# Why has my boiler got heating but no hot water (or vice versa)?

*David · Founder, Datum · Datum · Boilers*

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.

> One side working is good news — the boiler makes heat fine. The fault is in what routes it, and that is a component job, not a new boiler.
>
> — David, Founder, Datum

## Related questions

### Is a stuck diverter valve expensive to fix?

It's a mid-range component job on most combis — more than a seal, far less than a new boiler. We quote it fixed once we've confirmed that's the cause rather than a cheaper flow sensor.

### Why do I get hot water for a few seconds then it goes cold?

Classic diverter or flow-sensor behaviour on a combi — the boiler starts to divert to the tap then loses the signal. It's a book-it job; it won't fix itself and tends to get more stubborn.

### Could it be my thermostat?

If you've a cylinder and separate controls, yes — a failed cylinder thermostat or programmer channel can leave you with heating but no hot water. It's one of the things we check, and one of the cheaper outcomes.

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