Why a Furnace Short Cycles

Short cycling means the furnace starts, runs for a few minutes and shuts off before the house reaches the thermostat setting, then does it again soon after. It wastes fuel, wears out ignitors and motors and leaves rooms uneven.

A dirty flame sensor

The flame sensor proves to the control board that the burner lit. When it gets coated with residue, the board cannot see the flame and shuts the gas off within seconds. Cleaning or replacing the sensor is a quick fix.

Overheating from poor airflow

A clogged filter, closed registers or a restrictive duct system make the furnace overheat until the high-limit switch shuts the burner off. When it cools, it starts again. Changing the filter is the first thing to try.

An oversized furnace

A furnace that is too big for the house heats the space very quickly and shuts off, especially on mild days. That is common in homes where the furnace was sized by square footage or matched to an older, larger unit.

Thermostat problems

A thermostat placed near a supply register, in direct sun or on an outside wall can shut the furnace off early. The wrong settings for the type of equipment can also cause short cycles.

Venting and pressure switch issues

On high-efficiency furnaces, a blocked intake or exhaust pipe, often from snow or a nest, or a clogged condensate trap can trip the pressure switch. Check that the pipes outside are clear after storms.

• Change the filter first.
• Make sure supply registers are open.
• Check outdoor vent pipes for snow or debris.
• If it continues, have the furnace tested.