Combi boiler service in London

A combi boiler has two heat exchangers. The main one heats the water in your radiators, which goes round and round and does not pick up much new scale. The second, a plate heat exchanger, heats fresh mains water as it passes through to your taps. That fresh water carries new minerals every time you run a tap.

Thames Water says all the water in its region is hard, because it passes through chalk and limestone. Its own scale puts hard water at 200 to 300 mg/l of calcium carbonate and very hard above 300. In the south, SES Water supplies parts of Sutton, Merton and Croydon from chalk wells and publishes hardness by postcode.

When that water is heated, some of the minerals drop out as scale on the plates. The gaps get narrower, flow drops and the boiler struggles to hold a steady temperature. You notice it in the shower first.

Common signs are hot water that runs hot then cold, a boiler that cuts out when you turn a hot tap down, weak hot flow next to strong cold flow, and a rumbling or kettling noise. These signs can have other causes, such as a faulty diverter valve or a flow sensor, so the engineer tests before deciding.

If the plate heat exchanger is badly scaled, the usual fix is to clean it or replace it. Replacement is often simpler on modern boilers. We give a price once the engineer has seen the boiler.

Scale is slow. A combi does not usually go from fine to failed overnight. That is why a yearly hot water test at the service is useful: the engineer can compare flow and temperature with last year's notes and spot a trend before you are left with a cold shower.

Thames Water suggests keeping hot water at around 60 degrees to reduce scale and mentions fitting scale reducers. On many combis you can set the hot water temperature on the front panel. Turning it down a little from the maximum can help, as long as it still suits your household.

Many London combis were fitted with a scale reducer on the cold feed. Some are magnetic or electrolytic and need little care. Others use a polyphosphate cartridge that runs out. The engineer checks which you have at the service.

Combis are the usual boiler in London flats because there is no room for a hot water cylinder. That often means a tight cupboard and a flue through an outside wall or up through the building. Tell us where the boiler is when you book, and clear the cupboard before the visit, so the engineer can get the case off and work safely.

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