You can run a hot tap, but the radiators stay stone cold. It is one of the most common combi and system boiler complaints — and often one of the simplest to pin down.
When hot water flows but the central heating won't fire, that split is actually a useful clue. It tells you the boiler is broadly working — it can heat water on demand — so the fault usually sits in the part of the system that sends heat to the radiators rather than the burner itself. Below are the most likely causes, which you can safely check yourself and which need a Gas Safe registered engineer.
This sounds obvious, but it is genuinely the most common cause — and the cheapest to fix. Many programmers have separate schedules for hot water and central heating, so the water can be timed "on" while the heating is "off". Check the following:
After a power cut, a clock change, or a flat thermostat battery, programmers often revert to a default schedule. Always rule out the controls before assuming a mechanical fault.
In a combi boiler, a diverter valve decides whether heated water goes to your taps or to your radiators. In a system or heat-only boiler with a cylinder, a motorised valve does a similar job. When you run a hot tap, the valve sends water to the taps as it should — so hot water is fine. But if the valve has stuck in that "hot water" position, it never switches over to the heating circuit, leaving the radiators cold.
A stuck or failing diverter or motorised valve is one of the classic reasons for "hot water yes, heating no". This is an internal heating component, not a homeowner job — diagnosing and replacing it involves working inside the appliance. If your controls are clearly set correctly and the radiators still won't warm up, book a Gas Safe registered engineer to inspect the valve.
If most radiators are cold, or they're warm at the bottom and cold at the top, trapped air may be stopping hot water circulating. Bleeding the radiators is a safe DIY task that often restores heat quickly. Turn the heating off and let the system cool, then use a radiator key to release the trapped air until water appears, and re-check the boiler pressure afterwards (see below). Our step-by-step guide on how to bleed a radiator walks through it.
If pressure drops too low, the boiler may keep producing hot water on demand but refuse to run the heating, or it may lock out. Check the pressure gauge: cold, it should read roughly 1 to 1.5 bar, rising to around 2 bar when hot. A reading below 1 bar is low.
You can usually top up the pressure yourself using the filling loop — the small braided hose or built-in valve beneath the boiler. Open it slowly until the gauge reaches about 1.2 bar, then close it fully. If the pressure keeps falling repeatedly after topping up, there is likely a leak somewhere in the system, which needs an engineer.
Stop and book a Gas Safe registered engineer if your controls are correct and the heating still won't come on, if you suspect a stuck diverter or motorised valve, if pressure keeps dropping (a likely leak), or if the boiler is locking out or showing a fault code. Never remove the boiler casing or attempt any work on the gas valve, internal components or pipework — that is gas work, and it is for a registered engineer only. If you ever smell gas, call the National Gas Emergency line on 0800 111 999.
If nothing at all is working — no hot water and no heating — see our wider guide to why your heating is not working. To understand what a repair-and-breakdown policy would and wouldn't cover here, read what is boiler cover and is boiler cover worth it.
| Symptom | Likely cause | Who fixes it |
|---|---|---|
| Heating channel off / thermostat low | Controls set to hot water only | You |
| Rads cold at top, warm at bottom | Trapped air | You (bleed) |
| Pressure gauge under 1 bar | Low pressure | You (top up) |
| Controls fine, rads still cold | Stuck diverter / motorised valve | Gas Safe engineer |
| Pressure keeps dropping | System leak | Gas Safe engineer |
Because the boiler can clearly heat water, the fault usually sits between the boiler and the radiators — most often the controls being set to hot water only, a stuck diverter or motorised valve, airlocked radiators, or low pressure.
Yes. The diverter valve in a combi (or a motorised valve in a system boiler) directs water to the taps or the radiators. If it sticks in the hot-water position, your taps stay hot but the heating never switches on. Replacing it is a job for a Gas Safe registered engineer.
It can, if trapped air is the problem — typically when rads are cold at the top but warm lower down. Bleeding is a safe DIY task, but remember to re-check and top up your boiler pressure afterwards.
Most repair-and-breakdown policies cover faults like a failed diverter valve, subject to the plan's terms, excess and any exclusions. Always check what's included before you buy — exclusions and call-out limits vary by provider.
A repair-and-breakdown policy can cover faults like a failed diverter valve. Compare boiler cover from our selected panel of providers — not the whole market — and check what's included before you choose.
Compare boiler coverThis article is general information, not advice. It reflects a selected panel of providers rather than the whole market. Prices and ranges are indicative for 2026. Always follow your boiler manufacturer's instructions and use a Gas Safe registered engineer for any gas work.