Furnace turning on and off

Why Does My Furnace Keep Turning On and Off


We often get calls from homeowners who are concerned because their furnace is constantly turning on and off, which is also known as short-cycling. The causes and results of short-cycling are varied and can be minor or severe, so we’ve broken down everything you need to know to help you get ahead of the problem.

What is furnace short-cycling?
During a normal furnace cycle, your furnace will turn on when the temperature inside your home drops below the thermostat setting, dispersing warm air around your home. Once the desired temperature is achieved, your furnace will turn off until there is another drop. A short-cycle is when your furnace repeatedly cycles on and off during a short period of time, and the desired temperature is never achieved. You can tell the difference between the two because, during a short-cycle, your furnace will likely only stay on for a couple of minutes.

What causes short-cycling?
Several things can cause your furnace to constantly turn on and off, but a few of the more common causes are:

  • Restricted Air Flow: The safety mechanisms inside your furnace will signal it to shut off if there is insufficient flow. Lack of flow is most commonly caused by dirty air filters, which is why it’s so important to maintain them and change them frequently. It can also be caused by blocked vents, so if your furnace is constantly turning on and off, take a walk around your home and check the vents. Make sure that the vents are open and clear away any furniture, curtains, or clutter that could be blocking their flow.
  • Improperly Installed or Malfunctioning Thermostat: Thermostats, just like any electronic device, can malfunction on occasion. If your furnace is getting incorrect readings due to a malfunction, it could be causing your furnace to frequently turn on and off. Lack of power due to drained batteries can also cause your thermostat to malfunction, so be sure to check that they have a sufficient charge.
  • Improperly Sized Furnace: A furnace that isn’t the right size for your home can also cause short-cycling. For example, an oversized furnace will turn off and on more often because it heats your home so quickly that it only needs to run for a short period. Although this sounds like a good thing, it isn’t because it leads to unnecessarily high bills and cold spots in most instances. Unfortunately, if this is the problem, the only solution is a replacement.

Think your furnace may be short-cycling? Call the experts at HAMCO for assistance!

HAMCO has been proudly serving the Greater Hamilton Areas HVAC needs since 1946. We are dedicated to providing our clients with dependable products and reliable service.

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