Furnace Short Cycling

Rapid on-and-off firing wastes fuel and puts a season of wear on ignition parts in a matter of weeks, so this one shouldn't ride out the winter.

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When this becomes an emergency

The burner runs a couple of minutes and quits, then fires up again shortly after.
Rooms far from the equipment never catch up on cold days.
The propane or oil delivery is coming sooner than the calendar suggests it should.
The system seems to be running all the time without holding a temperature.

What a pro does

Watching an actual cycle beats any description of one. The pattern of when it quits is what points at the cause, and there's a fork right at the start. If the unit gets hot and then trips its limit, airflow is being choked somewhere, by a filter, a collapsed flex duct, or returns blocked by furniture. If it quits before it gets hot, the flame is being lost or the control is dropping out. On hot water systems the aquastat differential and the size of the zone come into play, since a small zone calling alone will satisfy a large boiler almost immediately. In old houses with a century of additions and re-piping, mismatched zone sizes cause more short cycling than failed parts do.

Frequently asked questions

How does airflow cause the burner to shut off?

Restricted air lets heat build faster than it's carried away, the high limit opens to protect the exchanger, and the burner stops even though the house is still cold.

Why does a small zone make a boiler cycle?

A large boiler firing for one small loop reaches its target temperature almost immediately, shuts down, cools off, then fires right back up. That's short cycling caused by mismatch rather than by any fault.

Does this show up more in mild weather?

It often does. Shoulder-season calls are short, so a system that's oversized for the load spends most of its time starting and stopping instead of running.

What does it cost to leave it alone?

Fuel efficiency drops because the equipment never reaches steady operation, and the ignition components wear at several times their normal rate.

Furnace Short Cycling?

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