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Triple-O Heating, Cooling, Electrical & Plumbing Blog

Why Is My Furnace Short-Cycling?


As we approach the time of the year when furnaces will once again fire up to keep New York homes warm through the winter, we want to look at one of the more common problems these heating systems can run into: short-cycling.

This is a type of behavior from a furnace, a repair problem in itself, and a symptom of possible bigger problems that need immediate attention. We’ll look into the causes for short-cycling and why you’ll want to call us for furnace repair in Pittsford, NY if there isn’t a simple explanation for the problem.

What Short-Cycling Is Like

If you aren’t familiar with short-cycling, it means that the furnace has become trapped in a rapid stop-start cycle where it never completes a full heating cycle and instead prematurely shuts down. It then turns back on again a short time later, only to shut down again early, and so on and so on. 

Short-cycling is damaging to a furnace because it places excess strain on the parts. A furnace works the hardest when it starts up, and if it’s caught doing this over and over again rather than performing in steady cycles, it will wear down faster. This leads to more repairs and an early replacement. It also raises utility bills because the furnace draws on the most power during start-up.

The Possible Causes

Short-cycling is bad on its own, but it’s rarely the only problem the furnace has. Short-cycling usually occurs because of one of several malfunctions—some simple, some complex.

  • Dirty air filter: This is a simple one that you can correct on your own. A dirty filter will create many types of problems for a furnace, including causing it to overheat so that it shuts off. The air filter needs to be changed for a clean one every 1 to 3 months. Staying on top of these changes will help prevent this problem in the future.
  • Blower fan or ventilation problems: If the heat exchanger in the furnace (the part responsible for transmitting heat from the gas jets to the air) doesn’t cool down because of the blower fan sending air around it, it will trigger a limit switch to shut off the furnace early. The same will happen if the furnace isn’t properly exhausting combustion gas from the heat exchanger. Blower fans and ventilation problems need immediate attention from professionals. 
  • Faulty thermostat: The thermostat may be sending incorrect information to the furnace. If the thermostat is miscalibrated, it may cause the furnace to shut down early. The thermostat could also have malfunctioning programming.
  • Oversized furnace: If this isn’t a new problem, but one the furnace has had since the beginning, then the problem is likely that the furnace is not the right size for the house. A furnace that is too powerful will so rapidly raise the temperature that the thermostat will register that the furnace’s job is already done and shut off its cycle early. Unfortunately, the only solution to this problem is to have the furnace replaced—and make sure our professionals handle the job so a correctly sized furnace is installed.

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