Boiler Not Firing Up for Heating — Diagnose in 10 Minutes
Combi gives hot water at the tap but radiators stay cold? Classic diverter valve fault. Here is the diagnostic order UK engineers follow.
Step-by-step method
1. Check the thermostat is calling
Turn the room thermostat to max and listen for a click. If Nest/Hive, check the app shows heating on.
2. Check external timer/programmer
Old-school Drayton or Horstmann timers seize. Set to permanent on.
3. Diverter valve
On combis this is the most common cause. Actuator jams in DHW position. Tap gently — sometimes it frees. Replacement £180–£280 fitted.
4. Motorised valve (system boilers)
S-Plan/Y-Plan uses 3-port or 2-port motorised valves. Test manually by pulling the lever — if it doesn't spring back, replace head (£80).
5. PCB fault
Rare. Boiler code will usually flag PCB fault (F0, E168). PCB replacement £260–£400.
Frequently asked questions
Why does hot water work but not heating?+
90% chance it's the diverter valve on a combi — it's stuck in domestic hot water position and can't route flow to the radiators.
Can I fix a diverter valve myself?+
On some models the actuator (motor) can be replaced separately without draining. The full valve requires draining and is a plumber job.