What To Do When Your Home Furnace Is Short Cycling

feeling cold due to broken furnace

Winter is almost here. In fact, the cold breeze has already started to blow, and those extremely hot summer months are behind us. It won’t be long now before it’s uncomfortably cold. To stay warm during winter, this is the perfect time to check if your home furnace is functioning properly. There are certain problems that are quite easy to fix. However, there are some faults that require the attention of a skilled technician. For instance, with short cycling, you will definitely need to call a professional to assist you. Short cycling occurs when the furnace runs then shuts off and starts again repeatedly.

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How To Know If Your Furnace Is Short Cycling

A furnace that is working properly will start running and continue to do so up until it reaches the temperature you want. It will then immediately turn off, allowing the desired temperature to be maintained. When the temperature begins to fall, it will start running again, and continue with that process all throughout the day and night. However, if the furnace develops a short-cycling problem, it will basically be turning on and off repeatedly in quick sequences. It will not get to the desired heat levels. The turning on and off sequences will eventually result in the wear and tear of the unit. Furthermore, even though the furnace will be on, your home will be far from warm.

What Causes Furnace Short Cycling?

why does a furnace short cycle

If your furnace begins to short cycle, you should immediately call a reputable HVAC company to fix the problem. This is because apart from causing the wear and tear of individual parts of the system, a system in this condition wastes a lot of energy. A furnace usually short cycles because of some of the reasons below:

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1. Overheating

HVAC units have in-built components that protect them from damage. The purpose of a home furnace is to provide warmth, but it can actually get too hot. If a dangerous level of heat is reached, the furnace will immediately turn itself off. If it does not switch off immediately, cracks can form in the heat exchanger and dangerous carbon monoxide can start to leak. So, in a sense, shutting off right away when it starts to overheat is actually a good thing. Overheating is usually a result of airflow blockages caused by congested exhaust vents and filthy air filters.

2. A Defective Flame Sensor

furnace burner

Another reason is a defective flame sensor, which can cause the flame to turn off the moment it lights. The main purpose of the flame sensor is to control the gas valve. It helps to prevent gas from entering your house. If there is no flame, it turns the valve off. If the sensor develops a fault, it will turn the valve off even in instances where it is in fact required. Sometimes, simply cleaning the sensor can actually help. But, if that doesn’t fix the issue, you will have to replace it. A licensed HVAC technician can assist you with this.

3. Blocked Heat Grates

A furnace works by warming up the air and then pushing that air out through the heat grates. If the grates are blocked, the air will not be able to flow outside the grates. The trapped air will eventually cause the unit to overheat. As long as that problem is not fixed, the system will continue to short-cycle. These heat grates also have a control device called a damper which lets you control the temperature in your home. This actually helps to save energy. Also, make sure that you do not close too many vents as this can also cause the system to overheat; 75%  or more of the vents should always be open.

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4. A Dirty Furnace Filter

replacing furnace filter

An HVAC filter that is dirty can also cause short cycling. Whereas a defective flame sensor will cause the furnace to turn off instantly, dirty filters will only turn off after a couple of minutes. Dirty filters can cause heat grates to pump out air that is hotter than usual. The dirt on the filters will make it difficult for air to flow back to the heating unit to allow it to cool down. What simply happens is that the heat will gather up and reach levels that are uncontrollable. As a safety measure, the furnace will just switch off. The filters need to either be cleaned or replaced.

5. Blocked Exhaust Vent

In order for your heating system to work effectively, the exhaust vents must be kept open at all times. But they can be blocked by things such as snow, vegetation, and nests, among other things. Make sure that these blockages are removed on a regular basis. It can also happen that a dangerous animal can actually block the opening of the vents. If that is the case, make sure that you contact an animal control expert to deal with the situation.

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6. The Thermostat Has Been Wrongly Placed

thermostat and comfort control

Thermostats have sensors that regulate the temperature inside the house based on what it picks up in the environment. But, if the thermostat is not in the correct location inside the house, it might not operate the way it is supposed to. For example, if the thermostat is placed near a heat grate, a window, or some other hot area, this will affect the functioning of the sensors. The sensors will pick up the nearby heat and assume that the interior of your home is already hot, and will immediately turn off the unit. The best spot would be the one that is not near anything that can affect the temperature readings.

8. The Furnace Is Too Big

You might think that getting a big furnace will give you more value for your money as it will probably offer more when compared to a smaller one. However, this is not the case; you might actually have a furnace that is too big for the house you live in. A bigger than required heating unit will result in excess capacity, and this can cause short-cycling. The wear and tear of different parts of the unit will happen much quicker, temperature fluctuations will be more noticeable, and you will have an increased energy bill. Although it might cost you quite a lot of money, the best solution in this instance is to have a technician install a new furnace which will be the ideal size for your heating needs.



Taking care of any issues that your furnace might have early will result in you having a comfortable winter. The moment you detect short-cycling, try, and figure out the reason for that based on the points discussed above. An experienced HVAC technician will be able to give you an accurate diagnosis and a lasting solution.

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