Boiler Won't Fire Up After A Power Cut

What's happening?

A gas boiler still needs electricity for the fan, the pump, the controls and the ignition, so when the supply goes off everything stops, and when it comes back the boiler has to restart its sequence from cold. Usually it does that by itself. When it does not, the reason falls into four groups. The power has returned but the boiler is not getting it, because the switch beside it was knocked off or a breaker tripped in the surge. The controls lost their settings and are not calling for heat, because the timer clock reset to midnight or a smart thermostat lost its connection. The boiler tried to start and locked out. Or the return surge damaged something, most often the circuit board or the fan. Working through them in that order finds most causes in minutes.

What usually causes it?

• The fused spur switch beside the boiler, or the RCD in the fuse box, tripped as the power returned
• The programmer or boiler clock reset to midnight, so it thinks it is not time for heating yet
• A wireless or smart thermostat that lost its Wi-Fi or its link to the boiler
• A lockout from a failed ignition sequence during the outage, shown by a red light or fault code
• A surge damaging the circuit board, fan or pump
• A condensate pipe or gas supply problem that has nothing to do with the outage but was hidden until it restarted

Is it dangerous?

Not usually. A boiler that is dead after an outage is a nuisance, not a hazard, unless you smell gas or fumes, in which case leave the property and call the National Gas Emergency Service on 0800 111 999 (0800 002 001 in Northern Ireland). Do not keep resetting a boiler that fires and drops out, and do not reset a trip that goes straight back off.

Can I fix it myself?

Check the fused spur near the boiler is on and the fuse box has no tripped switch. Set the timer to the right time and day, set it to constant for a test and turn the room thermostat well above room temperature. Reconnect the smart thermostat if it dropped off. Reset the boiler once by the manual. If the display stays blank after a trip or the RCD will not stay on, stop.

When should you call a heating engineer?

Call a Gas Safe registered heating engineer if the boiler is still dead after the checks, if it locks out again after one reset, or if you suspect a surge. Call an electrician if the RCD will not reset with the boiler switched off at the spur, because then the fault is the circuit.

What will the heating engineer actually do?

• Check the supply to the boiler and the fuse in the spur and in the boiler
• Read the fault history on the boiler to find the stage it stopped at
• Test the board, fan and pump for surge damage
• Check controls, programmer and thermostat power and settings
• Confirm gas supply and flame sensing on a full start cycle
• Replace the failed component and run a full heating and hot water cycle

Who fixes this

• Boiler repair
• Heating not working

Questions people ask

Sources & References

1. Vaillant: how to reset a boiler
2. Vaillant: how to set a boiler timer
3. Vaillant: most common boiler problems
4. GOV.UK: Approved Document P, electrical safety