November 29, 2017
A furnace that starts then stops is a common issue also known as short cycling. The furnace starts to heat the home, but turns off again quickly after only a short amount of time has passed. It is not normal for a heating cycle to be so fast, and short cycling can cause damage to the furnace. Furnace repair may be needed to correct the underlying issue causing short cycling.
A furnace that is the right size for the home should run three to eight heating cycles in an hour period (this will vary based on the characteristics of the home and preferences of occupants). When the furnace cycles more often than this, short cycling is possible – it may also be possible that extreme temperatures are causing the need for more frequent cycles, or your furnace is undersized or requires additional insulation.
What causes the furnace to start then stop?
Common causes of furnace short cycling include:
- Restricted airflow. Airflow can be limited through the system when an air filter is dirty, is not installed correctly, or a filter that is too restrictive is in place. Blocked or broken room vents can also prevent air from moving properly through the system and into the home, causing short cycling. Debris caked on the evaporator coils can also cause airflow restrictions.
- Thermostat issues. Poor placement of a thermostat may contribute to short cycling if a thermostat is installed in an area that experiences frequent temperature fluctuations, including on exterior walls, near windows and doors, or near heat sources.
- Furnace overheating. Overheating causes the furnace’s safety devices to trip, shutting down the heating cycle to prevent damage and danger. The high-limit switch will shut the furnace off if temperatures get too high, a common cause of short cycling.
- Flame rod failure. When the furnace’s flame rod fails to sense the burner flame, the furnace will shut down. Cleaning the flame rod may resolve this issue.
What should you do?
If your furnace is running, shutting off and then starting again it is best to call an HVAC professional to come take a look. They can help fix any of the above causes or identify additional items that need repair.