An annual boiler service that finds the fault before the cold snap does

A service follows the manufacturer's schedule for the boiler in front of the engineer, which is why it is not the same on every make. The common ground:

• A visual check of the boiler, the pipework, the flue run and the terminal, inside and out.
• Flue gas analysis with a calibrated analyser: CO, CO2 and the ratio between them, which says whether the burner is combusting cleanly.
• The gas rate and, where the boiler allows it, the working pressure at the burner.
• Case seals and flue seals, because a modern boiler is room-sealed and depends on them.
• The condensate trap, cleaned out, and the condensate pipe checked for a fall and for frost protection outside.
• The expansion vessel charge, the pressure relief valve, the system pressure and the filling loop.
• The magnetic filter, if there is one, emptied and cleaned. If there is not one, that is noted.
• Controls: the thermostat, programmer and any zone valves checked for correct operation.
• Safety devices: flame supervision, overheat stat, and the boiler's own fault log read for anything that has happened since the last visit.

Everything measured is written on the service record and in the boiler's logbook, with the readings, not just a tick.

A service and a safety check overlap but are not the same thing. The safety check confirms the appliance is safe to use: gas tightness, flue integrity, ventilation, combustion within limits. The service goes further into the condition of the boiler and its components and does the cleaning and adjustment that keeps it that way. A homeowner wants both once a year. A landlord is required by law to have the safety check done and certificated annually, and is well advised to have the service at the same time, because the engineer is already there with the case off.

A service does not include stripping and cleaning the heat exchanger unless the combustion readings say it is needed. On a modern condensing boiler running cleanly, opening the combustion chamber unnecessarily does more harm than good. Where the readings are off, the burner and heat exchanger are cleaned and the readings taken again.

A service often finds things. A pump that is noisy, a diverter valve that is sticking, an expansion vessel that will not hold a charge, a fan bearing on its way out. You are shown the problem, told whether it is urgent or can wait, and given a price for the repair. Where the part is on the van, it can be done there and then; where it needs ordering, the return visit is quoted. Nothing is done without agreement, and nothing found is left unsaid. Where a boiler is failing beyond economic repair you are told that too, with the reasons, rather than a service sticker and a surprise in December. Larger faults are handled as a boiler repair on their own visit.

Worcester Bosch, Vaillant, Ideal, Baxi, Viessmann, Glow-worm, Potterton, Alpha, Intergas, Ferroli, Vokera, Main and the older Ravenheat, Halstead and Saunier Duval units still running in London flats. The engineers carry the manufacturer's service instructions for each and the analyser settings that go with them. Where a boiler is obsolete and parts are no longer made, you are told at the service so a replacement can be planned rather than forced.

A service is a planned visit, but a boiler does not always wait for one. Where a boiler has locked out, is leaking or is showing a fault code, that is booked as a repair rather than a service, attended as quickly as the day allows. Where a boiler is producing fumes, a smell of gas or soot marks around the case, turn it off, open windows and call the National Gas Emergency number on 0800 111 999 first; the repair follows once it is safe.

Once a year, and late summer is the sensible time: the boiler has had its rest, the engineer has time, and anything found can be fixed before the heating is needed. A boiler that has been serviced in September and is running cleanly has the best possible chance of getting through to March without a call. New boilers need their first service within twelve months of installation to keep the warranty, even if they have barely run.