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Why Does My Furnace Keep Turning On and Off?

Why Does My Furnace Keep Turning On and Off?

January 9th, 2023


A furnace will turn on and off during the winter months, but it shouldn’t do so every few minutes. Let’s take a look at this problem and what’s causing it.

When you’re home during a cold day, you can expect to hear your furnace kick on and then run for 10–15 minutes or more before shutting off. Your house will retain enough heat for the furnace to remain off for about the same amount of time before coming back on. On an average winter day, you can expect the furnace to go through three heating cycles per hour. 

When a furnace begins to cycle more frequently, staying on for only five minutes or even less and then turning back on a short time later, it’s said to be short-cycling. You don’t want this to happen for several reasons:

  • The furnace will not run for long enough during each abbreviated cycle to evenly spread heat around the house. You’ll encounter cold spots throughout the rooms.
  • The constant stress of turning on and off will cause the furnace to wear down faster, leading to more frequent repairs and likely an early replacement.
  • Because a furnace draws on the most power when it starts up, starting up more than twice as often as usual means a massive waste of energy. You can expect to see higher heating bills from a short-cycling furnace.
  • Short-cycling is usually a symptom of another furnace problem that needs prompt attention.

We’ll look closer at that last point, since it will also help answer the question…

What Causes Furnace Short-Cycling?

Short-cycling is bad news on its own, but often the source is far worse. You might even say that short-cycling is the warning you need to call for furnace repair in Acworth, GA. What are some of these troubles?

  • The simplest problem is one you can fix yourself: a dirty air filter. Clogged air filters create all types of problems for a furnace, including short-cycling. Replace the dirty filter with a clean one and keep up with regular changes every 1–3 months.
  • The furnace is overheating, leading to the furnace limit switch shutting off the burner and the blower to prevent damage and avoid safety issues. Or the limit switch itself is malfunctioning, turning the furnace off when nothing is actually wrong. 
  • Another safety feature, the flame sensor, is malfunctioning and failing to sense that the gas jets have ignited. It will shut off the furnace to prevent unburned gas from flooding the combustion chamber, even though gas is burning.
  • The thermostat isn’t working correctly. Its connection to the control board that monitors when to turn on the burners and the blower fan is causing the system to continue to turn off too early. The thermostat may also be miscalibrated and sensing the home is hotter than it is. 
  • The furnace has worn down so much with age that it cannot maintain regular cycles. This often occurs in a furnace over 15 years old. The best “repair” in this case is to replace the furnace. 

This isn’t an exhaustive list, but it’ll give you a good sense of why you want professionals on the job when you’ve got a short-cycling furnace.

Dayco Systems has been Metro Atlanta’s choice for three generations. Contact us when you need heating system repairs.