 

Common causes behind a bad blower motor are insufficient power, burnt out, or weak capacitor.

A malfunctioning blower motor’s symptoms are burning smells, humming sounds, and furnaces not turning on. 

If the furnace shows a blinking green light, it indicates something is wrong with the blower motor.

Overheating can also signify dirt buildup in the motor, making the heat unable to reach the vent. 

How to fix it?

Fixing a blower motor is quite tough if you are not a professional. 

If the problem is insufficient power, turn off the unit, and check the motor on the right side of the unit. 

If the blower has tried to turn off multiple times, it will be hot. 

Replace the motor by consulting with the HVAC expert.

8. The blower capacitor is dead

The capacitor is a battery-like part of the furnace attached to the blower. 

The blower capacitor signals the ignitor when the thermostat tells it to provide the power required by the blower to start the fan.

Try checking the capacitor.

Check the capacitor with a meter to confirm that the level matches the label. If not, you have to replace the capacitor with the help of an expert. 

Some common signs of a bad capacitor are:

• Overheating – It will trigger the safety switch, and the unit won’t turn on. 
• Burning smells – It happens when the furnace has frayed or corroded capacitor wires and a faulty power supply.
• Slow and overheating furnace blower – It indicates that the capacitor does not have the changes required by the blower for running. 

A weak and faulty capacitor does not let your blower motor function properly.

How to fix it?

A faulty capacitor is common and easy to fix, provided it is broken and not something else. 

• Turn off the furnace power at the breaker. 
• Open the panel to check the capacitor furnace. 
• Get an identical capacitor matching the make and model of your existing furnace. 
• Remove the old one by removing the screws and then plug in the new one. 
• Fix the wires properly. Take a photo before removing the capacitor to remember all the positions.
• Close the panel and check the furnace.

Inspecting the capacitor during the annual maintenance may confirm the problem. 

Changing during that time will prevent the problem and encourage smooth functioning.

9. Damaged or cracked heat exchanger

The furnace heat exchanger separates the combustion from the air exchange. 

It contains metal chambers and passages from the burner assembly to the chimney vent. 

If the exchanger is faulty or damaged, the furnace won’t start. Even if it starts, it will fail to fire up properly. 

Some common symptoms are:

• Cracks near the heat exchanger’s seams
• Discoloration due to mold and doot escaped from the cracks
• A jumping flame due to the cracks that allow the air to mix with the air

A bad heat exchanger will cause ignition failure due to toxic carbon monoxide leakages. 

A cracked exchanger can also indicate other issues like low airflow.

How to fix it?

The best solution is to replace the heat exchanger. Hire an HVAC expert.

If the damage on the heat exchanger is due to an aged furnace, buy a new furnace system. 

10. Too much gas burning

Furnace manufacturers include specifications about the volume of gas a furnace must burn. 

A house owner regulates the furnace settings for more heat when the house is frigid. 

So, it will burn more gas than the recommended amount. 

It will overheat and automatically trigger the safety switch, which won’t allow your unit to turn back.

Besides, more gas means high expenses. 

How to fix it?

You cannot do much except for the thermostat setting. 

Ensure the annual maintenance of your unit. 

It will have a combustion analysis to check the amount of gas the furnace burns. 

If you set your furnace to high heat levels, turn it back to the recommended level. 

Ask for the HVAC’s advice if you are unsure about the problem. 

11. Condensate pan not draining

The condensate drain pan inside the furnace unit works like a container collecting condensed moisture. 

It has a drain and a pump to remove the moisture and send it to the wastewater drainage system.

If the condensate pan’s pump does not work properly or there is a blocked drain, the float switch will get triggered and stop your furnace from turning on. 

How to fix it?

To fix the problem, confirm that the condensate pan’s drain is clogged or the pump is jammed. 

Jammed pump

If the pump is jammed due to dirt, you must clean it and move it if it gets stuck. 

Clean your hands after working with the pump, as the condensed water will have acid. 

If cleaning does not fix your furnace, replace the pump with the HVAC expert’s help. 

A clogged condensate pan drain

Clean and remove the dirt clogging the drain pan. 

Adding condensate pan tablets can help in cleaning the drain. 

For example, SimpleAir Clean Flow HVAC Drain Line Treatment tabs, 6 counts. 

These tablets get activated by moisture and prevent clogging for at least 2 months. 

Once you have cleaned the pump and the pan drain, turn on the furnace switch. 

If everything is fine, the unit should turn on within a minute. 

12. Blockage in the airflow

The furnace must have enough airflow inside and outside for efficient functioning. 

Shift your attention to the airflow if you have checked all the above reasons. 

An outside element might block the airflow at the PVC pipe. 

If the fumes cannot exit, the furnace will stop working. 

