Emergency Boiler Repair in Bexley (DA5)

The 2021 Census counted 6,867 households across DA5 on mains gas central heating alone, 87% of the district and the second highest share of the eighteen DA districts. A smaller share heats by electricity alone than in any of the other seventeen. So when a boiler here stops, there is rarely a second system in the house to fall back on. If yours is a combi, there is usually no hot water cylinder and no immersion heater behind it either. Every drop of hot water and every warm radiator passes through the same box on the wall, and through the same valve inside it.

When that valve fails, you get half a boiler, not none.

Counted by when they went up, a third of DA5 dates from the 1930s: 3,230 of the district’s 9,880 homes, on the Valuation Office’s figures. No house of that age was built with a combi, so any combi in one went in later, and the age of the house says nothing about the age of the valve. The boiler’s installation date does.

Diverter valve inside a combi

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A combi has one burner and one pump, and it cannot heat the radiators and the hot taps at the same moment. The diverter valve is how it chooses between them. With the heating on and every tap shut, the valve sends the boiler’s water round the radiators. Open a hot tap and the tap wins: Potterton’s manual for its Promax combi says a demand at a tap or shower overrides any central heating demand. The valve moves across, the heat goes to the hot water, and when the tap closes the valve moves back. It does that every time a hot tap runs while the heating is on, for as long as the boiler is on the wall. When it starts to fail, Vaillant and Viessmann describe the same handful of signs: hot water at the taps but radiators that stay cold, taps that will only run lukewarm, or hot water that only turns up while the heating is running. When the valve is to blame, each of those comes from it no longer moving cleanly between its two positions. You tend to notice it as a pattern rather than a breakdown: the shower is fine but the bedrooms never warm, or the kitchen tap runs tepid all evening. Both manufacturers are clear about what to do next, which is not to attempt the repair yourself. The valve sits inside the casing beside the gas and electrical parts of the boiler, and that casing is for a gas engineer to take off.

Lukewarm water is not always the valve.

Among the causes Worcester lists for hot water that will not get hot are two simple ones: the tap running faster than the boiler can heat it, and the hot water temperature set too low on the boiler’s own control. Turn the hot tap down a little and check that setting before anything else.

Hot taps with cold radiators need a different check.

Make sure the heating is being asked for at all, with the programmer in an on period and the room thermostat turned up past the temperature of the room. If the taps run hot, the settings are right and the radiators still sit cold with every tap shut, the valve moves to the top of the list. Tell us which of the three signs you have when you ring.

On the visit, the engineer confirms the boiler is being asked for heat, then watches what happens as the demand switches from heating to hot water and back. Whether the cure is the whole valve or only part of it varies by make, and you hear which, and why, before anything is ordered.

A diverter valve is one part, not a verdict on the boiler, but if it is the latest of several faults on an ageing combi, we also arrange boiler installations, and the replacement itself is explained there, from sizing to the day it goes in.