April 26, 2021

How to Diagnose a Short Cycling Furnace

After a fairly warm start to the winter, temperatures are finally dropping. A short cycling furnace is the last thing you need to run into at this time of the year. When a furnace short cycles, the furnace repeatedly turns on and off resulting in a poor way to heat your home and quite the costly expense. Before you reach out to a South Jersey Heater Repair technician, take a look at different elements of your HVAC system to determine if professional intervention is needed rather than a simply blocked vent. There are multiple reasons why a furnace may short cycle. We have created a list of some of the most common causes of a furnace to short cycle.


The first explanation that could result in a short cycling furnace is overheating. When a furnace overheats, it shuts itself off prematurely to avoid extreme damage. This issue can cause cracks in the heat exchanger over time, which could put your home or business at risk of carbon monoxide exposure. You are able to tell if a furnace is overheating by checking for odd odors near where your furnace is located as well as the short cycling.

Bad Flame Sensor

A bad flame sensor, also known as a flame rod, can cause a furnace to short cycle as well. A bad flame sensor will force the furnace flame to shut off immediately after lighting. The best way to troubleshoot this potential problem is to safely remove the cover of the furnace to look for the burner firing. The ear test of listening in to the inducer motor fire can be a great way to check your flame sensor as well.


Two main blockages of the heat grate or exhaust vent can call for a furnace to short cycle. Heat grates allow heat to be dispersed throughout your home properly. These grates have dampers on them in most cases, which allow you to close off the grates or allow less heat to be omitted. If over 75% of your grates are obstructed, the air will be trapped in your furnace resulting in a shut down after about 3 – 10 minutes upon startup. It is important to check your grates in case furniture is blocking them or the dampers are activated. The exhaust vent may be blocked as well. Usual blockages of the exhaust vent come from nests of animals or buildups of snow and dirt. Airflow is extremely important to a furnace, so it is important to have all vents clear of debris to allow a furnace to breathe and function properly.

Dirty Filter

Everyday luxuries that we use like automobiles and swimming pools have filters that need to be cleaned or replaced, so it only comes to reason you will need to check your furnace for a dirty filter as well. A dirty filter in any of these things can lead to problems. A dirty furnace filter will not allow cool air to return to the furnace, which results in a premature shutdown and hotter air than usual to be pushed out of the heat grates. Furnace filters should be changed routinely to avoid this issue and can easily be done by anyone at your home or business.

If you have any questions or concerns about a short cycling furnace or troubleshooting, please reach out to Fante’s PHVAC or at 1-609-835-1512!

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