14 Reasons Your Furnace Won’t Turn Off (+Fix)

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.

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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:

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. 

Reference: Wikipedia.

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Arthur Smith

Howdy! I am Arthur Smith, an electrical engineer who is extremely passionate about electronics. I have lived in different parts of the US and currently reside in Wisconsin. I am one of those rare children who knew what he wanted to study and do in life right from the start. I was a curious child who wanted to know how switches work and how the AC works, and I would always observe my dad whenever he would handle the wires and fix things around the house. I currently work as an electrical engineer at a reputed company and write for this blog. And I read loads of books or play video games in my free time.

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