Boiler service checklist: what a Gas Safe engineer actually checks (and what it should cost)

A boiler service is the single most important thing you can do to keep your heating safe, efficient and reliable. But there's a big gap between a proper Gas Safe boiler service and the £40 "visual check" you sometimes see advertised. This guide walks through exactly what a full service should include, how long it takes and what it should cost across Nottingham and the surrounding areas in 2026.

Why an annual service matters

Manufacturers require an annual service to keep the warranty valid — miss one and you can lose cover on parts that would otherwise be replaced for free. More importantly, servicing catches problems early: a blocked condensate, sooted-up burner or degraded seal will get worse over winter and can lead to a mid-January breakdown or, at worst, a carbon monoxide risk. A well-serviced boiler also runs more efficiently, which shows up on your gas bill.

The full boiler service checklist

Here's what should happen when a Gas Safe registered engineer services your boiler. If any of these steps are skipped, you're not getting a full service.

• Visual inspection of the boiler, flue and pipework for damage, leaks or corrosion.
• Removal of the boiler casing to inspect internal components (heat exchanger, burner, injectors, fans, seals).
• Cleaning of key components where required — burner, condensate trap and combustion chamber.
• Gas rate test to confirm the boiler is burning the correct volume of gas.
• Flue gas analysis (FGA) with a calibrated analyser — measuring CO, CO₂ and the CO/CO₂ ratio to prove safe combustion.
• Operating pressure and flow checks, plus expansion vessel charge if accessible.
• Safety device tests — flame supervision, overheat cut-out and pressure relief.
• Flue integrity check and inspection of ventilation to the appliance.
• Reassembly, controls test and a written service report with readings.

Flue gas analysis — the difference-maker

Flue gas analysis is what separates a real service from a quick look. Using a calibrated analyser, the engineer measures the exact make-up of the gases leaving your boiler. The readings show whether combustion is clean, whether the boiler is burning the right amount of gas, and whether any carbon monoxide is being produced. Manufacturer benchmarking books require FGA readings on every service; if your engineer doesn't do one, ask why.

How long a service takes

A proper service on a modern combi boiler takes around 45–60 minutes. A system or regular boiler with a hot water cylinder can take a little longer. Anything under 20 minutes almost certainly means the casing hasn't been removed and no FGA has been done.

What it should cost in 2026

Across Nottingham, honest fixed pricing for a full boiler service typically sits between £70 and £110. Landlord CP12 gas safety certificates that include a boiler service tend to be a little higher because there's more paperwork and additional appliance checks. Beware of any price well below this range — the maths only works if corners are being cut.

Red flags to watch for

• £40–£50 “services” that only include a visual look at the boiler and no FGA.
• No written report or flue gas readings handed over at the end of the visit.
• The engineer refuses to show a Gas Safe ID card, or the number doesn't match the Gas Safe Register.
• The casing is never removed — a proper service requires internal inspection.

When to book

Late summer and early autumn are the ideal windows — the boiler is quiet, engineers have better availability, and any faults uncovered can be fixed before you rely on the heating every day. If you can't remember when yours was last done, it's due.