11 Reasons Your Furnace Won’t Stay Lit (+Fix)

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.

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