Why Does My Furnace Keep Turning On & Off?
All furnaces turn on and off throughout the day. How often this happens depends on several factors, including the layout and design of your home and the outside temperature. A typical furnace that is properly sized and functioning efficiently may turn on three to eight times per hour, more if it is very cold outside. The issue to be concerned about is not how often your furnace is turning on and off, but why it is happening and, even more importantly, how long it runs when it does turn on. An efficient furnace will run for a few minutes. A “short-cycling” furnace runs for only a minute or two (or less) before shutting off. This could indicate a dirty air filter, a need for maintenance or a problem with the thermostat.
Dirty air filter
A dirty air filter is the most common reason for short cycling, and the easiest to fix. When the air filter is clogged with dust and debris, your furnace has to work harder to produce the same amount of heat. Replacing the air filter every 30 to 60 days will not only keep your furnace from short cycling, it will help keep your whole system clean and functioning well. This means less chance of breakdowns and costly, inconvenient repairs, longer life for your furnace, lower utility bills and even cleaner indoor air.
Your thermostat could be malfunctioning. Make sure it is set properly, that the batteries are fresh or there is power getting to the unit, and that the wiring connections are secure. Another issue could be the location. The thermostat should be on an interior wall away from drafts and heat sources such as direct sunlight or lamps.
Dirty flame sensor rod
The flame sensor is a safety feature that prevents gas from flowing when the flame is not igniting. A dirty sensor rod will not get an accurate reading (it won’t sense a flame when there is one) and will shut the furnace down. Cleaning the sensor is a maintenance task that should be done at the beginning of the heating season.
Improper venting or a plugged vent
Blocked or closed vents can cause the furnace to short cycle. Make sure all the vents in your home are open and clear. Move any furniture or other household items that may be blocking airflow. Use a vacuum to clear dust and dirt from around and just inside the vent covers.
When it comes to furnaces, bigger is not necessarily better. A furnace that is too big for your home will cycle on and off too quickly to maintain the proper temperature. Your HVAC professional can do a heat load test to determine if the furnace is the right size.
Other areas to check
If none of the above solutions fix the problem, check for:
- Water in the tubing or pressure switch
- A cracked heat exchanger
- A malfunctioning inducer
- A faulty furnace blower motor
- The flue
How you can prevent this in the future
The best way to avoid problems with your heating system is to schedule a fall tune-up. Annual heating maintenance can extend the life of your equipment, diagnose potential problems early (before they become big problems) and keep your system running at maximum efficiency. Some manufacturers require annual maintenance for the warranty to remain in force.