10 common boiler problems (and how to spot them)

Some boiler faults have an obvious cause you can check in a minute. Others only look simple. This is what tends to be behind the calls we get, and where the line sits between something worth trying yourself and something that needs a Gas Safe engineer.

If you smell gas, do not touch any switches. Open the windows, turn the gas off at the meter if you can reach it safely, get everyone out, and ring the National Gas Emergency Service on 0800 111 999.

1. No heat or hot water

The most common call of the lot. It can be the thermostat, low pressure, an airlock, or a failed diaphragm or motorised valve. Check the thermostat is actually calling for heat and that the pressure gauge sits around 1 to 1.5 bar. If both look right, it needs diagnosing.

2. Leaking or dripping

Usually a failed internal seal or the pressure relief valve letting by. Turn the boiler off, catch the water, and call an engineer. Don’t keep running it while it leaks, because water finding the electrics turns a repair into a replacement.

3. Banging, whistling or gurgling

This is kettling. Limescale or sludge has built up on the heat exchanger, or there is air trapped in the system. A flush and a magnetic filter normally clear it. Worth sorting early, because the noise is the heat exchanger being worked harder than it should be.

4. Low pressure

If the gauge reads below 1 bar the boiler will often shut down. You can top it up yourself with the filling loop, following the instructions in the manual. If it keeps dropping, the system is losing water somewhere or the expansion vessel has gone, and topping it up over and over just hides the fault.

5. Frozen condensate pipe

A winter one. The boiler locks out, usually with a fault code, because the condensate pipe running outside has frozen. Pour warm water along it, not boiling, then reset the boiler. Lagging the pipe stops it happening again next cold snap.

6. Radiators not heating properly

Cold at the top and warm at the bottom means trapped air, and bleeding the radiator fixes it. Cold at the bottom and warm at the top is the other way round: that is sludge settling in the radiator, and it needs flushing rather than bleeding.

7. Pilot light keeps going out

Older boilers only. Normally a faulty thermocouple or a draught getting to it. If it will not stay lit, leave it off and get it looked at rather than relighting it repeatedly.

8. Boiler keeps switching itself off

Low pressure, a failing pump or a thermostat fault will all do this. Check the pressure first since that is the easy one. Repeated lockouts are the boiler protecting itself, so they are worth diagnosing rather than resetting each time.

9. Thermostat problems

If the heating runs at the wrong times or never reaches temperature, check the batteries first, then where the thermostat is sitting. One mounted above a radiator or in direct sun reads the room wrong. Older units can be swapped for a smart control.

10. Controls not responding

Before assuming the boiler has failed, check the batteries in the controls, the fused spur beside the boiler and the consumer unit. A reset clears it more often than you would think.

When to stop and call us

Bleeding a radiator, topping up pressure and thawing a condensate pipe are all fine to do yourself. Anything involving the gas supply, the casing coming off, or a leak inside the boiler is Gas Safe work by law. If a fault keeps coming back after you have fixed it, that is the boiler telling you something else is wrong.