Boiler Keeps Firing Up When The Heating Is Off

What's happening?

A boiler running with the heating and hot water both off is rarely random. It is answering a call you cannot see. Frost protection is the most common: if the temperature near the boiler or in the pipes falls to a few degrees above freezing, it fires briefly on purpose. Combis have a pre-heat or keep-hot mode that fires a short burst to keep the plate warm. A smart thermostat or app may be calling for heat on a schedule you forgot. A slow drip from a hot tap or a leaking hot pipe makes a combi think hot water is being used, so it fires. And a boiler pump that keeps running for a short time after the flame goes out, the pump overrun, is a quiet pump rather than the burner. Working out which is the question.

What usually causes it?

• Frost protection in a cold house, loft, garage or on an outside wall
• A pre-heat, keep-hot or comfort mode on a combi set to fire regularly
• A smart thermostat, timer or app schedule that is still calling for heat
• A dripping hot tap, a leaking hot pipe or a dishwasher valve passing water
• A diverter valve that does not seat, so the boiler sees flow
• A stuck room thermostat contact or a faulty programmer output

Is it dangerous?

No. It wastes gas rather than harming anyone. A boiler that fires with the heating off and that also smells, makes odd noises or locks out is a different matter and should be checked.

Can I fix it myself?

Look at the boiler display for a heating, hot water or frost symbol when it fires. Check the app and thermostat schedule. Turn off pre-heat or comfort mode in the boiler menu if it has one. Check every hot tap for a drip. Warm radiators with the heating off are a separate fault, covered on the radiators hot when the heating is off page. Do not open the boiler.

When should you call a heating engineer?

Call a heating engineer if the boiler fires every few minutes with no heating or hot water called, if the radiators warm up with it, or if you have ruled out frost mode, pre-heat, schedules and drips. A diverter valve or control fault will need the casing off, which is Gas Safe work.

What will the heating engineer actually do?

• Watch the boiler fire and read which call, demand or frost routine started it
• Check programmer, thermostat, app and wiring for a demand that should not be there
• Test the diverter valve and flow sensor for a false hot water call
• Look for a leaking or dripping hot water outlet drawing flow
• Adjust pre-heat, frost and pump overrun settings to suit the household

Who fixes this

• Boiler repair
• Heating not working

Questions people ask

Sources & References

1. Vaillant: what is a combi boiler?
2. Vaillant: how to fix a boiler diverter valve
3. Vaillant: how to set a boiler timer
4. Vaillant: most common boiler problems