Furnace keeps turning on and off?

What usually causes this

These are the common possibilities, not a diagnosis. A page cannot tell which one it is — that takes a technician testing the system.

Overheating from restricted airflow

A clogged filter, closed vents or a weak blower lets heat build up until the limit switch shuts the burners off. It cools, restarts, and repeats.

Dirty flame sensor

Burners light and shut off within seconds, several times in a row.

Thermostat placement or settings

A thermostat near a supply vent, in direct sun, or with an incorrect cycle setting can shut the furnace off early.

Oversized furnace

A furnace much larger than the house needs heats it in short bursts. It's hard on the equipment and leaves rooms uneven.

Venting or pressure-switch problem

If the furnace can't prove its venting is clear, it shuts down for safety.

What you can safely check yourself

• Replace the filter and make sure all vents and returns are open.
• Check that the thermostat isn't next to a supply vent, a lamp or in direct sunlight.
• Note how long each cycle lasts — seconds or minutes. It tells the technician a lot.

What we check when we get there

1. 1The technician times the cycles, measures temperature rise and static pressure, and checks the limit, flame sensor, inducer and venting.
2. 2The heat exchanger is inspected, since repeated overheating is exactly what cracks one.

What it is likely to cost

A filter or thermostat fix costs little. Flame sensor, pressure switch and blower repairs are priced before work. If the furnace is badly oversized, the long-term answer is at replacement time, sized with a load calculation.

Heating diagnostic

$89

Flame sensor

From $149

Pressure switch

From $199

Blower motor

From $499

Prices are starting points for standard work in the Savannah area. Final price depends on the system and site, and is confirmed before work begins. Permits and code-required upgrades may be additional.

Questions people ask about this

How long should a furnace run each cycle?

It varies with the weather and the house, but repeated cycles of only two or three minutes are a sign something's wrong.

Can short cycling damage my furnace?

Yes. Frequent starts wear the igniter and other parts, and repeated overheating stresses the heat exchanger.

Where to go next