Boiler isn’t working: 10 common boiler problems

If your boiler isn’t working, it’s most likely due to low boiler pressure, leaks, a blocked condensate pipe, faulty valves and pumps, and internal limescale & debris. Here is a list of the most common boiler issues, what causes them, and what can be done to fix them.

10 Common boiler problems – Contents

Low boiler pressure

Combi and system boilers require a pressure of 1 – 1.5 bar when cold in order to work. If the pressure is lower than this, the boiler won’t work and you won’t have any hot water or central heating.

Low boiler pressure is one of the most common boiler problems and it can have many possible causes. For example, bleeding the radiators may depressurise the system enough to trigger a low pressure fault. Topping the system back up via the filling loop should resolve the problem.

Main article: Boiler filling loop: the 2 different kinds and how to use them

Boiler leaks

A boiler which keeps losing its pressure may indicate a leak in the system, either from radiators or pipework or from one of the boiler’s internal components, such as the expansion vessel or pressure release valve. Faulty components can easily be replaced by a qualified engineer.

Main article: Boiler keeps losing pressure: 7 possible reasons why

Blocked condensate pipe – one of the most common boiler problems in the winter

Instead of expelling hot combustion gases straight out into the atmosphere, condensing boilers draw so much heat out of them that the water vapour within them condenses back into a liquid. This slightly acidic water is discharged into the sewer via a plastic pipe known as the condensate pipe. A blockage in the pipe will typically trigger a fault code and stop the boiler from working.

A frozen condensate pipe is one of the most common boiler problems and reasons for a boiler breakdown during a cold winter snap. Warm water and/or a hot water bottle applied to the suspected location of the blockage should quickly resolve this issue. If it doesn’t, a Gas Safe registered engineer should unblock it.

Main article: Frozen condensate pipe: how to fix

Faulty diverter valve

Combi and sealed system boilers are equipped with a diverter valve. This valve controls whether the boiler heats water for the radiators or for the hot taps. If your central heating is working normally but you don’t get any hot water, the diverter valve may be faulty or jammed. The same is likely if you open a hot tap and the radiators get hot even though the central heating is off.

The diverter valve is not to be confused with the motorised valve. In installations with a hot water cylinder and a heat-only boiler, this typically refers to a valve in the airing cupboard which controls whether hot water from the boiler heats the radiators or water inside the cylinder.

Boiler isn’t working / making strange noises

You may hear your boiler make strange noises, such as banging or whistling sounds. These are collectively referred to as “kettling” because the noises may sound like a boiling kettle. (In some ways, the word “boiler” is a bit of a misnomer, as the boiler doesn’t actually boil the water.)

Blocked/frozen condensate pipes can cause this sound due to the water gathering in the condensate sump. Another common reason is the buildup of limescale and corrosion inside the heat exchanger. This can be removed by a powerflush, where anti-corrosion and de-liming chemicals are pumped through the system. Faulty thermistors – electric components used to regulate the boiler’s heat output – may also cause the boiler to overheat, as well as faulty or jammed pumps.

Main article: Boiler kettling

Boiler isn’t working – pilot light keeps going out

Older gas boilers feature a pilot light. This is a small flame kept alight permanently in order to ignite the main burner when needed. A misdirected gas flow, blockages in the gas supply pipe to the pilot light, or frequent draughts may interfere with this. Another possible cause is a faulty thermocouple, an electrical sensor which closes the gas supply to the pilot light if it detects that it’s gone out. Pilot light faults can be extremely dangerous – if you smell gas, do not use any electrical appliances, open doors and windows, and call the emergency gas helpline on 0800 111 999. 

Hot water not hot enough

In combi boilers, this problem can be caused by a faulty diverter valve – energy which should be heating water for the taps is instead heating water for the radiators, even though the central heating may not necessarily be on. The end result is that the hot water isn’t hot enough.

This is also a common problem with older combi boilers, which lose their effectiveness over the years. Not opening the hot taps fully in order to get hotter water is a classic example of this. Due to the lack of availability of parts, replacing the boiler may be the only viable option.

You may also notice this problem in the winter. This is because mains water is significantly colder, and requires more heat to be brought up to temperature. Check to see if the boiler is on an economy setting – removing this may help during the winter months.

See also: Hot water not hot enough: 3 possible causes & how they can be fixed

Faulty pump

If you have a combi or system boiler, a faulty pump will stop hot water from being distributed through the radiators, even if the boiler appears to be working fine. In such cases, it’s common for the boiler to switch itself off shortly after coming on. A sensor detects that the heat generated by the boiler isn’t being distributed around the system, and shuts it off to prevent it from overheating.

Thermostat issues

A faulty or improperly configured thermostat may cause the boiler not to come on when needed. Check that the thermostat is on and that the correct programme is selected. You should also compare the current room temperature with the thermostat’s target temperature. It could be that the room is warmer than it feels, and the temperature isn’t low enough for the thermostat to call for heat. Increase the target temperature above room temperature to check if the boiler will come on.


Airlocks may cause your boiler to shut down. Bleeding the radiators is an effective way to release any air inside the system.

Remember: only qualified engineers listed on the Gas Safe register may carry out work on gas appliances. If you believe a gas appliance is unsafe, it is illegal to use it.