Short Cycling Furnace Problems

Restricted Airflow

• Lack of sufficient airflow can cause a furnace to short cycle. Dirty or clogged air filters are one of the most common causes for restricted airflow. Filters should be replaced every two to three months to reduce dirt and clogging problems. If air filters accumulate an excessive amount of debris from the ducts, clean the ventilation system to lessen the debris entering the furnace. Dirty air filters result in short cycling because the blower may not supply enough air to the heat exchanger to cool it sufficiently. Dense, high-efficiency filters can also cause this problem because they overly restrict flow.

Blocked Air Registers

• A similar situation involves blocked air vent registers. Improper room design may result in blockage of air vents due to furniture or other objects placed over supply or return registers, which can also restrict airflow.

Air Conditioning Coils

• Another condition that can affect airflow is dirty air conditioning coils. Evaporator coils that help cool a building are usually located above the furnace. The function of these coils is to remove heat from the atmosphere. Debris on the coils blocks airflow and could cause short cycling in both the heating and cooling seasons.

Thermostat Location

• To ensure proper operation, mount thermostats on an inside wall in a frequently occupied area of the home. Thermostats should be located away from supply registers as these can affect the amount of heat produced, causing the unit to shut itself off before sufficient heating occurs. The area should also be free of drafts, sunlight and other effects caused by windows.

Overheating

• Newer furnaces have safety devices that monitor overall temperatures, including a thermostat, also known as the high limit device, mounted inside the unit that shuts down the furnace if the device thinks the furnace is overheating to the point where cracks could occur in the metal or a fire might start. Loose or broken blower fan belts and dirty fan blades can also cause this condition.

  Thermostats can also accumulate dirt, which can interfere with moving parts inside the furnace and lead to shut down. Electrical switches on most thermostats can also fail with age, leading to short cycling.