Call-out · S-Plan · Y-Plan
Boiler will not fire: the controls work, nothing heats
“The programmer's on and I can hear things whirring, but the boiler never fires - no heating and no hot water.”
In short
The controls respond but the boiler never fires, for heating or hot water. 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 programmer's on and I can hear things whirring, but the boiler never fires - no heating and no hot water.”
- “There is no hot water, and the radiators are cold too. I hear the valve click and move, but the boiler never lights for either.”
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 zones fail the same way, and the controls respond. So look at what the zones SHARE after the valves.
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
Both zones fail the same way, so look at what they SHARE after the valves.
- 03
With a valve open: meter the switched-live terminal in the wiring centre, then the boiler's SL.
- 04
SL live and the boiler dead? Check the boiler's own supply: L-N, L-E, N-E.
- 05
The fault lies between the LAST point with 230 V and the FIRST without.
- 06
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.
- 07
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: Y-Plan
No hot water
- 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.”
- Radiators hot with everything off
“The radiators get warm when nothing's asking for heat - it's all off at the timer and they're still hot.”
Back to every call-out.
Practise it in the simulator.
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