For those of us living in the really cold parts of the country, our furnaces are our best friends during the winter months. However, what would you do if you suddenly noticed that your furnace is turning on and off more often than it ought to, and is heating your home as well as it should?

This is a sign of your furnace’s short cycling, and it isn’t necessarily a bad thing. Continue reading to learn what short cycling actually is, what causes it, and how you could fix it.

What does short cycling mean?

inspect furnace

Your furnace is an important part of your home’s HVAC system, along with air conditioners. While air conditioning keeps your home cool during the summer, the furnace is part of the heating system, responsible for distributing heat throughout your home and keeping it warm.

Here’s how a furnace works. Every furnace has a heat exchanger that is made of tubes that contain burning fuel in them. A blower fan sends air across these tubes, and this hot air is distributed across your home via vents.

Normally, a regular heating cycle for a residential furnace lasts anywhere between seven to 20 minutes, which means your furnace, on average, switches on and off between three and eight times in one hour.

If your furnace starts switching on and off more frequently, like once every couple of minutes, for example, it is a sign that your furnace is short cycling.

A furnace short cycling is not particularly a bad thing, though. It is a sign that the safety mechanisms are functioning perfectly, and preventing more serious accidents, such as your house burning down, from happening.

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Causes and fixes

Let’s now take a look at some of the causes for your furnace’s short cycling and what you as a homeowner can do.

A dirty air filter

Replace furnace filter

An unclean air filter is one of the most common causes of furnace short cycling. What happens is that when an air filter is dirty, air will not flow properly into the heat exchanger, which causes the furnace to overheat.

To prevent anything dangerous from happening, one of the furnace’s safety mechanisms, the high limit switch, switches off the furnace. Once it cools down, the cycle repeats again.

The solution is to simply clean your furnace and the filter periodically and change it when required as well. This is a DIY activity, just as long as you follow safety procedures and turn off the furnace and the gas valve before you do anything.

Faulty thermostats

There are a couple of reasons why your faulty thermostat may be telling your furnace to shut down. For one, if your thermostat is in direct sunlight or close to a heat source, it is capable of misjudging the temperature and sending your furnace the wrong message, therefore shutting it down.

Another possible reason for your thermostat shutting the furnace down is if the batteries are weak. Being a mechanical device, it is prone to making errors. Simply replace your thermostat batteries with new ones, turn it back on and observe it for an hour to make sure it is functioning properly.

Poor insulation

caulking a door

Air leaks are another common cause for furnaces to short cycle. Because hot air keeps escaping, the furnace keeps firing up repeatedly.

In most cases, the air leaks happen around your door and window frames. So check the caulking in those areas and if necessary, recaulk them to seal them properly. 

It is also advisable to check the ductwork in your home for air leaks and address any that may exist at the earliest opportunity.

Furnace size

This is something you might experience if you’ve moved into a new house. If your furnace is too big for the size of the home, it will not only heat up the area really quickly, but it will also switch on and off more often, increasing your energy bills.

The only solution in this situation is to replace the furnace with one that has a capacity more suited to your needs. However, it might be best to consult a professional to learn exactly what you need.

Mechanical parts problems

furnace inspection

Thus far, we’ve discussed common causes for your furnace’s short cycling. Now, let’s look at some mechanical problems. 

If your blower motor isn’t functioning properly, air won’t be getting warmed by the heat pump, nor will hot air be blowing through your vents. This will cause the furnace to shut down. If you do not feel air coming through the vents, this is probably the reason.

Your furnace is also liable to shut off prematurely if the high limit switch is malfunctioning. 

If your exhaust vent, or flue, as it is also called, is blocked, the heated gasses will not be able to escape properly, causing the flue switch to turn off the furnace before the vent overheats. 

The furnace will also be shut down by the flame sensors or flame rod if the ignition system does not produce a flame within four seconds of turning on.

You better address these issues with a professional HVAC technician.

Regular maintenance is the key

As with anything that has mechanical parts, regular maintenance is the key to keeping a furnace functioning properly. 

While the lifespan of a furnace may be around 15 to 20 years, regular maintenance and changing parts periodically can extend that to a few more years, while making sure you save money on energy bills as well. Even if you do not have the time, it is worth your money to call upon a home services company to maintain your furnace for you. 

Why is my electric furnace short cycling: Causes and fixes was last modified: June 22nd, 2022 by Narayan Shrouthy
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