Diagnosing boiler issues

Likely reasons and how to fix them

Boilers are often taken for granted until they go wrong. When this happens, it’s important to have the problem fixed as soon as possible.

Common boiler issues can be easy to diagnose, but you should always call out a Gas Safe Registered Engineer to confirm the cause of the problem and make any necessary repairs.

1 – Hissing sound

Likely reason: “Kettling” – Hardness salts have precipitated on the surface of the heat exchanger causing a thin layer of scale or there has been an influx of other deposit into the boiler.
Fix: Addition of a noise-reducing chemical. In severe cases, using an appropriate cleaner (preferably whilst powerflushing) will remove scale and sludge from throughout the system. This will ensure that there is no remaining debris and will help to maintain your boiler in the long term.

2 – Unexpectedly turning on and off

Likely reason: Unreliable thermostat – Thermostats can lose efficiency and develop faults with age.
Fix: Check that the date and time are correct on the thermostat: daylight savings or a power cut could affect the settings. If these settings are correct, call out an engineer who may be able to clean and recalibrate the thermostat. If this is not possible, it may need to be replaced.

3 – Pilot light goes out

Likely reason: Faulty thermocouple, a draught or a build up of deposits – A faulty thermocouple is restricting the gas supply to the pilot light, a back-draft is blowing down the flue or the gas valve is blocked or faulty.
Fix: Try to reignite the pilot light yourself using the instructions in the user manual. If this does not work you should call out a Gas Safe Registered Engineer to identify the issue and make the necessary repairs.

4 – Boiler leak

Likely reason: Faulty part or corrosion – A broken part or corrosion is allowing water to escape.
Fix: Leaks should always be fixed by an engineer but you may be able to identify the source of the leak yourself. It may be that a faulty part needs replacing, but corrosion throughout the system could require a system clean or even component replacement.

5 – Losing pressure

Likely reason: Pressure loss – There is a leak in the system or the pressure relief valve is faulty.
Fix: Call out an engineer to attend to the cause of the pressure loss. Re-pressurise the system using the instructions which can be found in the user manual. This is a fairly simple process but if you have any doubts, call out an engineer.

6 – Boiler break down in winter

Likely reason: Frozen condensate pipe – If this terminates outside the house, the pipe can freeze which causes a blockage.
Fix: If the condensate pipe is at ground level, and easy to access, you may be able to thaw this yourself using heat wraps or a hot water bottle. If in any doubt, call out an engineer.

7 – Whistling noises

Likely reason: Trapped air – Air has been drawn into the system through a defective valve or pressure leak.
Fix: Call out an engineer, who will be able to investigate how the air entered the system and make any necessary repairs to ensure that the same issue does not reoccur.

8 – Radiators are not heating up

Likely reason: Radiator sludge – Corroded radiators and pipes cause a build-up of sludge which causes cold spots.
Fix: To remove sludge, the system should be cleaned and preferably powerflushed with an appropriate cleaner. This will allow water to correctly flow through the whole radiator again.

Are You experiencing any of these symptoms?