Boiler Service Checklist — What Engineers Check in London 2025
A boiler service should take 45-90 minutes and cover 15+ checks. Many homeowners do not know what a proper service includes — meaning they cannot tell whether they received value. This is the complete checklist for a Gas Safe boiler service in London.
What a Proper Boiler Service Covers
An annual gas boiler service carried out by a Gas Safe registered engineer should take between 45 minutes and 90 minutes. Anything taking less than 30 minutes is almost certainly not a full service.
Visual Checks (Before and During Disassembly)
- System pressure check (should be 1-1.5 bar cold)
- Visual inspection for any visible leaks, corrosion, or damage
- Check flue terminal is unobstructed and undamaged (external inspection)
- Confirm correct clearances around boiler and accessibility
Gas Checks
- Gas inlet pressure at the gas valve (should match the manufacturer's specification — typically 20 mbar for natural gas)
- Check for gas soundness at connections (leak test at gas valve and burner connections)
Combustion Analysis
This is the most important check. The engineer inserts a flue gas analyser probe into the flue to measure:
- CO (carbon monoxide) — should be below 200ppm in the flue gas; higher indicates incomplete combustion or heat exchanger failure
- CO₂ — confirms correct gas/air ratio and combustion efficiency
- Efficiency percentage — allows comparison with boiler specification and previous readings
Without combustion analysis, a "service" is largely visual and does not meet Gas Safe Register service requirements. Always confirm the engineer uses a flue gas analyser.
Burner and Heat Exchanger
- Remove burner and visually inspect for damage, scale, or sooting
- Clean burner with brush — remove scale deposits
- Inspect heat exchanger for signs of leakage, cracking, or corrosion
Controls and Safety
- Check main heat exchanger condensate trap is clear (on condensing boilers)
- Test all safety devices operate correctly — overheat thermostat, pressure relief operation
- Check diverter valve operation (combi boilers)
- Check ignition electrode condition — clean and check gap
- Inspect expansion vessel pre-charge and top up if needed
System Checks
- Check inhibitor concentration in system water (should be at manufacturer's specified level)
- Check magnetic filter for sludge accumulation — clean if present
- Confirm programmer and thermostat operate correctly
What You Receive After a Proper Service
- A signed service record in the boiler logbook or digital equivalent
- Combustion analysis readings (ask to see these)
- Any advisory notes about repairs or system issues found
Frequently asked questions
How much does a boiler service cost in London in 2025?
A proper annual boiler service by a Gas Safe registered engineer in London costs £80-150. Boiler cover plans from British Gas, Homeserve, and similar providers include annual servicing. Services below £60 are usually abbreviated visual checks without combustion analysis — not a full service. Services above £180 without a specific reason are overpriced.
How do I know if my boiler service included combustion analysis?
Ask the engineer to show you the flue gas analyser readings before they leave. They should show you CO and CO₂ percentages from the flue test. If the engineer did not use a flue gas analyser, they did not carry out a full service — a visual-only service does not meet Gas Safe Register guidelines.
Does a boiler service include replacing parts?
No — a service includes inspection, cleaning, testing, and adjustments. If the engineer identifies a component that needs replacing (a worn ignition electrode, a failing pump, a degraded sealing gasket), this is additional work quoted and charged separately. Budget £50-200 for common service parts on top of the service fee if the boiler has not been serviced for 2+ years.
Is an annual boiler service a legal requirement in London?
For owner-occupiers: no — it is not legally required, only strongly advised for safety and warranty purposes. For landlords: an annual gas safety check (CP12) is a legal requirement covering all gas appliances including the boiler, but this is a safety check rather than a full service. A service is separate from the Gas Safety Certificate inspection, though both are often done together.