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Furnace Not Staying On/Lit? 13 Common Causes (+ What to Do)

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So, your furnace is firing up but the burners won’t stay lit for the standard 10-15 minutes cycle? Well, that’s no good news in the deep of winter. If this is your case, your furnace will not reach the target temperature and while you curl up to resist the biting winter cold, your one question will be why your furnace is not staying on.

Your furnace will not stay on if it is overheating, large-sized, old, the thermostat is malfunctioning, or the flame sensor is dirty and the igniter faulty. Other reasons your furnace will not stay lit include a defective draft inducer motor, a broken thermocouple, or a bad control board.

As we tell you in this article, it’s not advisable to try troubleshooting a furnace that’s short-circuiting and not staying on. You should instead find a trusted HVAC professional and let them handle the repairs. Nonetheless, it won’t hurt to understand why your furnace isn’t staying lit. You can use the table of contents below to jump to each potential issue.

Your Furnace Isn’t the Right Size

Some homeowners may have the idea that the bigger your furnace is the faster and better it will heat the home. Not necessarily!

It is true that a larger furnace will produce more heat, but that does not guarantee the warm air will be distributed evenly in your home.

What happens is that the furnace will keep going off in short cycles to try and have the temperatures in your home distributed. 

Also, if warm air is not evenly distributed, your furnace will keep getting the signal to start heating more often than it should.

How to Fix a Wrong Size Furnace

The best way to avoid having a furnace that won’t stay lit because it’s too large for the home is to get the right size furnace from the beginning. 

According to the U.S. Department of Energy, the rule of thumb in determining an HVAC system size is by estimating that your cooling and heating system should provide 12,000Btu (1 ton) of cooling/heating for each 400-500 square feet of home area.

This general rule, however, does not factor differences in the climate and the home’s existing insulation.

As such, when installing your home’s cooling and heating system, you should ensure that the HVAC expert uses the Air Conditioning Contractors of America (ACCA) Manual J to calculate the appropriate size of a cooling and heating system for your home.

Your Furnace is Overheating

A furnace that doesn’t stay lit can be caused by an overheating exchanger. That also means there’s limited air flow getting into the heat exchanger, which can be caused by these 3 issues:

  • A dirty air filter, making the blower struggle to bring in sufficient air over the heat exchanger.
  • Closing air supply vents, which slows down the blower and reduces airflow. 
  • A dirty blower wheel that slows down and limits airflow. 

If your furnace overheats, the inbuilt safety features like the high limit switch will trip and shut it down to protect the furnace, which is why you’ll notice your furnace not staying lit.

How to Fix an Overheating Furnace

While you can clean a dirty air filter or unclog blocked vents, an overheating furnace is a threat for you and your family.

The worst-case scenario is that the safety features could also be faulty, and if the overheating furnace doesn’t shut down, that could start a fire. 

As such, the best way to deal with an overheating furnace is by calling an expert HVAC professional immediately. Just don’t take chances by trying to troubleshoot. 

You Have an Old Furnace

Just like people, machines also get old and tired. So, if your cooling and heating system was the one used by your grandparents before your parents and now you, it’s definitely time to change it.

As heating systems get older, they will not only become less efficient and so more costly, but they’ll also manifest more and more functional issues, including burners not staying lit. 

How to Fix an Old Furnace that Won’t Stay Lit

There’s no negotiating with an old dysfunctional furnace. You simply need to replace it. The U.S. Department of Energy advises that a furnace that’s 20 years or older should be replaced even if it works fine

Replacing an old furnace will reduce energy bills by using newer high-efficient models and ensure you are not running a ticking bomb for a furnace.

The Thermostat is Malfunctioning

A thermostat tells the furnace to turn off when the set heat is achieved and turn on when the temperature has gone down. 

If the thermostat is placed wrongly so that the temperature in the surrounding ambiance is high and affects the thermostat’s functioning, then it can send wrong signals to the furnace.

This will make the furnace turn off before the needed heat is achieved or turn on while the heat is still sufficient. 

A simpler issue with your thermostat is that it could be set wrongly. That could mean forgetting to raise the temperatures when seasons change.

How to Fix a Malfunctioning Thermostat

A lame advice on how to fix a malfunctioning thermostat is to replace it and see if the problem is resolved. What if that is not the issue? You’ll have made an unnecessary expense.

So, call an expert HVAC technician to check if your thermostat has issues and if it’s the one causing the furnace not to stay lit. The HVAC pro will advise if you need to replace the thermostat or a simple fixing or adjusting is required. 