Additionally, furniture can block the airflow at the vents and prevent the warm air from entering your house, causing overheating.

As a result, the furnace will shut off and not turn on again due to overheating.

How to fix it?

Check the warm air outlets and the vents for blockage. 

See the outdoor vents to check that the PVC pipes are not near any objects or furniture, as they may block the air. 

Make sure that the vents are clear. 

13. The furnace power is switched off.

You will see the furnace not turning on for silly reasons like switched off. 

Sometimes the furnace has its power switch above or near the unit. 

House owners mistake this switch as the light switch and turn it off. 

The fix for this problem is simple. 

You need to be aware of the furnace switch and turn it on when you need it. 

Turn on the switch to start your furnace. 

14. Tripped high-limit switch

When your furnace does not turn on, the result could be a tripped high-limits switch. 

This switch helps communicate the temperature inside the furnace to the thermostat to turn on the blower motor. 

It is a safety feature that turns off your furnace and does not turn it on when the unit gets overheated. 

The switch will also trip if it is dirty, damaged, or requires replacement. 

Hire an HVAC expert to replace the switch. 

If the switch has tripped due to overheating, let your furnace cool down for a bit and then turn it on, or find out the reason behind the overheating. 

15. Triggered pressure switch 

The pressure switch is another safety feature that turns your unit off when it senses the exhaustion of the flue gas. 

When the switch gets activated, it detects a negative pressure in the unit’s flue pipe or draft inducer motor.

As a result, it stops the harmful exhaust back drafting, which can make you sick and even kill you. 

The automatic shutdown will save these gas leaks and your health. 

If the pressure switch gets dirty or worn out, it will wrongly trigger your furnace to shut down. 

Call an HVAC expert to look into the matter.

When should I call an HVAC team?

Most furnace problems need an HVAC expert’s attention, and some don’t. 

Here are 5 furnace problems where you must call the expert without any second thoughts:

• If you have cross-checked all the issues and cannot figure out anything, hire a professional.
• If the furnace releases a gas or propane smell, shut down your unit and call a professional. 
• Avoid doing anything yourself, especially if things are related to electrical wirings.
• Immediately call an HVAC professional if the furnace problem is related to the bad motor. 
• If the circuit breaker trips more than 2 times, shut down your unit and hire an HVAC team to deal with it. 

Final thoughts

Not having routine maintenance of your furnace unit is one of the common reasons behind the unit having so many problems. 

Regular checks once or twice a year will inform you what changes your unit requires. 

It will help the unit’s smooth functioning for years without any problems. 

Since there are multiple reasons behind a furnace not turning on, you must first confirm the problem and troubleshoot it. 

Reasons like safety switches get triggered whenever they get a signal that something is wrong with the unit. 

These switches save you and your unit from severe problems. 

If you are confused about what to do, call an expert. 

FAQs

How to reset the furnace?

The furnace will have a reset button attached to the unit. 

Press it for a few seconds. 

The reset button’s location may vary based on the furnace type. 

But it is mostly inside the blower compartment or on the wall near the unit.

If it does not work, reset the breaker.

Also read: Furnace Only Works After Reset: 9 Problem+Fix

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Reference: Wikipedia.

The reset button in a furnace is a quick way to solve a short-term problem in the furnace. But, if you always need to reset the furnace to start it, there might be some deeper issues with the unit. 

Dirty filters, faulty sensors, and controllers, closed dampers, low fuel, corrosion, and aging are common reasons behind a furnace starting only after a reset. To solve it, clean the filters, replace the controller, check the dampers and gas level, and look for error codes in the thermostat.

In this article, we will go through these problems in detail with some solutions for why the furnace starts only after resetting. These problems are mostly seen in oil furnaces but apply to other furnace types. 

Why does a furnace start only after resetting?

You will sometimes face a situation where the furnace will start, but it won’t fire. 

Once you flip the power switch off and turn on the furnace again, the unit will work fine. 

You will either face this problem very few times or frequently. 

It is the furnace’s way of telling that the unit is facing some issues. 

You need to detect it and solve it quickly. 

A furnace is an important part of the house during the winter. 

It helps warm the surrounding air and spread it throughout the house. 

If there is any serious issue, you must fix it. 

If your furnace starts only after you reset it, the problem can increase and progress to something serious over time. 

Here are a few reasons behind a furnace starting only after resetting:

1) The control board is bad.

A furnace starting only after resetting could be due to the control board failure. 

Maybe the board is in bad condition, so the unit keeps shutting down and does not start after resetting. 

When you shut off the main power to reset your furnace again, it clears the error code and allows the furnace to work great, but temporarily. 

If you delay analyzing what is wrong with the control board, the problem will persist and progress over time. 

