12 Common Boiler Problems & Fixes (Troubleshooting Guide)

Boilers are reliable — until they're not. Here are the 12 most common boiler problems we see at Sanju's Heating, what they mean, and what you can do about them.

1. Low boiler pressure

Symptom: The pressure gauge reads below 1.0 bar, and the boiler won't fire.

Cause: Water has been lost from the system, usually through a small leak or after bleeding radiators.

Fix: Repressurise the system using the filling loop to between 1.0 and 1.5 bar. If the pressure keeps dropping, you have a leak — call a Gas Safe engineer.

This shows up as different fault codes depending on your boiler brand: Vaillant F.22, Glow-worm F22, Ideal F1, Baxi E118.

2. No heating but hot water works

Symptom: You have hot water but the radiators are cold.

Cause: A faulty diverter valve, a stuck zone valve, air in the system, or a pump issue.

Fix: Bleed your radiators first. If that doesn't work, the diverter valve or pump may need replacing — call a Gas Safe engineer.

3. No hot water but heating works

Symptom: Your radiators are hot but you have no hot water from the taps.

Cause: A faulty diverter valve (on combi boilers), a blocked plate heat exchanger, or crossed pipework.

Fix: This needs a Gas Safe engineer — the diverter valve or plate heat exchanger may need cleaning or replacing.

4. Boiler kettling

Symptom: Your boiler makes a rumbling, banging, or popping noise — like a kettle boiling.

Cause: Limescale build-up on the heat exchanger causing localised boiling of water.

Fix: The heat exchanger needs descaling or the system needs a power flush. Call a Gas Safe engineer — this won't fix itself and will damage the boiler if left.

5. Boiler won't ignite

Symptom: The boiler tries to fire but fails, and displays an ignition fault code.

Cause: Gas supply issue, low gas pressure, faulty ignition system, blocked condensate, or frozen condensate pipe.

Fix: Check your gas supply and other appliances. In cold weather, check for a frozen condensate pipe. If the fault persists, call a Gas Safe engineer.

Common ignition fault codes: Vaillant F.28, Baxi E133, Worcester Bosch EA, Ideal L2.

6. Radiators cold at the top

Symptom: The top of the radiator is cold while the bottom is warm.

Cause: Air trapped in the radiator.

Fix: Bleed the radiator using a radiator key. This is a simple DIY job.

7. Radiators cold at the bottom

Symptom: The bottom of the radiator is cold while the top is warm.

Cause: Sludge or rust build-up in the bottom of the radiator.

Fix: The radiator needs removing and flushing, or the system needs a power flush. Call a heating engineer.

8. Boiler leaking water

Symptom: Water is dripping or pooling from the boiler.

Cause: A faulty internal component — often the heat exchanger, a pump seal, or a pressure relief valve.

Fix: Turn off the boiler and call a Gas Safe engineer immediately. A leaking boiler can cause serious damage and indicates an internal fault.

9. Boiler losing pressure

Symptom: You repressurise the boiler, but the pressure drops again over hours or days.

Cause: A leak somewhere in the system — could be a radiator valve, pipe, the heat exchanger, or the pressure relief valve.

Fix: You need a Gas Safe engineer to find and repair the leak. Don't keep repressurising without fixing the underlying issue.

10. Boiler making strange noises

Symptom: Gurgling, clicking, humming, or banging noises from the boiler.

Cause: Air in the system, low water pressure, a faulty pump, kettling, or a failing fan.

Fix: Check the pressure and bleed radiators. If the noise persists, call a Gas Safe engineer — some noises indicate serious problems.

11. Pilot light keeps going out

Symptom: The pilot light won't stay lit.

Cause: A faulty thermocouple, a draught blowing the pilot out, or a blocked pilot jet.

Fix: This is more common on older boilers. Call a Gas Safe engineer — the thermocouple or pilot assembly may need replacing.

12. Boiler displaying a fault code

Symptom: The boiler shows an error code and won't operate.

Cause: A specific component or system fault — the code tells you which.

Fix: Look up the fault code for your boiler brand. We have guides for Vaillant, Worcester Bosch, Ideal, Baxi, and Glow-worm. Some codes you can fix yourself (like low pressure), others need a Gas Safe engineer.

Need help with your boiler?

Sanju's Heating covers Newbury, Basingstoke and Reading. We're Gas Safe registered and can diagnose and repair all common boiler faults with transparent pricing.

Call us on 01635 74 50 40 or book online at sanjusheating.co.uk/book