If the issue with your thermostat is that you forgot to change its temperature settings with the change of seasons, simply raise the temperature a bit and have someone by the furnace tell you if it clicks and starts as the change on the thermostat is made.

Your Furnace has a Dirty Flame Sensor

If your furnace turns on several times and each time it goes off after a few seconds, you probably have a dirty flame sensor rod. 

Any time your furnace flame sensor does not detect the presence of a flame, it will shut off the gas valve.

A flame sensor that directs the gas valve to shut off when no flame is detected is a good safety measure and preempts gas poisoning or an explosion. 

However, a flame sensor that shuts the gas valve simply because it’s covered with soot and doesn’t detect an existing flame is a problem to be resolved ASAP.

How to Fix a Dirty Flame Sensor

A dirty flame sensor needs to be cleaned. But this is not as easy as it sounds as you’ll need to:

  1. Turn off your furnace. 
  2. Open the combustion chamber panel door to access the sensor. 
  3. Remove the sensor and clean it to remove grit, dust, or rust using emery cloth, a piece of light-grit sandpaper, or improvise with steel wool.
  4. Replace the sensor.
  5. Test to see if the issue is resolved. 

We recommend you call an HVAC expert to check if your flame sensor is dirty and if it is, clean it. Calling an HVAC pro is also important because he/she can proceed to replace the sensor if that’s what is required. 

If you feel up to the task, you can check this visual demonstration on how to clean a furnace flame sensor which is what the HVAC pro will be doing.

The Furnace Igniter/Pilot Light is Damaged

Newer furnace models have a hot surface igniter while older models have a pilot light. If the igniter stops working or the pilot light goes out, the gas valve shuts off to preempt gas leak, which means your furnace won’t stay lit.

There are a number of issues that can cause problems to your furnace igniter:

  • An old igniter that’s worn out or spoilt.
  • A faulty temperature limiting switch which is the safety feature that turns of the burners when the furnace is too hot.
  • A wrong igniter that is not compatible with your furnace’s voltage and consequently causes it to fail. This usually happens with DIY repair attempts.
  • A power surge that burns your igniter, especially with hot surface igniters.

How to Fix a Damaged Igniter

If your furnace power is on and you’ve tried to relight your furnace but still won’t stay on, it’s best to call a pro.

Furnace issues, especially those related to power and gas supply require expert handling. You should, thus, call an expert HVAC and avoid DIY trials as these can cause dangerous gas leaks and related accidents.

Your Furnace has a Defective Draft Inducer Motor

A draft inducer motor turns on a minute before the burners and works to clear any remnant combustion gases from the previous cycle, venting them out from your home. Once the burners are on, the draft inducer motor continues to supply oxygen to the burners. 

If your draft inducer is working properly, the pressure switch that measures the amount of air flowing through the heat exchanger closes. 

If instead, the draft inducer motor is defective and insufficient air is circulating in the furnace, the pressure switch will not be able to close and will shut down the furnace.

How to Fix a Defective Draft Inducer Motor

Not every furnace has a draft inducer motor. But current standards require new gas furnaces to have one.

If your furnace’s inducer motor is defective, the wisest thing to do is to call a reliable HVC expert. Most draft inducer designs are not easy to repair and more often than not require replacement, which is better done by a pro.

The Furnace’s Thermocouple is Broken or Malfunctioned

A furnace thermocouple is a sensor that detects the functioning of the pilot light. If the pilot light goes off, the thermocouple shuts off the gas valve to preempt any accidents.

If your furnace’s thermocouple is not working right, it could shut the gas valve at abnormal intervals, which is why the burners may not stay lit. 

How to Fix a Broken or Malfunctioned Thermocouple

Telling whether the thermocouple has malfunctioned and if it needs replacement requires the expertise of an HVAC technician. 

You should, therefore, call a reliable expert and not try troubleshooting a bad furnace thermocouple.

The Furnace’s Condensate Pan is Full

High-efficiency or condensing furnaces produce condensate that drips into a condensate pan and is drained down a condensate pipe into the main drainage.

If the drainage line is blocked and the condensate pan fills up, the evaporator coil sensor detects rising liquid levels and the furnace shuts off. 

How to Fix a Condensate Pan that’s Filled up

The first thing to do is to drain the condensate pan using an absorbent duster. You’ll then need to unclog the drainage line. 

You can improvise an unclogging tool using a straight wire and passing it to remove any debris that’s causing a blockage in the drainage line. Alternatively, you can get a plumber’s snake for the job.

