Explained
Heating wiring, explained
In short
Short written explanations of heating wiring and fault finding, in the words the lessons use: one question each, with the diagram, the part or the call-out it belongs to.
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.
Wiring the system
How S-Plan, Y-Plan and combi controls are wired, block by block and core by core.
- How the wiring centre’s numbered blocks tell the story of an S-PlanAn S-Plan wired in the order it works: the supply on blocks 1 to 3, heating on 4 and 5, hot water on 6 and 8, the valves’ orange cores on 10.Updated 5 November 2026
- Why grey is the permanent live on a 2-port zone valve, and brown is notA 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.Updated 5 November 2026
- Wiring a wireless thermostat to a combi boiler: the receiver, the link and the room-stat terminalsA combi needs only its supply and a call on its room-stat terminals. The receiver’s relay replaces the link; check volt-free against mains-switched first.Updated 5 November 2026
- Y-Plan explained: what white, grey and orange do on a mid-position valveA Y-Plan’s mid-position valve: white calls for heating, grey says hot water is not wanted, orange fires the boiler. None of its cores is a permanent live.Updated 5 November 2026
Finding faults
Finding the fault with the meter: the order, the readings, and what each one proves.
- Heating on but no hot water: how an engineer approaches itThe 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.Updated 5 November 2026
- Last 230 V, first 0 V: the rule that finds a fault with three readingsHeating 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.Updated 5 November 2026
- Prove a part with two readings before you condemn itA 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.Updated 5 November 2026
Safety and testing
Working dead, proving dead, and proving a system works before you hand it over.
- Safe isolation in five steps: prove, confirm live, isolate, test every pair, prove againHeating wiring is done dead. Prove the tester, confirm the supply is live, isolate and lock off, test every pair, prove the tester again. Why each step matters.Updated 5 November 2026
- What a truth table proves when you commission a heating systemCommissioning proves every state of the controls before handover. The truth table’s rows, the boiler interlock row by row, and the dead tests that come first.Updated 5 November 2026
Reading the makers’ diagrams
The makers’ terminal maps on programmers and thermostats, and how to read them.
Practise it in the simulator.
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