Furnace Short Cycling

Short cycling wastes fuel and hammers the ignition components with starts they were never meant to take, so it's worth a call before something quits for good.

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When this becomes an emergency

The burner runs three or four minutes and quits, over and over, all evening.
The house never reaches the setpoint even though the furnace seems busy constantly.
Propane use has jumped without the weather explaining it.
The unit fires again within a minute or two of shutting down.

What a pro does

Short cycling is a symptom with a wide differential, so the useful thing is to watch several cycles back to back and time them. A limit switch tripping on high heat is the classic finding, and behind it sits restricted airflow: a filter nobody's changed since the leaves came down, a closed-off return in a room shut for the season, or a blower running slower than it should. Bad flame signal produces a similar pattern with a different rhythm. So does a thermostat mounted where a woodstove warms it early while the far bedrooms stay cold, which is common in houses running two heat sources at once. Sizing is the other one. A furnace picked for the coldest week of the year will satisfy a mild day in a handful of minutes, That isn't a fault, though it does explain the pattern.

Frequently asked questions

Could a woodstove really make the furnace cycle badly?

If the thermostat is anywhere near it, yes. The stove satisfies the sensor while the rest of the house stays cold, so the furnace fires briefly and shuts down again and again.

How many minutes should a normal heating cycle run?

On a cold day, a healthy furnace usually runs ten to fifteen minutes or longer per cycle. Anything under about five, repeating, is worth investigating.

Is it expensive on fuel?

Startup is the least efficient part of any cycle, so a system that mostly starts and stops burns noticeably more propane than one that settles into long steady runs.

Can a plugged filter alone cause this?

It's one of the most common single causes. Starved return air makes the exchanger overheat, the limit opens, and the burner shuts down well before the thermostat is satisfied.

Furnace Short Cycling?

Get connected with a local pro now.

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