No Heating But Hot Water Still Works? Here's Why
Taps running hot but radiators stone cold — the diverter valve is almost always the cause. Here's how to tell, and what fixes it.
If your taps run properly hot but every radiator in the house is stone cold, the fault sits in one specific place on a combi boiler: the diverter valve. Its job is to switch flow between heating and hot water — when it sticks in the "hot water" position, that's exactly the symptom you get.
Why it happens
Diverter valves wear over time, and limescale (common across the SW London and Surrey border area) accelerates it. Sometimes it's just the valve's motor or cartridge that's failed, not the whole assembly — a much cheaper fix if caught before the valve itself is damaged.
What to check first
Confirm the programmer's actually set to "heating on" — a knocked setting after a power cut causes the same symptom and costs nothing to rule out. If the heating schedule looks right and radiators are still cold with hot taps working, it's a diverter valve fault and needs a Gas Safe engineer.
Is it worth fixing?
Almost always yes — it's a component repair, not a boiler replacement, on a fault that's usually resolved same-day once diagnosed.