What is short cycling
Short cycling is a term used to describe a furnace or heating appliance that is not heating efficiently or completing a full heating cycle – for example: your thermostat is set for 70 but will only reach 64 degrees. This would be because the furnace is not able to run long enough to reach the desired temperature setting.
What causes short cycling
There are many different reasons that a furnace could experience short cycling. We will cover some of the most common causes in this article.
Dirty or blocked air filter
A dirty or blocked air filter will cause a host of issues related to lack of airflow. When your furnace has a blocked air filter, the furnace will overheat and shut down on a high limit. This is because the blower motor can’t pull enough cold, return air through the air filter fast enough to sufficiently send air through the heat exchanger.
Blower motors have an internal thermistor attached to the windings. When the windings overheat, the thermistor causes the blower to stop running until the windings cool down, it will then allow the motor to begin to run again. If your motor cooling plate is blocked, this can happen rapidly. When the motor overheats and shuts down, it causes the furnace to overheat and trip the high limit switch. I recall a job one time during the cooling season where the air conditioner kept short cycling on high pressure and I could not figure out what was happening. The unit would run fine and then randomly shut down. The refrigerant pressures were normal, then randomly go through the roof. Five minutes later, the unit was running fine again. I went around to the front and into the basement and checked the filter, it was brand new, the blower was running, and the vents were all open. After about 10 trips back and forth, I finally caught the blower motor not running! It was going off on thermal protection and shutting down mid-cycle. Then after about 90 seconds, it would cool off and start back up again. So, between the time that I was outside and noticed the compressor shutting down on high pressure and the time it took me to walk all the way around the home, enter the home and get down the basement to the furnace, the blower had already cooled off and began working again!
Blocked or dirty evaporator coil
If you have air conditioning, you will have an evaporator coil located directly on top of your furnace. If you have an ill-fitting filter, don’t change your filter often enough, or even run your furnace without an air filter, the evaporator coil may become clogged with dust and debris. This is often overlooked with a furnace that is short cycling. If you need to change your filter prematurely to stop short cycling, have a new filter and still short cycling, and have all your supply and return vents open and unobstructed, this is likely your issue.
Closed or blocked supply or return registers
Each heating system is sized specifically to a specific heat load. When you close down or block registers as shown in the photo above, It begins to mimic a system that is oversized. If you block too many registers, the furnace can’t bring enough cold, return air and the furnace will overheat and trip the high limit switch. When a high limit switch (shown above) trips, it shuts the burners off and activates the blower motor until the limit switch reaches its reset temperature, at which point, it then allows the burners to re-ignite.
The heat exchanger is the heart of any furnace. This part of the furnace is responsible for the heat production by “exchanging” the cold air from the return for heated air as the air passes through the heat exchanger, hence the term “heat exchanger”. High-efficiency furnaces have two heat exchangers, a primary and a secondary. If the primary heat exchanger has a crack or hole, (shown in the photo above) it will trip the high limit safety switch and cause a short cycle. If you have a high-efficiency furnace, then you have a secondary heat exchanger. This can become clogged (shown in the photo above) just like an evaporator coil and cause the furnace to overheat and short cycle as well.
The inducer motor or “Draft inducer” (in some cases) pulls the exhaust through the heat exchanger and out through the exhaust, flu pipe. If the exhaust pipe or air intake becomes, even partially blocked, then the furnace may short cycle. The photo above shows an air intake pipe removed from the furnace housing to reveal a partially blocked air intake by a rodent.