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Gas combi boilers: servicing checks and common problems


All being well, your boiler should provide heating and hot water when you need it. But boilers need regular checkups to ensure that they are in good working order and safe to use.

Householder adjusting gas boiler settings

How to be boiler savvy

Get your boiler checked every year

Always use Gas Safe-registered engineers for all gas-related work.

Getting your boiler checked annually by a Gas Safe-registered engineer will keep it running smoothly and prevent problems or costly repair bills in the future.

A straight forward service will cost around £60 to £80 and should include a number of routine checks (see below).

However, some faults, such as pump failure, can happen at any time and are impossible to foresee during a service.

We asked Che Richard Meek from Which? Local Business of the Year award-winning The Hampstead & Highgate Heating & Plumbing Co for his advice on annual central heating checks and common boiler problems.


Annual boiler checks

Note: All details are for a gas combination boiler – the most common type of boiler in the UK.

All prices are estimates and include parts and labour but exclude VAT. Exact cost will depend on the make and model of your boiler, the condition of your central heating system and your location in the UK.

Gas condensing combi boiler diagram showing parts

1. Test air pressure in expansion vessel

This ensures that water has space to expand when it gets hot.

Solution: If pressure is low, pump up the expansion vessel.

2. Emissions test

Test air/gas ratio to check safety.

Solution: In the unlikely event that levels are unsafe, the boiler would be disabled. The engineer might suggest cleaning the gas chamber (£180 to £240) or you might need a new boiler.

3. Remove and clean magnetic filter (if present)

A magnetic filter stops debris getting into your central heating system. Most engineers will recommend fitting one to a new boiler.

Solution: This should be carried out as part of regular service. To supply and fit a new magnetic filter costs around £200.

Common boiler problems

4. Faulty automatic air vent

Dripping or build-up of scale at the vent that releases air from boiler to outside. Should be spotted during a service.

Solution: Replace vent (£120).

5. Limescale build-up in central heating pipes

This is most common in hard-water areas. It can lead to blockages and cause fluctuation in temperature of hot water taps.

Solution: Fit a scale reducer, ideally when boiler is installed (£60 to £80).

6. Faulty or blocked pressure relief valve

This may cause constant dripping from the copper pipe leading to the external wall.

Solution: Replace pressure relief valve (£120 to £200).

7. Pump failure

If this packs up, it would leave you with no heating or hot water. Caused by sludge in the system, or old age.

Solution: Replace pump (£250).

8. Faulty diverter valve

The system may get 'stuck' on hot water or central heating.

Solution: Replace diverter valve (£240).

9. Sludge in system

Radiators are hot at the top with cold spots at the bottom.

Solution: The system may need a powerflush, but this is costly (£330 to £450) and not always the best solution. Don't get one until you have got several opinions to be sure it's needed.


About our expert trader

Boiler engineer Che Richard Meek from The Hampstead & Highgate Heating & Plumbing Co in London

Our expert: Che Richard Meek

Boiler engineer Che Richard Meek is the owner of Which? Local-recommended The Hampstead & Highgate Heating & Plumbing Co. 

The business is also an endorsed  Which? Trusted Trader - passing an assessment process to establish that his business has met the high standards set by Which?

With this high accolade, the business is able to use the Which? Trusted Traders logo similarly to the best buy icon, and stand out from the crowd.