Why Does My Furnace Turn On And Off Every Few Seconds

Your furnace has to continue running until the temperature you set it to is reached. In the middle of winter, a furnace turning on and off every few seconds means that your house will get cold. You can't possibly fix this problem without knowing why it happens, so what exactly causes this issue? Keep reading, because we've researched the problem and have answers.

A furnace turning on and off every few seconds is probably short cycling, which may cause your unit to become damaged. There are several reasons for short cycling, which include:

  1. Dirty air filter
  2. Thermostat problems
  3. Poor insulation
  4. Faulty furnace parts
  5. Incorrectly sized furnace

A short-cycling furnace fails to produce enough heat and turns off often. How should you resolve this problem? Read on to learn more about why your furnace is turning on and off, the causes of short cycling, and how you can fix the issue. 

A home high energy efficient furnace in a basement - Why Does My Furnace Turn On And Off Every Few Seconds

Signs That Your Furnace Is Short Cycling

If you still aren't sure if your furnace is short cycling or if it's just the cold temperature because of winter, there are several other signs you could look for.

Typically, a short-cycling furnace will come on and then turn off after about four to seven seconds. But, to be sure short cycling is the problem, here are some of the other signs you should look for:

  • Low room temperature: If your furnace has turned on quite a few times and you've set your thermostat to give you enough warmth, but your house still feels too cold, your furnace may be short cycling.
  • Too frequent heating cycles:  How long your furnace's heating cycle takes will depend on the temperature your area needs. For instance, a house you haven't used during winter will have your furnace run for hours. On the other hand, homes that are well-insulated and heated have short heating cycles. 
  • Difference between room and thermostat temperature: Even if your house doesn't feel that cold, if the room temperature falls short of what you set on your thermostat to, your unit is probably short cycling.

Causes And Solutions For Furnace Short Cycles

Technician repairing Gas Furnace

Now that you've made sure that your furnace's issue is nothing else but short cycling, you can proceed to learn about what causes it.

There are several reasons for short cycling, and each has its solution. Here is a list of the causes and fixes for a furnace that is short cycling:

1. Dirty Air Filter

Dust on filter-concept and ideas-Dirty automobile interior cabin air filter

A clogged air filter hinders airflow to the heat exchanger. Because of this, the furnace overheats. When your furnace detects this overheating, it takes this as a sign to shut off the appliance.

It then cools down after several seconds and will restart. Change your furnace's air filter or clean it regularly, depending on which of these your manufacturer suggests to stop the short cycling.

Each furnace has specific directions you should follow in cleaning or replacing filters, so keep this in mind when you're trying to resolve the problem.

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2. Thermostat Problems

Man regulating heating temperature with a modern wireless thermostat installed on the wall at home.

Your furnace's thermostat may not be working correctly, and this can cause your furnace to short cycle. To check the thermostat, turn it off and replace its batteries. Then turn on your furnace after a while. The appliance's problem will be resolved if it stops short cycling after an hour of running.

If batteries aren't the issue, check the location of your thermostat. A thermostat located near a heat source may affect the heat cycle of your furnace.

Because thermostats are sensitive to heat and control your furnace, an external heat source near it will cause it to detect that the temperature is high enough around the area and causes the furnace to short cycle. You will have to move your thermostat to resolve this issue.

Check out this thermostat on Amazon.

3. Poor insulation

Hands Applying Weather Seal Caulk to Window Frame

If your furnace has heated your home, but the heat it generates leaks outside because of unsealed windows, the furnace will turn on again prematurely. Add caulk around your window frames to keep warm air from leaking out.

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4. Faulty Furnace Parts

Technician servicing heating boiler

Your furnace's components may be causing the short cycle, so you will also have to check if you've ruled out all the other possibilities stated above. To assess if this is the cause, here are some malfunctions that can cause this issue:

  • Your furnace blower's flue or flame sensor acting up causes your heat exchangers to not warm air or become too hot, making your furnace shut off too often. If this happens, air won't be able to go through your vents. Even if it does blow air, it will just be weak. Place your hand in front of the vent to determine if this is what's causing the short cycle.
  • Blocked flues may also be the culprit of your furnace's issue. Blocked flues can prevent heated gases from escaping. This will make the flue limit switch shut off the furnace to prevent overheating.
  • A faulty ignition flame can cause your furnace to shut off if it detects that the ignitor isn't making a flame within four seconds. A clogged or damaged thermocouple or damaged ignitor may be what's causing this lack of ignition. 

Unlike problems with the thermostat and air filter issues that cause short cycling, these malfunctioning furnace parts require the help of a professional, as they can be quite challenging to resolve on your own.

5. Incorrectly Sized Furnace

A home high energy efficient furnace in a basement

A furnace that's too big for your space will short cycle because its system has more heating power than you need. It generates heat inside your house faster and turns off immediately.

This can cause shorten the lifespan of your furnace because it's running too frequently. In addition, your electricity bill will go up since your system uses power too often.

Contact an expert and replace your furnace so that you can ensure that your unit fits your space.

How Do You Prevent A Furnace From Short Cycling?

Short-cycling furnaces can cause discomfort, especially if your house needs to be warm during cold weather. If you don't want to spend your time trying to investigate and fix your furnace, make sure to follow these steps:

  • Check and change your furnace's air filter once a month.
  • Ensure your house's air vents are open and clear of furniture.
  • Have an expert tune up your unit each year.

Doing this will save you from the trouble of a cold home and save you money on expensive furnace repairs.

How Much Does It Cost To Repair A Short-Cycling Furnace?

On average, a short cycling repair for a furnace costs from $100 to $400, depending on what causes the short cycling.

Furnace repair costs in general can range from $125 to $480. Your furnace's brand, size, condition, and product warranty will impact how much you may spend to fix it.

In Closing

As furnace problems go, short cycling is better than overheating, because the appliance doesn't turn off when it overheats. But this doesn't mean that you should ignore a short-cycling furnace.

You can make sure that short cycling is the reason why your furnace is frequently turning on and off by looking for several signs. If your house feels cold when you're running your furnace, or if your furnace has frequent heating cycles, short cycling is probably the cause of the problem.

There are several reasons why a furnace will short cycle. While you can fix thermostat and air filter issues to stop short cycling, a professional repair is necessary if its other parts malfunction.

But, if an oversized furnace is causing the trouble, then your best bet is to replace your unit with one that's the appropriate size for your space. A furnace that's the wrong size can cause you to spend a lot of money because it will increas your electricity bill.

Before you go, read these posts about how to deal with a short-cycling furnace for more information:

Short Cycling Furnace – What To Do?

Furnace Tries to Start Then Shuts Down – What To Do?

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