Boiler Won't Turn Off: Causes & Fixes

If your boiler keeps running even when the thermostat is turned down and the hot water isn't being used, there's a fault that needs attention. Here's what could be causing it.

Common causes of a boiler that won't turn off

1. Faulty thermostat

If your thermostat is faulty, it may keep sending a demand for heat to the boiler even when the set temperature has been reached. This is one of the most common causes.

What to check: Turn the thermostat down to its lowest setting. If the boiler keeps running, the thermostat may need replacing. If you have a smart thermostat (like Hive or Nest), try restarting it.

2. Stuck diverter valve

On combi boilers, the diverter valve switches between heating and hot water. If the valve is stuck in the heating position, the boiler may keep running for central heating even when there's no demand.

What to check: If you have heating but no hot water (or vice versa), the diverter valve may be stuck. This needs a Gas Safe engineer.

3. Faulty programmer or timer

If your programmer or timer is faulty, it may keep sending a demand signal to the boiler. Check that the programmer is set correctly and isn't stuck in a continuous 'on' state.

What to check: Turn the programmer to 'off' or set all periods to off. If the boiler keeps running, the programmer may need replacing.

4. Stuck zone valve

If you have a system or regular boiler with zone valves, a stuck zone valve (in the open position) will keep water flowing through that circuit and the boiler will keep running to heat it.

What to check: Feel the pipes on either side of the zone valve — if both are hot when the heating should be off, the valve is stuck open.

5. Faulty PCB

In rare cases, the printed circuit board (PCB) may be faulty and sending incorrect signals to keep the boiler running. This needs professional diagnosis.

6. Pre-heat function

Some combi boilers have a pre-heat function that keeps a small amount of water hot for instant hot water. If the pre-heat is on, the boiler will fire periodically even when no taps are running. This is normal, not a fault — but you can switch it off if you prefer.

What you can do immediately

1. Turn the thermostat to its lowest setting — See if the boiler stops.

2. Turn the programmer/timer to 'off' — See if the boiler stops.

3. Switch the boiler off at the spur — If the boiler won't stop, turn it off at the fused spur switch to stop it running.

4. Call a Gas Safe engineer — If the boiler keeps running despite the controls being off, there's a fault that needs professional diagnosis.

Need help with a boiler that won't turn off?

Sanju's Heating covers Newbury, Basingstoke and Reading. Call us on 01635 74 50 40 or book online at sanjusheating.co.uk/book