What happens in a boiler service?

Last checked 21/10/2026

Boiler makers say you should have your boiler serviced once a year. A service checks that it burns gas safely and works well. It can also spot small problems before they turn into a breakdown in winter.

Who can service a boiler

In the UK, only a Gas Safe registered engineer may work on a gas boiler. That’s the law. Ask to see their Gas Safe ID card when they arrive. The back of the card lists the types of gas work they’re qualified to do. You can also check them on the Gas Safe Register website.

What the engineer checks

Every maker’s manual has its own service steps, and a good engineer follows them. Most services include these checks.

Before opening the boiler

• A look at the boiler, the flue and the pipes for damage, leaks or signs of overheating.
• A check that the flue is fitted right and nothing blocks it outside.
• A run of the boiler, to see how it lights and how it heats.

The flue gas test

• The engineer puts a probe from a flue gas analyser into a test point on the flue.
• It measures the gases coming out, including carbon monoxide (CO) and carbon dioxide (CO2).
• The results show whether the boiler is burning gas safely and cleanly. The manual gives the limits for each model.

Inside the boiler

• Checks on the burner, the heat exchanger and the electrodes, with cleaning or new parts where needed.
• A clean of the condensate trap. This is where the waste water collects before draining away.
• Checks on the seals, so no fumes can leak into the room.
• Checks on the gas pressure, and on the safety devices that shut the boiler off if something goes wrong.

Afterwards

• The engineer fills in the service record. On most boilers, that’s the Benchmark section at the back of the manual.
• They tell you about anything that needs fixing, and how urgent it is.

Why a yearly service matters

• Safety. A badly burning boiler can make carbon monoxide. You can’t see or smell this gas. A service checks for this. A CO alarm near the boiler adds extra protection.
• Warranty. Most makers only honour the warranty if the boiler has a service every year. Keep the records.
• Fewer breakdowns. Worn parts and blocked traps get found before they stop the boiler.

A service is not a gas safety check

People often mix these up.

• A service is a full check and clean of the boiler, as described above.
• A gas safety check is a safety test of all the gas appliances in a home. Landlords must have one done every year by a Gas Safe engineer, and give a copy of the record to their tenants. It’s often called a CP12.

Many engineers can do both on the same visit. Ask for both if you’re a landlord.

Before the engineer arrives

• Clear the space around the boiler.
• Have the user guide ready, if you have it.
• Note any problems, such as noises, fault codes or low pressure.

Not sure of your model? See how to find your boiler’s model and GC number.