Boiler Short Cycling On and Off
The burner lights, runs for a minute or two, drops out and relights, over and over..
The burner lights, runs for a minute or two, drops out and relights, over and over.
What is actually going on
The burner lights, runs for a minute or two, drops out and relights, over and over.
The realistic causes, in order
An oversized boiler on a small load — the system can't absorb the minimum output, especially in mild weather Restricted circulation from sludge, a blocked filter or a stuck thermostatic valve Controls fighting each other, such as a boiler thermostat set low against a room stat set high
What we test on site
Burner run time and cycle count over a real heating period, not a two-minute test Flow and return differential to see how quickly heat is being carried away Control strategy, including whether weather compensation or load compensation is available and unused
Get a boiler that short cycles diagnosed properly
Correcting flow temperature, control strategy and circulation usually settles it without replacing anything.
Questions we get asked
What causes a boiler that short cycles on a gas boiler?
An oversized boiler on a small load — the system can't absorb the minimum output, especially in mild weather
What normally puts a boiler that short cycles right?
Correcting flow temperature, control strategy and circulation usually settles it without replacing anything.
Is it safe to keep using the system with a boiler that short cycles?
Short cycling wastes gas and wears ignition components. It is a system and controls problem more often than a broken part.