Annual boiler service in London

A boiler service is a set list of checks laid down in the maker's service instructions, not a quick look at the pressure gauge. The engineer works through the gas side, the water side and the flue.

On the gas side they check the working pressure at the boiler, make sure the burner is clean and lighting cleanly, and run a combustion test. That test uses a flue gas analyser, a handheld meter that reads carbon monoxide and carbon dioxide in the flue gases. It tells the engineer if the boiler is burning safely and efficiently.

On the water side they check the system pressure, the expansion vessel (a small tank that soaks up the water as it heats and expands), the pressure relief valve and the condensate trap. The trap collects acidic water from the boiler and drains it away. If it clogs, many boilers lock out.

The flue is checked where it can be seen, inside and out. The terminal outside must be clear of nests, plants and new obstructions such as a neighbour's extension or a new window.

Thames Water says all the water in its region is hard, because it passes through chalk and limestone. That matters most in combi boilers, which heat mains water on demand. Scale builds up inside the plate heat exchanger and you notice it as weak or swinging hot water. At a London service we run the hot tap and check flow and temperature, not just the heating.

Access is the other London factor. In converted Victorian houses the boiler is often in a tight kitchen cupboard, in a hall cupboard or up in the loft. Newer flats along the river and in town centres sometimes have the flue hidden in a ceiling void. The HSE flues in voids guidance says that since 2013 an engineer who cannot inspect a concealed flue should treat the boiler as at risk. If your flat has no inspection hatches, tell us when you book.

Book about a year after your last service. If you are not sure, the date is usually on the service record, on a sticker inside the boiler cover, or in the Benchmark checklist at the back of the boiler manual.

Spring and summer are the easiest times. Google search data for London shows demand for boiler services in October at about four times the level of May and June, because people turn the heating on and find a problem. A summer service means a checked boiler before the cold arrives and more choice of days.

Gas work in the UK must be done by a Gas Safe registered engineer. Ask to see the ID card when the engineer arrives and check the licence number on the Gas Safe Register.

You get a written service record with the readings from the combustion test and notes on anything found. Keep it with your boiler paperwork. Boiler makers may ask for it if you make a warranty claim, and buyers' solicitors often ask about heating service history when you sell.

If the engineer finds a fault, they explain it in plain words and give a price before doing any repair. If they find something dangerous, such as a flue leaking fumes, the law requires them to follow the gas industry unsafe situations procedure, which can mean turning the appliance off with your permission until it is fixed.