No heating · S-Plan · Needs a proper trace
Programmer: heating channel failed
“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. The display says CH ON but terminal 4 never goes live: the heating relay inside the programmer has failed.
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 programmer says heating ON, light's on and everything.”
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.
How to find it in the simulator
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 a real system: 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 heating only, then hot water only. Watch the valve(s), boiler and pump.
- 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 (stop the simulation) before you touch anything. Continuity only on a dead circuit.
- 08
Click Report Fault, then the wire or part you have PROVEN faulty.
What the meter shows
Where to probeProgrammer shows CH ON. Meter its terminal 4 (CH ON) to neutral, then its terminal 3 (HW ON) for comparison.
The readingYou are looking for a device whose display says ON while its OUTPUT terminal is dead. A display is not an output.
The faultThe display says CH ON but terminal 4 never goes live: the heating relay inside the programmer has failed.
Common wrong guesses
Programmer display says CH ON but terminal 4 reads 0 V (its L and N are good). Verdict?
- Faulty room stat
- Faulty valve
Show the right answer
Faulty programmer
A display is not an output. Only the meter tells you whether the relay made.
Related
More no heating faults
Components involved
The system
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