Is Your Furnace Short Cycling? Here’s Why

Most homeowners probably don't pay too much attention to their heating and cooling system until something is amiss. If your furnace is turning on and off too frequently, it's suffering from short cycling, and it can impact your comfort right away. If you're dealing with this problem, here's what could be causing it and how you can fix it.

Wrong Size

If your furnace is new or if you've just moved into a new home and you notice the furnace short cycling the first night you're there, it's possible that the HVAC system has too much power for the space you're living in. In other words, it's too big.

There's no doubt that most homeowners want to heat up their space as fast as they can. And there's really nothing wrong with that so long as it's heating up efficiently and economically. But when a furnace is over-sized, it uses a lot of energy to heat up your home because it's turning on too often. It can also keep the air from being distributed correctly.

The end result is a higher electric bill and increased wear and tear on your system. If the furnace is new and you learn that it's not the right size, implement your warranty to replace it with the right size system. If you have issues with this, find a contractor who will perform an inspection of the new unit. Sometimes, county officials do this at no charge or for a reduced fee. If your furnace doesn't pass inspection and you have a warranty in hand, you should not be responsible for the replacement.


When an HVAC system gets too hot, it can cause cracks to form in the heat exchanger, ultimately leading to carbon monoxide leaking into the home. If your furnace is working at least partially as it should, it will shut off when the elements start to overheat.

Even if your furnace shuts off when overheated, it may not turn off quickly enough. So it's important to have this problem inspected as soon as possible. There are many things that can lead to overheating, and consulting with an HVAC contractor is the best solution.

Thermostat Issues

The furnace receives instructions from the thermostat, and where it's mounted plays a vital role in how often it runs. Your thermostat may have been installed in the ideal spot when it was first put in. But over time, things can change, particularly if you've done any remodeling. Ideally, it should be installed in a room that's frequently used (except for the kitchen) but as far away from windows, vents, and drafts as possible.

To test that the issue is the location of your thermostat, construct a homemade shield out of paper or cardboard. If the cycling returns to normal, call an HVAC professional to have it relocated to a better spot.

Blocked Air Flow

If the air flow in the furnace becomes blocked or restricted, it can cause the system to short cycle. There are a few things that might block air flow, but fortunately, most of them are relatively easy to fix.

Dirty Filter. A filter that has accumulated too much dust and dirt can quickly block air from flowing through your furnace. Replace the filter as often as the manufacturer recommends. And be sure to only use filters that are appropriate for the model you have.

On average, thin 1"– 2" filters should be changed monthly, and thicker 5" filters last up to 3 months.

Blocked Registers. If your home has floor registers, be sure that furniture, plants, and other items aren't covering them up and blocking air flow. This could be what's causing your furnace to short cycle.

Blocked AC Coils. In most central AC units, evaporator coils are located just above the furnace, and their function is to turn cold air into warm air. But if they get covered in dust and debris, they stop working efficiently, causing your furnace and air conditioning to short cycle. It's best to have them cleaned professionally to fix the problem.

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