Call-out · S-Plan · Y-Plan · Combi
No heating: hot water fine, radiators cold
“Hot water's fine but the radiators are stone cold, whatever I do with the thermostat.”
In short
Hot water works; central heating 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
- “Hot water's fine but the radiators are stone cold, whatever I do with the thermostat.”
- “We've got hot taps but no heating at all since yesterday.”
- “The radiators only warm up when the hot water's on as well. On its own the heating does nothing: I hear the valve click, but the boiler never fires for it.”
- “We only get heating when the hot water's on as well - it won't do heating 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
Hot water works, so the supply, boiler and pump are PROVEN good. The fault is on the heating path only: programmer heating output → room 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
Hot water works, so supply, boiler and pump are PROVEN. Stay on the heating path.
- 03
Follow the live, each reading to NEUTRAL: programmer CH ON → room stat in → room stat out → valve brown (or white).
- 04
Valve has its live but does not move? Check ITS neutral before blaming the motor.
- 05
Valve open but boiler not firing? Compare the valve's GREY (in) with its ORANGE (out).
- 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
The parts on the path, and where to practise
Parts on the path
Practise it
- Fault Vault jobs: S-Plan
Heating dead · Heating only with the hot water on
- Fault Vault jobs: Y-Plan
No heating · Heating and hot water only together · Radiators cold at heating · Valve never reaches heating only
- Random Fault
One hidden fault at a time, Easy, Medium or Hard
Other call-outs
- 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.”
- 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
- Why grey is the permanent live on a 2-port zone valve, and brown is not
A 2-port zone valve: brown drives the motor, grey feeds the end switch all the time, orange fires the boiler. Why grey, not brown, is the permanent live.
- Last 230 V, first 0 V: the rule that finds a fault with three readings
Heating fault finding with a meter: follow the demand point to point, probe to a neutral, and the fault lies between the last 230 V and the first 0 V.
- Prove a part with two readings before you condemn it
A motor needs a live and a neutral. Read live to earth, then live to the part’s own neutral, before you change a valve, a pump or a fuse. Then prove the repair.
Practise it in the simulator.
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