Why Your Furnace Is Short Cycling? Cause, Troubleshooting And Fixes

Furnace Is Short Cycling

One of the best things about winter is that we get to stay cooped up inside the house with the furnace on full blast- feeling perfect and warm inside and out. There is only one thing that can interrupt this dreamy haze: a malfunctioning heat furnace.

More specifically, nothing can be more irritating than a short-cycling furnace system that never lets the room get warm enough for you to relax. 

Short-cycling, for a furnace, is a process wherein your unit switches on and off frequently without ever reaching the set temperature. Usually, a smoothly working furnace will work in such a way that it first switches on, and increases the temperature of the room to the one you’ve set on the thermostat. 

Once it is quite clear, through its temperature detecting quality, that the room has reached the desired temperature, the furnace system will turn off automatically.

However, when the temperature of the room is again naturally influenced by the outside weather and subject to change, the furnace system will start running again to warm up your room.

Typically, there’s a hours’ long gap between the switching on and off of the furnace in this process. However, in short-cycling, you’ll find that this happens very frequently and quickly without the room even being warmed up.

This can be a sore spot for many owners, who simply seek some comfort from their furnace system. To understand this problem better, below are listed some reasons behind a short cycling furnace.

Furnace Short Cycling Causes

Overheating Furnace

Overheating can be one of the most common, and also quite a dangerous cause behind a faulty furnace system. If your furnace is overheating by itself, one of the reasons for it could be that the temperature monitoring components inside your furnace might be working improperly. 

An overheated furnace system can create cracks in your heat exchanger. The heat exchanger usually comprises dangerous combustion fumes including carbon monoxide, which could spread outside the furnace through the cracks. This is why it’s advisable that you should be careful with an overheating furnace system.

Solution: For an overheating furnace unit, it is generally recommended that you contact a technician right away, as this is a serious problem. Getting hands-on with your unit at a time like this can do more harm than good. It’s possible that your heat exchanger or any other temperature regulating component might need to be repaired or replaced.


A flame sensor plays its part in heating up your room by detecting a small flame, which, when lit by the gas produced by the gas valve, releases heat into the room.

A bad flame sensor with impaired judgement clearly fails to recognize a flame, and sends a signal to the gas valve to shut off, because it assumes that gas is being released without any flame- which is dangerous because it can lead to a gas leak that can end in explosions.

A flame sensor might be deteriorating in its functioning also if it hasn’t been cleaned well enough.

Solution: For a bad flame sensor, regular inspection and cleaning should do the trick. It’s possible that your heat sensor is being blocked off by dust and debris. It could go back to working properly once cleaned.

However, it is also possible that the small amount of water released near the flame sensor might have caused it to corrode. This could have deteriorated the sensor itself, in which case it is a good idea to simply replace the flame sensor.

Dirty Furnace Filter

A dirty filter can also lead to the overheating of your furnace, and ultimately: Short Cycling. When an air filter is clogged up or contaminated, it reduces airflow from outside to enter the furnace system. This means that there is no new air for the system to heat. 

On the other hand, the warm air already inside the furnace is held back instead of being sent out to heat the room. This process is called backup, and it will cause heat to accumulate inside the walls of the furnace, and ultimately lead to overheating of the unit. Finally, the unit will start short cycling to protect itself from releasing dangerous gases.

Solution: For a dirty air filter– clean, clean, and clean! Not just when you run into a problem like this, but monthly and yearly! There is a reason air filters are designed to be washable and reusable.

They contribute to the maintenance of the unit, and their timely cleaning will keep the internal components secure, and release only clean air for you to enjoy. If you have been using the same furnace filter for way too long, it is subject to wear and tear. In such cases, it might be a better idea to replace it.

Faulty Thermostat

Of course, one of the leading causes behind a faulty furnace is nothing more than a faulty thermostat. A well-functioning thermostat is absolutely essential because it sends a reading of the temperature back to the furnace to determine how warm a room should be. 

However, if your thermostat is performing inaccurately, it’s causing the furnace to work incorrectly as well. Sometimes, this might simply be a matter of making sure that your furnace is turned to the heating mode, as we tend to forget.

Solution: For a faulty thermostat that doesn’t seem to be working properly, you should get a professional to take a look at that. Usually, a simple repair works.

Oversized Furnace

If your furnace is even a bit oversized for your house, there’s a chance that it is going to malfunction. This is because furnaces, like Air Conditioners, have a set area footage that they can heat accurately and thoroughly. 

A furnace that is too big for your home might heat up your room quickly and inadequately because it aims to keep the temperature even throughout your home. 

This often leads to short cycling and temperature swings. In case of an oversized furnace, the best option is to go for a full replacement, but talking to your HVAC technician before making any decisions might yield the best results.

Additionally, if your house isn’t well insulated. That is, if it has cracks and gaps that haven’t been sealed, it’s likely that heat will escape out of your room to quick, and this will also cause the furnace to switch on and off too quickly.

Solution: If you’re dealing with a poorly located thermostat, the most obvious solution is to place it somewhere else. Or, if you can, you can also shift moveable heat sources like the lamp and put them out of the way. It is recommended that you put the thermostat in a location that isn’t subject to sunlight, or air from the heat grate

Misplacing The Thermostat

You might not even realize that this was the reason behind your furnace’s short cycling all along, but a poorly placed thermostat might be sneakily sabotaging your furnace’s working.

This is because your thermostat might be located at a place where temperatures are higher, and thus, it’s being heated up way before the rest of the house. 

This, of course, leads to the device sending back a reading to the furnace of its own temperature, causing the furnace to shut off assuming the house has reached its set temperature.

This phenomenon is especially common in thermostats that have been set up by the owners themselves. If your thermostat is set up, say, above a heat grate or a utility closet, or close to a lamp or a tube light, it’s likely that this is where the problem lies.

Solution: As discussed, for a furnace too big, the best solution might be to simply replace it. This is because it is likely that you will keep running into problems if you stick with a furnace that is simply unsuitable for your home.

There are companies out there that will help run tests and see if your furnace is truly incompatible with the house. So, you can always contact them to be sure before making any decisions.


All in all, a short cycling furnace can be really uncomfortable to live with. It fails to give you the comfort that you were promised as it doesn’t heat the air well enough to keep you warm. This is why you should be quick in diagnosing and treating your furnace so you can enjoy it again. 

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