Boilers
Why has my boiler got heating but no hot water (or vice versa)?
3 min read · Reviewed for accuracy before publishing
When one works and the other doesn't, the fault is usually in the part that decides where the heat goes — not the boiler's ability to make heat. On a combi, heating-but-no-hot-water (or hot-water-but-no-heating) most often means a stuck diverter valve, or a failed flow sensor that isn't telling the boiler you've opened a tap. On a system or heat-only boiler with a cylinder, no hot water can be the cylinder's immersion, a motorised valve, or the programmer. It's rarely a whole-new-boiler problem — but it does need an engineer to pin down which component.
On a combi boiler
A combi diverts its heat between your radiators and your taps. A diverter valve that sticks toward heating leaves you with warm rooms and cold taps; a flow sensor that's failed means the boiler never realises you've turned a tap on. Both are known, quotable component jobs — not a reason to replace a sound boiler.
With a hot-water cylinder
If you've a cylinder, no hot water can be a motorised (zone) valve that's stopped opening, a programmer or thermostat fault, or the immersion heater. The heating side working tells us the boiler itself is firing — so we look at the controls and valves that route hot water to the cylinder.
What we check first
We confirm the boiler is making heat (it is, if one side works), then test the diverter, sensors and valves in order — cheapest and most likely first. You get the fixed price for the actual fault, not a guess, and we'll always tell you honestly if a repeat-offender component means a replacement is the better spend.
David founded Datum after years on the tools across heating and plumbing. He writes the boiler and cost guides from what actually happens on real installs.