Call-out · S-Plan · Y-Plan
Hot water scalding: the cylinder keeps on heating
“The hot water's scalding - far hotter than it used to be, whatever the dial on the cylinder says.”
In short
The hot water keeps heating after the cylinder is up to temperature: the hot water path never switches off. 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 hot water's scalding - far hotter than it used to be, whatever the dial on the cylinder says.”
- “The taps are dangerously hot. My daughter burnt her hand.”
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 cylinder is hot and the hot water path is still live. Follow it back: is the cylinder stat breaking? Is the programmer's hot water output dropping? Is the valve shutting?
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
Heat the cylinder up (drag Cylinder water on the Conditions card above the stat's setting) with hot water ON. The hot water valve should shut and the boiler stop.
- 03
Everything switched and the valve still open: isolate and watch it spring shut - or not.
- 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
Practise it
- Fault Vault jobs: S-Plan
Hot water scalding · Hot water heating with the timer off
- Fault Vault jobs: Y-Plan
Hot water scalding
- 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.