Before the boiler is opened
The engineer asks how the system has been running, listens for anything you've noticed (banging pipes, a rad that won't heat, short-cycling), takes a baseline reading of pressure and any error history. Five minutes; sets up the rest of the visit.
The eight checks
- Flue gas analysis. A calibrated flue gas analyser samples the combustion exhaust and reports the carbon monoxide ratio, oxygen, and combustion efficiency. Any reading outside the manufacturer band is a service-stopping issue. This is the single most important test of the visit; if you don't see an analyser come out, that's a thin service.
- Combustion and safety check. The flame is visually inspected. Yellow or lazy flame is a sign of incomplete combustion; should be a steady blue cone. Safety controls (overheat thermostat, flame failure device, pressure switch) are functionally tested.
- Pressure and expansion vessel check. Sealed-system pressure should sit at 1.0-1.5 bar cold. The expansion vessel (the metal can usually behind the boiler) holds an air charge that absorbs the pressure rise as the system heats up. If the charge has leaked, the vessel needs re-pressurising or, less often, replacing.
- Heat exchanger inspection and clean. The heat exchanger transfers energy from the burner to the water. Carbon, scale and lime build up on it over time, lowering efficiency. Inspection includes a visual check and, where access allows, a brush or vacuum clean.
- Condensate trap clean (gas). The condensate trap collects acidic moisture from the flue gases. It blocks easily, especially on boilers used heavily, and a blocked trap is the single most common cause of mid-winter lockouts. Cleaned out with the trap removed.
- Nozzle and photocell (oil only). The oil burner atomises fuel through a nozzle that wears out by design. Nozzle change is part of every standard oil service; the photocell that confirms the flame is lit gets a clean and a sensitivity check.
- Seals, casings, joints. Every gasket and joint on the boiler is inspected for leakage. Any seep is identified and either fixed on the day if it's a standard part, or quoted for return.
- Pump pressure and current draw. The pump that circulates water is tested for pressure delivery and amperage. A pump approaching failure draws higher current and runs hot; catching this early is a £180 part swap rather than a £600 emergency.
After the checks
The boiler is reassembled. The engineer issues paperwork:
- A Gas Safe service record (gas) or OFTEC inspection sheet (oil), signed and dated.
- A landlord gas safety certificate (CP12) on the day, if you're a landlord and asked for it.
- A written report that flags anything marginal, with a recommendation for whether it needs action now, action this year, or just monitoring.
The engineer talks you through anything that came up. If repairs are needed, they're priced before any work starts; you decide whether to do it now, defer, or get a second opinion.
How long it takes
- Standard gas boiler: 45-60 minutes.
- Standard oil boiler: 60-90 minutes.
- Service plus landlord certificate: add 10-15 minutes.
- Service plus minor repair on the day: usually under 2 hours total.
What it isn't
A service is not the same as a repair. If the engineer finds a fault that needs a replacement part, that's quoted separately. The service fee covers the inspection, the standard consumables (oil nozzle, condensate trap clean), and the paperwork.
A service is also not a power flush. If your radiators are running cold at the bottom or you've got rusty water coming out when you bleed them, the engineer flags it but flushing is a separate service.
How to spot a thin service
- Done in under 15 minutes.
- No flue gas analyser used.
- Boiler casing never came off.
- No written report or certificate on the day.
- Same "all fine" verdict every year, even on an ageing boiler.
A real service catches small problems. If a system has been ticked off as fine for five years and then suddenly fails, the previous services were probably visual.