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If your radiators warm up fine but the taps run cold, the fault is almost always in the part of a combi boiler that handles hot water on demand — not the heating side. Here are the four usual culprits, what you can safely check, and what needs a Gas Safe registered engineer.
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.
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.
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.
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.
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.
Before you call anyone, run through these homeowner-safe checks — occasionally they solve it outright and at the very least they help the engineer:
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.
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.
| Job | Indicative cost |
|---|---|
| Diagnostic call-out / one hour labour | £70–£120 |
| DHW thermistor / sensor replacement | £90–£180 |
| Flow switch / flow sensor replacement | £120–£220 |
| Diverter valve cartridge / repair | £150–£280 |
| Diverter valve replacement (full) | £200–£400 |
| Plate heat exchanger flush | £120–£250 |
| Plate heat exchanger replacement | £250–£500 |
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.
Compare boiler cover plans with repairs, parts, labour and an annual service from a panel of UK providers — see exactly what each one includes and what you'd pay each month.
Compare boiler coverOn a combi boiler this almost always means a fault in the hot-water path rather than the heating side. The most common cause is a stuck diverter valve jammed in the heating position, so all the heat goes to your radiators and none to your taps. A faulty hot-water sensor, a failed flow switch, or a scaled plate heat exchanger can also do it. The heating side keeps working because it uses different components.
No. The diverter valve sits inside the boiler casing and is a Gas Safe engineer job to test, free or replace. Don't open the casing or attempt it yourself. Before calling out, you can safely check the boiler's hot-water temperature dial and that your mains stopcock is fully open, as those occasionally explain the symptom.
Gradually weakening hot water, especially in a hard-water area, often points to limescale building up in the plate heat exchanger. It restricts how much heat reaches your mains water, so the taps run lukewarm while the radiators (which don't use that exchanger) stay hot. An engineer can flush or replace it.
This specific "heating works, no hot water" pattern is mainly a combi issue, because combis make hot water on demand. System and heat-only boilers heat a separate hot water cylinder, so a hot-water-only failure there usually points to the cylinder, its immersion heater, the motorised valve or a timer setting rather than a diverter valve.
Most plans cover breakdown repairs including parts and labour for faults like a failed diverter valve, flow switch or sensor, subject to the policy terms and any exclusions. Always check the specific plan's inclusions, limits and excess before buying, as cover levels vary between providers.