If your taps are hot but the radiators stay cold, the boiler itself is almost always fine — the fault is in the diverter that sends heat to either taps or radiators. Six common causes account for 95% of "hot water but no heating" call-outs in London. This guide explains how to identify each and which need a Gas Safe engineer.
Combi vs system boiler — which do you have?
- Combi boiler (no hot water tank): a sticking diverter valve is the #1 culprit.
- System or regular boiler (hot water cylinder in airing cupboard): the issue is usually a stuck 3-port motorised valve or its actuator.
Cause 1: Stuck diverter valve (combi)
When you run a tap, the diverter sends hot water to the kitchen. It is supposed to spring back to send heat to the radiators when the tap closes — but limescale and worn springs cause it to stick. Symptom: hot taps work fine, radiators stay stone cold even with heating on. Fix: diverter cartridge or full valve replacement (£180–£320 fitted).
Cause 2: Stuck motorised valve (system/regular boiler)
The 3-port valve sends flow to either heating or hot water based on programmer demand. The actuator (the box on top) jams. Test: with heating on, the lever on the side of the valve should move. If it does not, the actuator needs replacing (£140–£220 fitted).
Cause 3: Programmer / room thermostat
Check the programmer is calling for CH (heating) — not just HW (hot water). Then check the room thermostat is set above current room temperature. A flat battery in a wireless thermostat blocks the call to the boiler entirely.
Cause 4: Low system pressure
On a combi, look at the pressure gauge — should be 1.0–1.5 bar cold. If it is below 0.5, the boiler may produce hot water but lock out on the heating circuit. Fix: open the filling loop to top up to 1.2 bar.
Cause 5: Airlock or sludge in the radiator circuit
If only some radiators are cold, bleed each one in turn. If the bottom is cold but the top is warm, it is sludge — needs a power flush (£450–£850). If the top is cold and the bottom warm, it is air — bleed and re-pressurise.
Cause 6: Frozen condensate pipe (winter only)
A boiler that worked yesterday and now will not fire after a frost is almost always a frozen condensate pipe running outside. Pour warm (not boiling) water over the white plastic pipe until the boiler restarts.
When to call a Gas Safe engineer
Anything involving the boiler casing, diverter, motorised valve or gas valve is Gas Safe registered work only — it is illegal for non-registered engineers to attempt. Our London engineers attend the same day, diagnose on a fixed-price first hour, and quote before any parts go in.
Cold radiators with the heating on? Call 07427 826446 — same-day Gas Safe diagnosis from £95 across London.