So, it is better to hire a professional and ask for help to fix the controller soon. 

2) Dirty air filters

Some error codes occur due to dirty air filters. 

As a result, your furnace won’t start unless you reset it. 

A furnace does not turn on if the air filter is very dirty. 

It blocks the airflow and does not allow the furnace to spread the warm air. 

Some furnaces have an in-built system of shutting off if the airflow gets blocked due to dirty air filters. 

Since resetting will clear the error, your furnace will work great after you reset it. 

But the furnace won’t start over time even if you reset it. 

Over time, the furnace will stop producing warm air and release yellow or even flames instead of blue. 

So, you need to change the filter. 

Shut down the main and open the furnace’s external panel. 

Gently remove the filter and try seeing through it for darkness and discoloration.

If you cannot see anything through the filter, it is time to replace it. 

If you buy a washable filter, you do not have to change it frequently. 

You must wash it with water, dry it, and install it back. 

While installing the filter back, install it in the airflow’s direction to let in enough air. 

3) Faulty or dirty flame sensor

When you turn on the furnace, the burner will turn on, but it will go out after a few seconds. 

Resetting will temporarily clear the error, so your furnace will start working fine like before. 

Consider checking the flame sensor once. 

There could be dust and debris that covers the end part of the sensor. Cleaning the flame sensor can fix the problem. 

Sometimes, it could be because the sensor casing got cracked. 

In that case, you have to replace the sensor. 

Replacing the sensor is a low-cost fix. 

You can handle it yourself if you are fine with working on the furnace. 

Otherwise, hire a professional. 

To clean or replace the sensor:

• Open the furnace access cover. 
• Based on the model, you need to unscrew some screws or bolts. 
• You will find the sensor outside the burner assembly. It looks like a small rod with porcelain surrounding the end connected to the burner. 
• Some sensors are straight, and some are bent at a 45 or 90-degree angle. 
• Disconnect the wires leading to the sensor and control board and loosen the screw to remove the sensor.
• Take an emery cloth and gently remove the dust and debris from the sensor. Clean the remaining with a stiff brush or steel wool.
• If it requires replacement, replace a new one in the same way as the old one. 
• Reconnect the wires to the sensor and control box, fix the burner assembly, and screw back the furnace cover. 
• Start the furnace to see if it is working or not. 

The sensors will cost less than $20. 

If you consult a professional team from HVAC, expect the total payment to be around $75 to $250. 

4) Too many closed dampers

Sometimes, people close some furnace dampers if they have a two-story house. 

Keeping the upstairs dampers open will make the upstairs very hot. 

However, closing too many dampers will harm the unit. 

A furnace is designed to keep heat registers and dampers wide open to spread enough warm air throughout the house. 

If you close too many dampers in the furnace, it will block the airflow, further burning out the motor. 

As a result, the furnace will require resetting 1-2 times to start working. 

If the problem persists, your furnace will constantly shut off.

If the furnace runs despite the closed dampers, the airflow will increase in the ducts, creating a whistling noise.

So, make sure that the dampers are open for adequate airflow. 

However, you can partially close or open the dampers according to the temperature of your house. 

5) Cracked furnace heat exchanger

A heat exchanger maintains the separation of the combustion process and oxygen for proper breathing. 

It stops carbon monoxide intoxication. 

If the heat exchanger is cracked, unburnt gasses like carbon monoxide, sulfur dioxide, and nitrous oxide will enter your house and intoxicate it.

When a crack occurs, the furnace will not start at first. 

But, after resetting, it may work fine, but only for some days. 

Over time, the furnace will keep tripping off, and you must reset it multiple times. 

You can understand that the heat exchanger has cracked when you smell bad odors, see discolored metal, or find a change in the flame color. 

A cracked heat exchanger will reduce the furnace’s efficiency, consume more fuel, and increase energy bills. 

To prevent the crack:

• Keep the vents and air inlets open and clean. Remove the blocks like furniture for proper airflow. 
• Change the air filter when dirty.
• Check the whole unit once a year to look for cracks and other damages. Even if you take proper care, there will be cracks over time. 

You need to replace the exchanger if you find cracks. Please consult a professional to do it. 

6) Insufficient fuel levels

If your furnace does not have enough fuel, it will turn off at the start but start working after you reset it once or multiple times.

Though the furnace will work after resetting, it may turn off again after some time. 

The furnace turning off immediately after a start is a sign that there is not enough fuel left in the unit. 

Check the oil storage tank and refill it immediately if only a little is left. 

To prevent this problem, contact your local company for regular service delivery. 

The oil requirement frequency depends on your tank’s size and the furnace’s usage. 

For fair tracking, you can track the oil and check the daily at the same time. 

