What does a boiler service include? (Gas Safe engineer checklist)

An annual boiler service is a routine health check, not a repair visit. A Gas Safe registered engineer works through a set of checks — visual, mechanical, and combustion — to confirm the boiler is operating safely and efficiently, and that its safety devices will activate correctly if something goes wrong.

Here's exactly what the engineer is checking and why each part matters.

Who can service a boiler?

Only a Gas Safe registered engineer. Gas Safe Register is the official UK register of engineers legally permitted to work on gas appliances. Ask to see the engineer's Gas Safe ID card before they start — it shows the type of work they're registered to carry out. You can also check any engineer at gassaferegister.co.uk.

What the engineer checks

Visual inspection

The engineer starts with a physical check of the boiler's casing, controls, and visible pipework — looking for signs of corrosion, water staining, or damage that might indicate an underlying problem.

Gas pressure and flow rate

The engineer checks that gas is arriving at the correct pressure and that the boiler is consuming the correct amount. Pressure outside the manufacturer's specified range affects efficiency and can indicate a supply problem.

Flue and ventilation

The flue carries combustion gases — including carbon monoxide — out of the building. The engineer checks the entire flue from the boiler to the terminal outside: routing, condition, seals, and that the terminal isn't obstructed. A faulty flue is a carbon monoxide risk and will fail the service.

Combustion analysis

Using a flue gas analyser, the engineer measures CO (carbon monoxide) and CO₂ levels in the combustion gases. This tells them whether the boiler is burning gas efficiently and whether any adjustment is needed. High CO readings indicate a fault that must be investigated.

Heat exchanger and burner

The heat exchanger transfers heat from combustion to the water. The engineer inspects it for cracks or scaling and often cleans the burner. Deposits on either component reduce efficiency and can indicate developing faults.

Condensate trap (condensing boilers)

Most UK boilers installed since 2005 are condensing boilers, which produce an acidic condensate that drains away through a plastic pipe. The engineer checks the trap is clear — a blockage is one of the most common causes of boiler lockout, especially in winter when external condensate pipes can freeze.

Safety devices

The engineer tests all the safety cut-out devices, including:

• the pressure relief valve — releases if pressure builds to a dangerous level
• the overheat thermostat — cuts the boiler if it overheats
• the gas valve — confirms it closes correctly when called upon

These are tested, not assumed working.

Seals and gaskets

All accessible seals and gaskets are checked for deterioration that could lead to a gas or water leak.

Controls and programmer

Thermostat, timer, and any smart controls are tested to confirm they're responding correctly. The engineer may also check that any boiler interlock with the heating controls is working as it should.

Gas tightness

A gas tightness test confirms there are no leaks on accessible pipework connected to the boiler.

The full checklist

Area checked	What the engineer is looking for
Casing and controls	Corrosion, damage, water staining
Gas pressure and flow rate	Within manufacturer's specification
Flue and terminal	Obstruction, leakage, correct routing
Combustion analysis	CO/CO₂ within safe limits; combustion efficiency
Heat exchanger and burner	Deposits, cracks; cleaned if needed
Condensate trap	Clear, no blockage or incorrect operation
Safety devices	Operational; will cut out under fault conditions
Seals and gaskets	No wear, deterioration, or leaks
Controls and programmer	All responding correctly
Gas tightness	No leaks on accessible pipework

What you receive at the end

At the end of the service the engineer should provide a service record documenting what was checked, any adjustments made, and any faults identified. This document is important to keep — most boiler manufacturers require an annual service record as a condition of the warranty. See how long is a boiler warranty in the UK? for the detail on why that matters.

If the engineer identifies a fault that requires repair, they'll advise you separately on parts and costs — that's outside the service fee.

How JustTaggit helps

Store each year's service certificate against the boiler's QR code so it's retrievable in seconds — whether for a warranty claim, a house sale, or to show a letting agent. JustTaggit also reminds you when the next annual service is due, which matters if your boiler warranty runs ten years and requires continuous servicing throughout. For the full maintenance picture, see looking after your home appliances.

---

Keep every service certificate one scan away. Tag your boiler free →