Call-out · S-Plan · Y-Plan
No hot water: heating fine, cylinder cold
“Heating's lovely and warm but there's no hot water - the cylinder's cold.”
In short
Central heating works; hot water does not. 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
- “Heating's lovely and warm but there's no hot water - the cylinder's cold.”
- “Radiators are fine. Hot water ran out this morning and hasn't come back.”
- “The cylinder only heats when the heating's on as well. Hot water on its own: the valve clicks, nothing fires.”
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
Heating works, so the supply, boiler and pump are PROVEN good. The fault is on the hot water path only: programmer hot water output → cylinder stat → valve.
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
Heating works, so supply, boiler and pump are PROVEN. Stay on the hot water path.
- 03
Follow the live to NEUTRAL: programmer HW ON → cylinder stat C → cylinder stat call → hot water valve brown.
- 04
Check WHICH cylinder stat terminal the valve is fed from: a cold cylinder makes the CALL contact.
- 05
Valve open but boiler not firing? Compare its GREY with its ORANGE.
- 06
The fault lies between the LAST point with 230 V and the FIRST without.
- 07
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.
- 08
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
No hot water · Hot water only with the heating on
- 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.”
- 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.”
- 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.
Read the explanation
- Heating on but no hot water: how an engineer approaches it
The radiators are warm, the cylinder is cold. What the working heating already proves, the hot water path to trace, and why you prove a part before changing it.
Practise it in the simulator.
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