Furnace Short Cycling

A furnace that keeps restarting burns fuel without heating anything and wears out its ignition parts quickly, so it's worth having looked at before something quits outright.

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When this becomes an emergency

The burner lights, runs for two or three minutes, then quits and starts over.
The house never gets to the set temperature no matter how long the system runs.
Propane use has jumped while the rooms feel no warmer than before.
You can hear the unit starting up far more often than it did last winter.

What a pro does

Short cycling is a symptom with four or five common parents. A tech watches an entire cycle with a meter on the limit circuit, because the difference between a unit tripping on high limit from restricted airflow and one dropping out on flame signal is the difference between a filter and a sensor, and guessing between them wastes a service call that took real driving to reach. Airflow is the usual culprit in older homes that weren't built with undersized return ducting. An oversized furnace installed for the coldest week of the year will also cycle short through most of the season, which is a sizing story rather than a repair.

Frequently asked questions

Could a furnace be too large for the house?

It happens often out here, where equipment gets sized for a brutal cold snap. The unit satisfies the thermostat before the house evens out, so it fires again minutes later.

Does short cycling actually cost much in fuel?

It does, because the first minute of every cycle is the least efficient one and a short-cycling unit runs almost nothing but first minutes.

Where should a filter be checked on an older forced-air system?

It usually sits in a slot at the blower cabinet or behind a central return grille, and on older units it's easy to miss because there's no obvious door.

Can a thermostat on an exterior wall cause this?

Yes, a thermostat mounted on a cold outside wall reads low, then recovers quickly once the register nearby warms it, and the result looks exactly like a furnace fault.

Furnace Short Cycling?

Get connected with a local pro now.

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