Most of the tanks use 4 gallons of oil per day. 

You may need 125 gallons of oil per month. If you have 250 gallons, fill the tank up every 2 months. 

7) Clogged fuel line

A clogged fuel line will encourage constant resetting. 

The furnace may trip out of safety due to the clogged line. 

After that, it will start working fine if you reset it once. 

But over time, you might have to reset it multiple times.

So, you need to check the fuel lines for your furnace. 

Check the valves in the fuel line to confirm that they are open. 

If they are closed, it means the valve directions are wrong. 

Turn the valve to open the gas valves. 

You will hear the flow from the fuel line again. 

Turn off the furnace’s main line before opening the gas valves. 

Otherwise, there could be potential fuel problems. 

8) A problem with the thermostat

The thermostat accesses modern furnaces. 

A thermostat guides you about the timing of turning the furnace on or off. 

So, if the thermostat fails to communicate with your furnace, your furnace reset button will start popping up. 

It can be tempting to press the reset button, but you must do it only once. 

So, before you flip the switch to the ‘On’ position, consider checking the thermostat to see if it is showing any error. 

9) Corrosion or aging

For older systems, corrosion and aging is a common thing. 

A furnace lasts for 10 to 20 years. 

The older systems may not show any aging signs, but they will trip off often. 

So, restart your furnace by resetting it. 

If your old system has evident rust signs, it is time to replace your furnace. 

Corroded metals will not only trip your furnace, but they can also lead to severe accidents. 

If your new system shows signs of corrosion, the reason is moisture deposit. 

Check for leakages and patch them. Call an expert to deal with it. 

How many times can I reset the furnace system?

Whenever a furnace detects a problem or danger, it will trip off. 

This system is new in most of the recent furnace models. 

When the furnace trips off, the reset button pops up. 

When you see the button, you can hardly resist pressing it. 

Resetting is an easy way to fix most problems in the unit. 

When the reset button pops out, the unit thinks resetting is the best solution. 

You need to reset your furnace once. 

If the problem is solved and never happens again, it is fine. 

But, if you always need to reset your furnace once every time you start the unit, the problem is severe, which the reset button cannot handle. 

Do not reset your furnace more than twice. 

If you press the reset button multiple times, it will constantly hit the ignition area with oil and make it ignite. 

If the oil does not ignite now, pressing the reset button multiple times will create a bigger flame when the furnace suddenly turns on.

How to reset a furnace?

Resetting a furnace is one of the easiest ways to troubleshoot most furnace problems. 

A furnace requires resetting mostly in winter because it works overtime during the winter and sometimes becomes very hot. 

Resetting the furnace will stop the unit and restart it again, preventing a popped circuit breaker. 

Resetting a furnace would be better than resetting the breaker. 

Here are the simple steps to reset a furnace:

• Find out the furnace power switch near the unit and turn it off. 
• If you do not find any switch, turn off the breaker. 
• The reset button might pop out. Press it for a few seconds until it clicks.
• Wait for 10 seconds. 
• Turn the switch or the breaker back to the ‘on’ position and check if the furnace is running well. 

If you have a gas furnace:

• Turn off the gas control setting to turn off the pilot light. 
• Set the thermostat to the lowest setting. 
• If the furnace has an electric ignition, switch off the circuit connected to the furnace. 
• Close the main gas supply, but keep the pilot gas supply line open. 
• Relight the pilot light with a matchstick, lighter, or piezoelectric spark ignition. 
• Turn on the main power and gas supply back. 
• Press the reset button for 30 seconds and release it. 
• Your furnace will start. After that, adjust the thermostat settings. 

What is short cycling?

When the heater turns off before it reaches the full heat cycle, it is called short cycling. 

It occurs when your furnace overheats, and turning off is the furnace’s way to indicate something is wrong. 

Overheating can occur due to low airflow or a large-sized heater. 

Different houses will need different sizes of furnaces. 

An oversized furnace can heat your room and shut off as a safety measure if the heating increases. 

In that case, you will have to restart your furnace. 

Your furnace will frequently shut off if you don’t replace it. 

Final thoughts

Though resetting a furnace is a good way to solve maximum problems, constant resetting indicates a deeper issue. 

When you see that you have to reset the furnace every time you start, it is time to analyze the real cause behind it and solve it. 

You can either solve it yourself or hire a professional. 

The common problems behind a furnace working only after reset are a bad control board, dirty air filters, closed dampers, cracked heat exchangers, insufficient fuel, clogged fuel lines, thermostat issues, corrosion, and aging.

The reset button popping can seem tempting to press but reset the furnace only once or twice. 

If your furnace still trips off, consult this article to find out why. If you can’t, ask for help from the HVAC team. 

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Reference: Wikipedia.