No Hot Water But Heating Works: The Likely Causes

This is a very specific and very telling symptom.

When the central heating works normally but you get no hot water from the taps, you've effectively ruled out a lot of common boiler problems. The gas, the burner, the pump and the main heat exchanger are all clearly doing their job — otherwise the radiators wouldn't get hot.

The fault lies in the components that divert and deliver domestic hot water (DHW) on a combi boiler.

On a combi, heating and hot water share the same boiler but follow different paths once heat is generated. When you open a hot tap, the boiler is supposed to stop sending heat to the radiators and redirect it to warm mains water passing through a second, smaller heat exchanger.

If anything in that hot-water path fails, heating carries on working while the taps stay cold. Below are the four most likely causes, roughly in order of how often they crop up.

1. A stuck diverter valve (the most common cause)

The diverter valve is the component that physically switches the flow of hot water between the radiators and the taps. At rest, it sends heat to your central heating. When you open a hot tap, a flow sensor tells the valve to swing over and prioritise the hot water instead.

Over time the valve can seize, stick or wear, often jamming in the heating position. The result is textbook: the radiators heat up perfectly because that's where all the hot water is going, but the taps never get warm because the valve never switches over. A diverter valve stuck the other way produces the opposite symptom — hot water but no heating.

The diverter valve sits inside the boiler casing, so testing or replacing it is Gas Safe engineer only. There's no safe homeowner fix; don't open the casing. An engineer can confirm it quickly, and either free a partially stuck valve or replace the valve or its internal cartridge.

2. A faulty DHW thermostat or sensor

Your combi uses a sensor (often called the DHW thermistor) to measure the temperature of the water heading to your taps.

If that sensor fails or gives a false reading, the boiler's control board can get confused — it may think the water is already hot enough and refuse to fire for hot water, even though the heating circuit, which uses a different sensor, works normally.

A failed hot-water sensor sometimes triggers a fault code on the boiler display. The diagnosis and replacement are engineer tasks, but it's a relatively inexpensive part.

Before assuming the worst, it's worth checking the obvious on the front panel first: make sure the hot water temperature dial (usually a tap symbol) hasn't been turned right down or to an "eco/off" setting by accident — that's a genuinely homeowner-checkable cause that catches people out.

3. A faulty flow switch or flow sensor

For the boiler to fire on hot water, it first has to know you've opened a tap. That's the job of the flow switch (or flow sensor) — it detects water moving through the system and signals the boiler to start heating the DHW.

If it sticks or fails, you can open every hot tap in the house and the boiler simply won't respond, because as far as it's concerned no one is asking for hot water.

A useful homeowner clue here: low mains water pressure or a partially closed stopcock can mimic a flow-switch fault, because the boiler needs a minimum flow rate to fire. Check that your mains stopcock is fully open and that hot water flow from the tap is reasonable.

If flow is fine but the boiler still won't fire on hot water, the flow switch itself is likely at fault — and that's an engineer job inside the casing.

4. Scale or blockage in the plate heat exchanger

The plate heat exchanger (PHE) is the small secondary exchanger that transfers heat from the boiler into your incoming mains water. In hard-water areas it gradually furs up with limescale, or it can partially block with system debris.

As it clogs, hot water output drops — first you get lukewarm water, or hot water that's weak and slow, and eventually it can fail almost completely while the heating (which doesn't pass through the PHE) carries on as normal.

Tell-tale signs of a scaling PHE: the hot water was getting progressively weaker over weeks or months rather than failing overnight, and you live in a hard-water region. Flushing or replacing the plate heat exchanger is an engineer job. If you're in a hard-water area, ask your engineer about a scale-reducing filter to slow it happening again.

What you can safely check yourself

Before you call anyone, run through these homeowner-safe checks — occasionally they solve it outright and at the very least they help the engineer:

• Hot water dial: confirm the tap-symbol temperature dial on the boiler hasn't been turned down or to eco/off.
• Mains stopcock: make sure it's fully open so the boiler gets the flow it needs to fire.
• Other taps: check whether it's every hot tap or just one (just one points to a tap/plumbing issue, not the boiler).
• Pressure gauge: it should read roughly 1–1.5 bar cold, rising towards 2 bar hot. If it's below about 1 bar, low pressure can stop the boiler firing — top up via the filling loop.
• Fault code: note any code on the display to give the engineer.
• One reset: a single front-panel reset, once.

Everything inside the boiler casing — the diverter valve, DHW sensor, flow switch and plate heat exchanger — is strictly for a Gas Safe registered engineer.

Never remove the casing or touch the gas valve, flue or sealed components yourself, and always check the engineer's Gas Safe ID card before work starts. (CORGI hasn't been the UK gas registration body since it was replaced by Gas Safe in 2009.)

If it turns out your taps and radiators are both cold, that's a different diagnosis — see our companion guide on no hot water from your boiler, which covers power, gas supply, pressure and timer faults.

Indicative repair costs

The figures below are broad UK ranges for 2026 and are indicative only — your actual price depends on the boiler make, the specific part and your area.

A single combi repair like a diverter valve can cost more than a year of monthly cover.

That's the core argument for boiler cover — a plan that bundles repairs, parts, labour and usually an annual service into a predictable monthly cost. If you're weighing it up, our guides to the best boiler cover and cheap boiler cover compare what's included at each price point, and it's worth reading whether boiler cover is worth it before you commit.

Frequently asked questions