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

*David · Founder, Datum · Datum · updated 2026-10-19 · 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, Vaillant, Viessmann and Baxi all name the diverter valve, which sends hot water to the radiators or to the taps: stuck one way, you get one without the other. On a boiler with a cylinder, other components in the hot-water circuit are involved. It's rarely a whole-new-boiler problem, but it needs an engineer to pin down which component.

## On a combi boiler

A combi diverts its heat between your radiators and your taps. Vaillant describes the diverter valve as the mechanism that directs hot water to either the radiators or the taps and showers, and lists hot water with cold radiators, and lukewarm taps, as symptoms of a faulty one. It is a known, quotable component job — not a reason to replace a sound boiler.

## With a hot-water cylinder

If you've a cylinder, no hot water means a fault somewhere in the circuit that sends heat to it — a valve, a control, or the immersion heater if one is fitted. 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

Baxi's first checks are pressure (it gives a normal range of 1.0 to 2.0 bar) and the timer, so we start there. Then we confirm the boiler is making heat (it is, if one side works) and test the diverter, controls 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. Vaillant advises against fixing it yourself and recommends a qualified Gas Safe registered engineer. We quote it fixed once we've confirmed that's the cause.

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

A partly working diverter valve is one possibility — Vaillant lists lukewarm taps as a symptom of a valve that is stuck and letting some hot water pass to the radiators — but other faults can behave the same way. It won't fix itself, so it's a book-it job.

### Could it be my thermostat or timer?

Yes, and it's the cheapest outcome. Baxi's first checks for no hot water are pressure, the timer and the diverter valve. If you've a cylinder with separate controls, a control fault can leave you with heating but no hot water — it's one of the things we check.

## Sources & verification

- [Vaillant — How to fix a boiler diverter valve (a combi's diverter valve sends hot water to the radiators or to the taps and showers; hot water with cold radiators and lukewarm taps are symptoms; call a qualified, Gas Safe registered engineer; accessed 19 Oct 2026)](https://www.vaillant.co.uk/advice/understanding-heating-technology/boilers/how-to-fix-a-boiler-diverter-valve/)
- [Viessmann — My boiler is working but I have no hot water (if heating works but hot water does not, the diverter valve may be stuck, and a trained Gas Safe engineer is needed; accessed 19 Oct 2026)](https://www.viessmann.co.uk/en/heating-advice/boilers/my-boiler-is-working-but-i-have-no-hot-water.html)
- [Baxi — What to check if you have no hot water (low pressure, the timer and the diverter valve; a faulty diverter valve means calling a heating engineer; accessed 19 Oct 2026)](https://www.baxi.co.uk/help-and-advice/faqs/what-to-check-if-you-have-no-hot-water)
- [Baxi — What should my boiler pressure be set at? (normal range 1.0 to 2.0 bar; repressurise below 1.0 bar; check your manual; accessed 19 Oct 2026)](https://www.baxi.co.uk/help-and-advice/faqs/what-should-my-boiler-pressure-be-set-at)

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