A furnace “short cycles” when it turns off before reaching the desired temperature. This means that if your thermostat is set to 72 degrees, the furnace may shut down at 65 or 68 degrees, but will never reach 72 degrees. Short cycling can also be used to describe a furnace that turns on and off too often. Most furnaces will turn on and run for several minutes before reaching the desired temperature and shutting back down. This can happen anywhere from three to eight times an hour, depending on the condition of the furnace and the outdoor temperatures. If it cycles too frequently or only runs for a minute or two before turning off, there may be a problem with the furnace.
What Causes Short Cycling?
Furnaces have built-in safety systems that prevent dangerous situations like fires, carbon monoxide leaks, or gas leaks. The most likely cause of short cycling is the fan limiter, which prevents fires and furnace damage by shutting the system down if the heat exchanger becomes too hot. The flue limiter can also shut the system down if the flue is clogged to prevent combustion gas accumulations, and the flame sensor can shut the system down if the igniter has failed.
Several conditions can cause furnace shutdowns, including:
- A malfunctioning thermostat – If the thermostat is not operating properly, it can cause the furnace to cycle too frequently. Some thermostats require batteries, which should be changed every six months.
- A clogged air filter – If the air filter is clogged, the blower fan may not be able to move enough air over the heat exchanger. This can allow the heat exchanger to overheat, tripping the fan limiter and shutting the system down.
- A clogged flue – If the flue or vent pipe has been damaged, an object has fallen into it, or a pest like a bird or rodent has made a nest in it over the summer, the flue limiter will shut down the furnace to protect you from carbon monoxide poisoning and other dangers.
- A dirty flame sensor – The flame sensor, or the thermocouple on older furnaces, senses the presence of flames at the burner and allows natural gas to flow. A dirty flame sensor or damaged thermocouple will shut off the gas valve and the furnace. This prevents natural gas accumulations that could cause a fire.
- A damaged igniter – If the hot surface igniter on newer furnaces is not working, or the pilot light goes out on older models, the furnace will shut down to prevent gas accumulations.
- A fan limiter failure – The fan limiter prevents the heat exchanger from becoming hot enough to start a fire or damage the heat exchanger itself, like cracks that could lead to carbon monoxide leaks. If the fan limiter is not working properly, it may shut the furnace off prematurely.
- An oversized furnace – If your furnace is too large for your home, it may cycle on and off frequently. While this is not a dangerous condition, it could lead to increased energy usage and higher utility costs.
Though it may be inconvenient when your furnace shuts down for something minor, like a clogged air filter, such safety features prevent harm to your family and your home.
Furnace Repairs & Maintenance in Maryland
If your furnace is short cycling, talk to our team at GAC Services. We can quickly repair most problems that would cause a furnace to short cycle, and we offer furnace repair services throughout Maryland, including Bethesda, Gaithersburg, and the surrounding areas. We can also prevent problems from happening in the first place with our annual furnace maintenance services, which include preventative maintenance like changing the air filter, cleaning the flame sensor, and testing the igniter.
Contact us online to schedule furnace services near Gaithersburg today!