When a furnace begins to short cycle it often is a sign of a problem in the heating system. Short cycling may be due to small issues that can be easily fixed or there can be a major problem. Depending on the cause of the short cycle some repairs can be done without the need of an HVAC technician while others may require deeper inspection of the furnace to locate the cause. Hal Mechanical will define furnace short cycling and some of the common causes.
Furnace Short Cycling Dangerous
A furnace has a built-in safety mechanism that is designed to prevent fire, carbon monoxide leaks or gas leaks in the case of a gas furnace. There are three primary safety mechanisms in a furnace and one is the fan limit, which shuts the furnace off if it becomes too hot. Another component is the flue limit which will shut down a furnace if the flue is clogged. The third component to the safety system is the flame sensor which will shut down a heating cycle if the igniter fails. Often it is one of these three components that causes the furnace to shut down early or before the temperature in the home reaches the designated setting. When a furnace starts up and quickly shuts down this is essentially short cycling.
One common cause for a short cycle is a problem with the thermostat. Sometimes the thermostat needs new batteries or the thermostat was set to the “on” setting which after a long period of time can cause the furnace to overheat. If the furnace overheats, the safety system will trigger and shut the furnace off.
Dirty Air Filters in House
Another common problem that leads to a short cycle is dirty air filters. When air filters are neglected, they become compacted with dirt. Dirty air filters restrict the air flow and causes the heat exchanger to overheat. This will trip the fan limit. Changing air filters is essential maintenance that should occur every 1 to 3 months.
Clogged Furnace Flue
A furnace is connected to the flue pipe which redirects carbon monoxide safely outside. At times the flue pipe can become clogged by birds or rodents. The flue pipe should be checked and cleaned at least once a year to prevent clogs. If a clog is present the flue limit is triggered, turning off the furnace early.
Bad Flame Sensor
A dirty flame sensor, or in an old furnace the thermocouple, often fails to light the igniter. If it is too dirty or damaged, the furnace will start up and then quickly shut down. Often the flame sensor needs to be cleaned or replaced.
Damaged Furnace Igniter
If the igniter is damaged it will fail to light the pilot light. If the igniter fails, the flame sensor will shut down the furnace upon start up.
Furnace Fan Limit Failure
A fan limit is an important part of the furnace safety system. If it is malfunctioning it may cause the furnace to shut off even though their isn’t a problem with the heat exchanger. Sometimes the fan limit needs to be replaced. However, there could also be a crack in the heat exchanger.