Boiler Service: What Happens, How Often, and Why It Matters

A boiler service is one of those maintenance tasks that’s easy to defer. The boiler is working, nothing is obviously wrong — is a service really necessary? The answer is yes, for reasons that go beyond warranty compliance. An annual service identifies developing problems before they become failures, maintains efficiency, and is legally required for gas appliances under landlord obligations.

How often should a boiler be serviced?

Annual servicing is the standard recommendation for all domestic gas boilers, regardless of age or condition. This is:

Required by most boiler manufacturer warranties (failure to service annually voids many warranties)
Required by law for landlord-owned properties (Gas Safety (Installation and Use) Regulations 1998)
Recommended by Gas Safe and British Gas

For oil-fired boilers, annual servicing is also standard. For LPG boilers, the same requirements as gas apply.

New boilers: Some manufacturers require the first service within 12 months of installation. Others allow 24 months for the first service on a new installation. Check your warranty documentation.

What happens during a gas boiler service

A full boiler service by a Gas Safe registered engineer typically takes 45–75 minutes and covers:

Visual inspection:

Condition of the boiler casing and any obvious external damage
Flue terminal (where exhaust gases leave the building) — clear, undamaged, correct termination distance from windows and doors
Gas supply pipe and connections — no corrosion, no signs of leakage
Any unusual soiling around the boiler (carbon staining indicates combustion problems)

Internal inspection: The boiler casing is removed. The engineer inspects:

Burner condition — clean, undamaged
Heat exchanger — free from scale, corrosion or cracking
Ignition leads and electrodes — correctly positioned, no damage
Combustion chamber — clean, no debris
Flue connection within the appliance — sealed, undamaged
Pump (if internal) — operating, no noise
Expansion vessel — checked for correct pre-charge pressure (deflated vessels are a common finding)
Diverter valve (combi boilers) — operating correctly
Gas valve — correct operation

Testing and adjustment:

Gas rate check — the engineer connects a flow meter to confirm the boiler is consuming gas at the correct rate for its output
Combustion analysis — a flue gas analyser is inserted into the flue to measure carbon monoxide (CO) levels, oxygen levels, and combustion efficiency. Abnormal CO readings indicate incomplete combustion and require investigation and adjustment
Boiler operation — the boiler is run through heating and hot water modes and observed
Safety controls — lockout and safety device testing
Flue integrity — for room-sealed boilers, a spillage test confirms no combustion products enter the room

Documentation: The engineer completes a gas boiler service record. This should include the date, engineer’s Gas Safe registration number, findings, combustion readings, and any recommendations.

What findings indicate a problem

High CO in flue gases: Combustion analysis reading above manufacturer’s specified limit indicates incomplete combustion. This can be caused by a dirty burner, partially blocked heat exchanger, incorrect gas pressure, or a fault with the gas valve. The engineer adjusts where possible and may replace the burner or escalate to a manufacturer-qualified engineer.

Low/zero expansion vessel pressure: The expansion vessel diaphragm has failed. The vessel needs recharging (it has a Schrader valve for this purpose, like a bike tyre) or replacing if the diaphragm has collapsed. A failed expansion vessel causes repeated pressure drops and eventually trips the pressure relief valve.

Cracked or corroded heat exchanger: A serious finding. Heat exchanger replacement is expensive (often approaching the cost of a new boiler for older models). An engineer finding a cracked heat exchanger will typically advise whether repair or replacement is more economical.

Worn or stuck diverter valve (combi boilers): Combi boilers use a diverter valve to switch between heating and hot water. A valve that’s stiff or not travelling fully can cause hot water to be warm but not hot, or heating to run when the DHW is calling. An experienced engineer can often free a stiff valve; a fully failed valve needs replacement (£80–£200 parts plus labour).

Carbon monoxide detector not fitted: Not a boiler fault, but a responsible engineer will note the absence of a CO detector and recommend installation. CO detectors should be fitted in any room with a gas appliance and should be replaced every 7–10 years as the sensor degrades.

Finding a Gas Safe registered engineer

All engineers working on gas appliances must be registered with Gas Safe Register. Verify any engineer’s registration at gassaferegister.co.uk before commissioning work. The registration card shows which types of work the engineer is registered to carry out — ensure gas boiler servicing is included.

Unregistered gas work is illegal, invalidates insurance, and can void the boiler warranty. Always verify, even if the contractor claims registration.

Servicing costs

Annual boiler service costs typically:

Standard gas combi boiler: £60–£120
System or regular boiler: £70–£130
Older open-flued boiler (increasingly rare): £80–£150

Boiler service contracts from energy suppliers and plumbing networks (British Gas, Homeserve, local plumber annual contracts) typically cost £150–£250/year but include emergency call-out cover, which may provide value depending on your boiler’s age and reliability.

A boiler that hasn’t been serviced for several years may cost slightly more for the first service as the engineer finds more to clean and check. Bringing it into a regular annual programme reduces this premium.