Call-out · S-Plan · Y-Plan
Boiler cuts in and out on its own
“It keeps cutting in and out. Works for a bit, the boiler drops off, then it comes back on its own.”
In short
The boiler drops out and comes back while the demand is steady. 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
- “It keeps cutting in and out. Works for a bit, the boiler drops off, then it comes back on its own.”
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 demand is steady, so watch which stage drops out. Everything upstream of the flicker is good.
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
Do not touch anything yet. WATCH for a minute: which stage stays steady, which one drops out?
- 03
Meter each side of the stage that flickers.
- 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.
Runs in the browser on a laptop or desktop. Cancel any time.