No hot water but the heating works? Here's why
The four usual causes when the radiators are hot but the taps run cold, which ones you can fix yourself, and which need an engineer.
14 July 2026 · 5 min read
If your radiators are warming up but the hot tap runs cold, the good news is that your boiler is almost certainly firing. Something is stopping the heat reaching the water, rather than the boiler failing outright.
That narrows it down considerably, and on a combi it usually points at one of four things.
1. The diverter valve
A combi heats your radiators and your hot water from the same heat exchanger. A diverter valve decides which one gets the heat. When you open a hot tap, the valve should swing over and send everything to the water.
When that valve sticks - and they do stick, usually with limescale or sludge - it stays stuck on heating. Radiators hot, taps cold. It is the single most common cause of these exact symptoms and it needs an engineer. A valve is typically £250 to £400 fitted.
2. The thermostat on the boiler
Worth thirty seconds of your time before you ring anyone. Boilers have a separate hot water temperature dial, and it is easy to knock down without noticing. Check it is set to about 60C.
3. Airlocks
Common after a system has been drained or refilled. If the cold tap runs strongly and the hot barely trickles, there may be air trapped in the pipework. Running the cold and hot together on a mixer tap sometimes clears it.
4. A failing heat exchanger
The one nobody wants. If the exchanger is scaled or partly blocked, the boiler can still push enough heat into the radiators but not enough to raise the water fast enough for a tap.
On a boiler under ten years old this is worth repairing. On one over twelve, a heat exchanger costs a meaningful fraction of a new boiler, and you would be putting an expensive part into a machine that is running out of road.
The honest answer on repair vs replace
If your boiler is under ten years old, fix it. Diverter valves are a routine job.
If it is over twelve, has already needed a repair or two, and now this - you are close to the point where the next repair pays for a chunk of a replacement that would come with a ten-year warranty and a lower gas bill. Getting a couple of quotes costs you nothing and tells you where you actually stand.