Boiler Keeps Turning Off

What's happening?

A boiler is built to switch itself off when something is outside its safe range, and that is the usual reason it keeps turning off. It stops when the water inside is getting too hot because not enough is flowing through it, when the pressure is too low, when the flame is lost, or when its exhaust cannot clear. Which one it is shows in the timing. A cut-out after a few seconds usually means the flame is not being proved, or the condensate pipe is blocked. Switching off after a few minutes points to overheating from poor flow. Switching on and off every minute or two is short cycling: the water reaches temperature faster than the radiators can lose heat. A fault code or red light is the boiler telling you which safety stopped it.

What usually causes it?

• System pressure that has dropped below about 1 bar
• Poor flow from sludge, a failing pump or radiator valves all closed, so the heat exchanger overheats
• A blocked or frozen condensate pipe
• Short cycling, because the flow temperature is set too high or the heating demand is small
• A faulty flow or return temperature sensor, or an overheat thermostat that has failed
• A weak ignition electrode or a flame sensor covered in deposits
• A tripped room thermostat or one with flat batteries ending each call for heat

Is it dangerous?

No, in itself. The shutdown is the safety working. What matters is not overriding it: pressing reset again and again on a boiler that keeps locking out hides the cause, and a blocked flue or a failing combustion can sit behind a flame that keeps dropping out. Switch the boiler off and call if you smell fumes or the CO alarm sounds.

Can I fix it myself?

Check the pressure gauge and top up to about 1.2 bar if it is low. Check that radiator valves are open and that you have not shut them all. Change the batteries in the thermostat. In freezing weather, look at whether the condensate pipe outside is frozen. Reset once using the button in the boiler manual and see whether it settles. If it cuts out again, leave it off.

When should you call a heating engineer?

Call a Gas Safe registered heating engineer if it cuts out more than once after a reset, if it shows a fault code each time, if the pressure was fine, or if it has been cycling for days. Do not open the casing: anything inside is gas work.

What will the heating engineer actually do?

• Read the fault history and fault code to see which safety is stopping the boiler
• Measure flow and return temperatures to show whether circulation is the problem
• Check the pump, the system for sludge and the radiator and bypass circuit
• Clear and test the condensate trap and pipe
• Test the flame signal, electrode and temperature sensors, and replace the failed part
• Rerun the boiler through a full heat cycle to confirm it holds

Who fixes this

• Boiler repair
• Heating not working

Questions people ask

Sources & References

1. Vaillant: most common boiler problems
2. Vaillant: how to reset a boiler
3. Vaillant: what is a heat exchanger?
4. Vaillant: getting ready for winter, your condensate pipe