Short-Cycling Furnaces and What to Do

Because winter is quickly approaching, you want to ensure your furnace is working properly at keeping your home warm and your family comfortable. If you feel like hardly any heat came from your vents and/or your unit shuts off shortly after firing up, it may be short cycling. Short cycling means that the furnace turns on and off very frequently with little time between on and off. This is a common issue that you may experience with your furnace this time of year. If you start having furnace problems (even if it doesn’t completely go out), go ahead and give us a call at Mainline HVAC in Alabaster, AL, so we can diagnose the problem and complete any furnace repair as soon as possible. We’ve already had some cold weather and are expecting more and we don’t want you to go without heat. Let’s take a look at the what and why of short-cycling– 

Why Does a Furnace Short Cycle? What causes it?   

Common short cycling causes include:

  • Overheating
  • Bad flame sensor
  • Dirty furnace
  • Blocked heat grates/ registers
  • Blocked exhaust vent
  • Thermostat is in the wrong location
  • Furnace is too big for your home

Thankfully, when we take a look at your system, we will be able to figure out which of these is causing the issues. You won’t have to worry about a thing, we’ve got it taken care of.


Like most equipment, furnaces have a way to shut off to prevent dangerous overheating and damage. Your furnace may turn off prematurely if it’s overheating, which is a good thing as far as safety goes, but bad for keeping your home warm. An overheating furnace can potentially cause cracks in the heat exchanger, which could cause carbon monoxide to leak into your home. You might be wondering, “Do I have to replace my whole furnace if it’s short cycling?” Not necessarily. There are many reasons a furnace may get overheated and cause short cycling and most don’t require a full replacement. Some of the causes actually require very simple corrections. Here are the most common reasons:

  • Blocked Registers

The registers in your floor, walls, or ceiling likely have a way to close them, allowing you to control which rooms get heat. If too many of these vents are closed your furnace can’t disperse enough heat, so the heat will build up in the furnace, causing it to overheat and shut down. Make sure that most of your registers are open and not being blocked by anything like furniture. If you make these adjustments and your furnace is still acting up, then this was likely not the main cause.

  • Blocked Exhaust Vent

This is a dangerous cause of overheating and short cycling since it can allow carbon monoxide through the vent. The exhaust vent becomes blocked by birds’ nests from birds, beehives, or something else. If you check it out and realize the cause is a nest, it may be best to have animal control or pest control handle the task.


  • Dirty Furnace Filter

A dirty filter can cause overheating, kind of like with a clothes dryer. A sign is that you feel hotter than usual air coming out of the registers- seems like hotter air would be nice when you’re trying to heat the house, but it’s actually a sign of overheating. If the furnace gets too hot, the switch will shut it off. A dirty filter in your returns prevents a flow of return air into the furnace, and will quickly overheat. The solution is super simple- change your filters out regularly (several times a year).

Can It Be Something Other Than Overheating?

Yes, sometimes short cycling can be as simple as bad thermostat placement. If your thermostat is getting warmed before the rest of the house, it will cause your furnace to turn off prematurely. This can be due to poor placement like right above a register or on the wall of the utility closet where the furnace is located. If placed here, the thermostat will heat quickly and reach its set point before it should. The solution is easy! Just relocate the thermostat to a location that isn’t near a vent or window.


If you read through this and realized it sounds like your current situation, your furnace may be short cycling or overheating. Contact Mainline HVAC for your furnace repair needs in Alabaster, AL and surrounding communities. While it could be something as simple as a thermostat issue, it could also be something more serious, so it’s always best to have our Mainline professionals take a look.

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