Call-out · S-Plan · Y-Plan
Boiler overheats and cuts out; the pump hums, radiators cold
“The boiler fires, gets really hot and cuts out. I can hear the pump humming but the radiators stay cold.”
In short
The pump runs but no water moves: the boiler overheats and locks out. In the simulator: confirm it, then follow the demand with the meter. The fault lies between the last point with 230 V and the first without.
If this is your home: this page is written for heating engineers, and for learners under the supervision of one. It explains how the wiring works; it is not training to do the work. In England and Wales, electrical work in a home must meet the Building Regulations (Part P), and some of it must be notified to building control or done by a registered electrician; Scotland and Northern Ireland have their own rules. For the wiring, call a registered electrician or a heating engineer competent in controls. Work inside a gas boiler is for a Gas Safe registered engineer only. If you smell gas, leave the building and call the gas emergency number: 0800 111 999 in Great Britain, 0800 002 001 in Northern Ireland.
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.
What the customer says
- “The boiler fires, gets really hot and cuts out. I can hear the pump humming but the radiators stay cold.”
- “It fires up, bangs and rattles for a minute, then goes off. Nothing gets warm.”
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
The pump has power and runs; the boiler still overheats.
The method
- 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
Watch the pump while the boiler fires: it runs. Watch the boiler: it kettles and locks out.
- 03
Meter the pump: L to N, 230 V and turning. The electrics are proven.
- 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
- 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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