Call-out · S-Plan · Y-Plan
Boiler fires with everything off, radiators cold
“The boiler keeps firing with everything off at the timer, but the radiators stay cold.”
In short
The boiler fires with no demand at all, and no zone is open: something is feeding its switched live directly. Confirm it for yourself, then follow the demand with the meter: the fault lies between the last point with 230 V and the first without.
What the customer says
- “The boiler keeps firing with everything off at the timer, but the radiators stay cold.”
- “It's switched off and I can still hear the boiler going. Rads are cold though.”
The customer’s words are where you start, not what you trust. Confirm the complaint for yourself before you pick up the meter: set one demand at a time and watch what each part does.
What it already proves
Both valves are shut and the boiler still fires. Only a valve's orange should ever feed the boiler's switched live: find what else is feeding it.
The method
If this is your home: this page is written for heating engineers. Mains wiring and gas appliances are not DIY jobs; call a Gas Safe registered engineer. If you smell gas, leave the building and call the National Gas Emergency Service on 0800 111 999.
In the simulator the meter is safe. On site: isolate and prove dead before you touch a terminal, and make live tests only if you are trained and equipped for them. The steps below are the method as practised in the simulator, in its own words, for competent persons and for learners under the supervision of one.
- 01
CONFIRM the complaint yourself: spur ON, run the simulation. Ask for one thing at a time: the heating, then the hot water if it has its own control. Watch what each part does.
- 02
Turn EVERYTHING off. Both valves shut, boiler still firing: what feeds its switched live?
- 03
Disconnect each valve's orange in turn (isolated!).
- 04
The fault lies between the LAST point with 230 V and the FIRST without.
- 05
ISOLATE before you touch anything: the fused spur OFF, locked off, proved dead with a proved tester. Then stop the simulation: the simulator only erases when stopped. Continuity only on a dead circuit.
- 06
Click Report Fault, then the wire or part you have PROVEN faulty.
Where to probe
- The black probe stays on a neutral for the whole trace. The red probe follows the live from point to point, in the order the demand travels.
- Between two points, one with 230 V and the next without, there is only a wire, a screw or a part.
- Prove a part with two readings before you condemn it. A motor needs a live and a neutral: measure across it, to its own neutral.
- Continuity and resistance only on a circuit that is isolated and proved dead.
Follow the path on the diagram
- S-Plan wiring diagram
S-Plan
- Y-Plan wiring diagram
Y-Plan
The parts on the path, and where to practise
Parts on the path
Practise it
- Fault Vault jobs: S-Plan
Boiler fires with nothing on
- Random Fault
One hidden fault at a time, Easy, Medium or Hard
Other call-outs
- No heating
“Hot water's fine but the radiators are stone cold, whatever I do with the thermostat.”
- No hot water
“Heating's lovely and warm but there's no hot water - the cylinder's cold.”
- Everything dead
“Nothing works. No heating, no hot water, and the controls are dead.”
- Boiler not firing
“The programmer's on and I can hear things whirring, but the boiler never fires - no heating and no hot water.”
Back to every call-out.
Practise it in the simulator.
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