Wondering, “Why does my furnace keep turning on and off?” If your furnace turns on, shuts off a few minutes later and then starts again shortly afterward, you may be dealing with what HVAC technicians call short cycling.
At Hamilton Home Comfort, our team of HVAC specialists covering the Hamilton and Greater Horseshoe Area in Ontario see this regularly during the heating season. The important thing is figuring out how quickly the furnace is shutting down and what is happening immediately before it stops.
A furnace that shuts off after ten seconds can point to a very different problem than one that runs for five or ten minutes and then shuts down.
Is It Normal for a Furnace to Keep Turning On and Off?
Yes. Your furnace is supposed to cycle. The thermostat calls for heat, the furnace runs until the home reaches the set temperature, then it shuts off until more heat is needed.
What is not normal is a furnace starting and stopping without properly heating the home.
If you notice the furnace cycling several times in a short period, or constantly starting again only a few minutes after shutting down, something may need attention.
If the Furnace Runs for a Few Minutes and Then Shuts Off
One of the first things we think about is overheating. Your furnace needs enough airflow to carry heat away from the heat exchanger and distribute it throughout the house.
If airflow is restricted, the temperature inside the furnace can become too high. The furnace’s high limit safety control can then shut the burners off.
Once the furnace cools down, it may start again.
That creates the repeated on and off pattern homeowners often notice.
A dirty furnace filter is a common cause, but we also find blocked return air, closed vents, dirty blower wheels, incorrect airflow settings and ductwork that cannot move enough air.
TIP: Replace your furnace filter regularly. This is an easy DIY furnace maintenance activity homeowners should be doing to keep their furnace running well.
An Oversized Furnace Can Short Cycle
Another reason why your furnace shuts off after a few minutes could be that it is too large for the home. It may heat the space very quickly, satisfy the thermostat and shut off before running a proper heating cycle.
It can then start again shortly afterward as the temperature drops.
A furnace can also be too large for the home’s duct system.
If the furnace produces more heat than the ductwork can properly move, it may overheat and shut down on its safety limit.
This is why proper furnace sizing matters.
A larger furnace is not automatically a better furnace. The equipment needs to be matched to both the heating requirements of the home and the airflow capacity of the duct system. This is why our team at Hamilton Home Comfort does a thorough assessment of your HVAC system before recommending a replacement furnace.
If the Furnace Shuts Off After Only a Few Seconds
If the burners light and then shut off almost immediately, there may be an ignition or flame proving issue.
A dirty flame sensor is one possibility.
The furnace needs to confirm that a flame is actually present after the gas valve opens. If it cannot properly prove flame, it shuts the gas off as a safety measure.
You may hear the furnace try to start several times before eventually stopping.
Other factors for the furnace starting and stopping after a few seconds can include issues with the:
- Igniter
- Gas valve
- Pressure switch
- Furnace venting
- Furnace controls
The timing of the shutdown gives an HVAC technician useful information when diagnosing the problem.
Your Thermostat Can Also Cause Frequent Cycling
Sometimes your furnace keeps turning on and off repeatedly because it is doing exactly what the thermostat is telling it to do.
If the thermostat is located near a heat register, fireplace, sunny window or another unusually warm area, it may think the entire house has reached temperature when it has not.
The furnace shuts off, the thermostat cools down, and then it calls for heat again shortly afterward.
Thermostat calibration, wiring and control issues can also contribute to unusual furnace cycling.
Why Short Cycling Should Not Be Ignored
A furnace starting and stopping constantly is more than an annoyance.
Every heating cycle involves components such as the inducer motor, ignition system, gas valve and blower motor.
If the furnace is cycling far more often than it should, those components are also operating far more often than they should.
More importantly, repeated shutdowns can sometimes mean a furnace safety control is doing its job.
If the furnace is overheating, for example, repeatedly resetting it does not fix the reason it overheated.
What Should You Check Before Calling an HVAC Technician?
There are a few simple things you can safely check yourself.
Make sure your furnace filter is clean.
Confirm that your supply vents and return air grilles are open and not blocked by furniture or rugs.
Check that your thermostat is set to HEAT and that the temperature setting is above the current room temperature.
Pay attention to how long the furnace runs before shutting off.
If possible, also notice whether the burners actually light before the furnace stops. Those details can help when diagnosing the system.
Why Does My Furnace Keep Turning On and Off? Get Answers and Have the Furnace Properly Diagnosed
If you have checked the basic things yourself and your furnace keeps turning on and off, it is time to have the system properly diagnosed.
Hamilton Home Comfort is a TSSA-registered contractor and is fully licensed and insured. Our licensed HVAC and gas technicians service furnaces throughout Hamilton, Stoney Creek, Ancaster, Dundas, Binbrook and surrounding areas.
When we diagnose a furnace that is short cycling, we are not simply looking for a part to replace.
We want to determine whether the furnace is shutting down because of restricted airflow, overheating, improper furnace sizing, thermostat operation, ignition problems or another safety control.
Finding the actual cause is what matters.
If your furnace keeps turning on and off before your home reaches temperature, call Hamilton Home Comfort and one of our licensed HVAC technicians can inspect the system, diagnose the problem and explain what is causing it before recommending a repair.
Call Hamilton Home Comfort at 905-643-098
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