Advice on Why a Furnace Cycles On and Off Frequently

It’s officially winter and you may be wondering why your furnace kicks on and off constantly. The heating experts at A&D talk are here to explain furnace short cycling causes.

First, what does it mean when a furnace is short cycling. A furnace short cycle is when your heating system repeatedly kicks on for brief periods of time on a repeated cycle. When your furnace cycles on and off frequently, with each portion of the cycle lasting from a few second or a minute or two, you may be dealing with an uncomfortably cool house due to ineffective heating. The good news is that if your furnace turns off and on repeatedly it is likely because one of the many safety features built in to your homes heating unit are being activated, which means that a situation where your furnace kicks on and off constantly likely isn’t inherently a dangerous situation. But, it is a situation that should be looked at and remedied. We are going to talk about some DIY tips that you can try as a furnace short cycling fix.

Before we address some of the things you want to check if your furnace cycles on and off frequently it is a good idea to briefly talk about how your heating system works and how some of the built in safety features function. Your home’s heating system is a pretty basic system. There are coils, or tubes, that are heated up by burning fuel inside of them. Thus heating up the air around the tubes. The blower on your furnace then blows and forces that heated air around your home. There are a number of safety checks that happen before the ignition sequence that burns the fuel inside the tubes.

  • Induced Draft Motor (IDM) turns on. The IDM is a small fan that cycles to blow out the air that is inside your furnace and forces that air up the exhaust flue. This step makes sure there is no gas build-up inside of your furnace, which when ignited could cause an explosion.
  • IDM pressure switch checks to ensure the IDM actually turns on. This fail-safe step is a second way to ensure there is no gas build up inside the furnace before ignition happens.
  • If the IDM motor has turned on and passed the independent check by the IDM pressure switch, the igniters are energized and fuel is introduced into the furnace, which creates a flame. Your furnace is equipped with a Flame Recognition System, which utilizes a flame sensor to ensure the presence of a flame so that fuel is not being dumped into the furnace without being properly burned. If no flame is present the furnace will shut down after about 4 seconds.
  • Only after all of these steps have taken place with your furnace begin to run properly and being it’s heating cycle.

In addition to safety features before ignition there are also built in safety checks for while the furnace is running.

  • The High Limit Switch automatically shuts down your furnace if the internal temperature gets too high.
  • The Flue Limit Switch also shuts down the furnace if air is not properly ventilating out of the flue. Your exhaust flue ensures that hot and newly combusted gas is not building up inside the furnace, which could lead to a potentially dangerous situation.

Now that we have a basic understand of how your furnace and the safety features work we can begin to talk about some of the troubleshooting steps you can take if you find your furnace cycling on and off.

Check the Filter

If your furnace cycles on an off frequently the first thing you want to check is the filter. Ideally, a filter should be changed at a minimum every 90 days, but if you live in a dry and dusty climate you may need to change it more often. A clean filter not only helps decrease dust and allergens in the air, but it also helps your furnace run more efficiently by filtering out dirt and dust from entering the internal components of your furnace. If the filter is clogged it does not allow for proper airflow, which can lead to a heat buildup over the coils, which can cause the high limit switch to shut off the furnace. A clogged air filter is the leading cause when a furnace cycles on and off repeatedly. Luckily, it is a very easy fix that does not require a service call from an HVAC technician.

Check the Thermostat

Like every other electrical device, a thermostat can eventually stop working properly, leading to a situation where your furnace keeps cycling. Many thermostats are battery powered, in which case a simple battery swap could eliminate your problem where your furnace keeps going on and off. If your thermostat is not battery powered you may want to consult a HVAC technician, as it may require installation and wiring of a new thermostat.

Verify That the Blower Is Operating Correctly

If you have a furnace cycling on and off too frequently it may be because your blower is not functioning properly. When you hear your furnace running simply check the airflow at one of the vents. If it is weak or nonexistent you may have isolated your problem. If the blower is not moving air out of the furnace until it is likely that the high limit switch is being triggered due to a build up of heat. While this was a simple step to diagnose this problem you will need to contact your local HVAC repair specialist, as this requires a professional technician and it not DIY friendly.

Check Your Flue

This step is not for the faint-of-heart. You will need to access your flue, which will likely be on the roof of your house. When the furnace is blowing you should be able to detect gas and heat escaping from the flue. Be very careful not to touch the flue and it could be very hot. If there is no gas escaping, or it is very weak, it may be that you have something clogging your flue, which is causing the flue limit switch to trip which means your furnace cycles on and off quickly.

There are other furnace short cycle causes, such as checking the flame sensor, but they are items that are best left to the HVAC professionals.

If you are having short cycle issues and have tried to find your furnace short cycle causes but have came up empty the professionals at A&D Heating & Cooling Inc. are here to help! Call us today to schedule and appointment and we will fix your furnace short cycling issues and have your heating system running correctly and efficiently before you know it!