Passing a mixture of hot water with vinegar and a bit of kitchen soap will get rid of any soot or debris attached to the drain pipe. You can also opt for an HVAC-safe algaecide treatment and pan cleaner product. 

Always remember that a trained HVAC technician will do a better job in resolving furnace issues. They’ll also give you advice on how to preempt future condensate drain clogs.

The Blower Motor is not Running

A blower motor moves warm air from the furnace into the vents and to your home. If it should malfunction, the furnace switches off to prevent the heat exchanger from overheating, which comes with other furnace problems.

There are several reasons that can cause a blower motor to stop running:

  • The motor is not receiving power.
  • The fan relay is broken.
  • The blower motor belt is worn out.

How to Fix a Non-functional Blower Motor

Repairing or replacing a faulty blower motor is best left to an expert furnace technician. The HVAC pro will find out what the issue is with the blower motor and decide if it needs repair or replacement. 

Note that a furnace with blower motor problems should be attended to immediately as this can cause the heat exchanger to overheat and break down, which means an altogether dysfunctional furnace.

The Fan Limiter has Failed

A fan limiter is a device that determines when the furnace blower turns on or off. As such, it controls when the warm air is blown through the vents to your home and prevents the heat exchanger from overheating and causing a fire.

If the fan limiter is not working as it should, it can cause your furnace to go off prematurely. Problems that could cause the fan limiter to fail include:

  • A dirty switch sensor. 
  • Clogged air filters that lead to higher temperatures and cause the switch to trip prematurely.
  • The blower motor overheats and the switch trips, causing the furnace to shut off.
  • Issues with other related parts of the furnace including the thermostat, contractors, gas valve, or circuit board can all cause the fan limit switch to fail.

How to Fix a Failed Fan Limiter

Ensure that your furnace undergoes the annual maintenance before you light it to keep your home warm in winter. The cleaning done during maintenance will ensure your fan limiter functions as it should.

If it has already failed, fixing a fan limiter may be beyond your skills. As such, it is best to call an experienced HVAC technician and have them replace a faulty fan limiter.

Your Furnace’s Circuit/Control Board is Bad

If none of the other issues discussed are the cause for your furnace not staying on, a bad circuit board is your last suspected cause for a furnace not staying on. 

The furnace circuit board distributes power to whichever part of the HVAC needs it. So if it is bad, then your furnace or part of it will not receive the power it needs and will consequently not stay on.

Most furnace circuit boards have a LED light that blinks if there is an issue. So, if your furnace is not staying on, check the light to see if it’s blinking.

A diagnostic chart should also be attached to the inside of the panel lid which should tell you what the warning light indicates. You can use this info to give the HVAC expert an idea of your furnace’s problem when you call them.

How to Fix a Bad Circuit Board

Any furnace issues that have to do with the power supply should necessarily be handled by an expert HVAC technician. 

You should, therefore, desist from troubleshooting a circuit board problem and call an expert to take care of it. As you can see in this video, troubleshooting a control board is no easy task and requires the expertise of a pro as well as the use of a meter.

The Weather is Extremely Cold

This is not necessarily a furnace problem. But if temperatures are extremely low your furnace will turn on but it won’t stay on for the maximum 15 minutes of a heating cycle. This is also described as cold cycling.

While that is the furnace’s way to cope with extremely cold temperatures, the consistent on-off cycling will not let your house reach the desired temperature and will leave you shivering with cold.

The fact is, if your furnace is working right, this should not be happening even if the temperatures are extremely low. But there are a couple of factors that could cause your furnace not to stay on with extreme temperatures:

  • A poorly insulated home that loses heat through the door, windows, or roof.
  • A home with extremely high ceilings, making more room that needs heating.

How to Fix a Cold Cycling Furnace

There’s not much you can do if the weather is below normal temperatures. But you can have your furnace checked to ensure it does not have other issues that are causing it not to stay on as required.

Also, if your home has poor insulation, it is will help to do some repairs and changes to ensure better insulation. For example, replacing single-pane windows with double-pane ones can improve your home’s insulation and stop your furnace from working too hard.

Final Thoughts

Furnaces are created to work in cycles that last 10-15 minutes under normal circumstances. Sometimes, however, your furnace runs for a few minutes then stops. 

While this can indicate a furnace that’s trying to cope with its heating task in extremely cold weather, a furnace that is not staying on long enough is usually an indication of bigger problems with the igniter, thermostat or other parts.

Most furnace problems require the expertise of a professional HVAC expert, which is why we recommend you seek the services of a reliable HVAC professional by filling out the form at the bottom of the article.