S-Plan · wiring diagram
S-Plan wiring diagram
In short
An S-Plan is a fully pumped heating system with two 2-port motorised zone valves, one for central heating and one for hot water, driven by a programmer, a room thermostat and a cylinder thermostat. Each valve’s end switch feeds the boiler’s switched live, so the boiler fires only while a valve is open: the boiler interlock.
A complete S-Plan: two zone valves, a room stat, a cylinder stat, the boiler and the pump.
This circuit, wired in the simulator. Wire colours as the lessons teach them. The meter reads what the circuit is doing.
The wiring centre, block by block
A block is a plain conductor: every wire landed on it is joined to every other, and to nothing else. The numbers are the habit the DrawWarm Method teaches, so the next engineer can read your work.
| Block | As the lessons teach it | Wired here |
|---|---|---|
| 1 | Permanent live from the fused spur, and both valves’ greys | Fused Spur L, Heat-Only Boiler L, 2-Port Valve (heating) G, 2-Port Valve (hot water) G, Dual Channel Programmer L |
| 2 | Neutral | Fused Spur N, Circulation Pump N, 2-Port Valve (heating) N, Heat-Only Boiler N, Room Thermostat N, 2-Port Valve (hot water) N, Dual Channel Programmer N |
| 3 | Earth | Heat-Only Boiler E, 2-Port Valve (heating) E, 2-Port Valve (hot water) E, Dual Channel Programmer E, Fused Spur E, Circulation Pump E |
| 4 | Heating demand from the programmer, out to the room thermostat | Room Thermostat L, Dual Channel Programmer 4 |
| 5 | The room thermostat’s call, out to the heating valve’s brown | Room Thermostat SL, 2-Port Valve (heating) B |
| 6 | Hot water demand from the programmer, out to the cylinder thermostat | Cylinder Thermostat C, Dual Channel Programmer 3 |
| 8 | The cylinder thermostat’s call, out to the hot water valve’s brown | Cylinder Thermostat 1, 2-Port Valve (hot water) B |
| 10 | The valves’ oranges in, the boiler’s switched live out | 2-Port Valve (heating) O, 2-Port Valve (hot water) O, Heat-Only Boiler SL |
Every wire, terminal by terminal
| From | Terminal | To | Terminal | Colour |
|---|---|---|---|---|
| Heat-Only Boiler | PL | Circulation Pump | L | Brown (live) |
| Heat-Only Boiler | E | Wiring Centre | 3 | Green/yellow (earth) |
| 2-Port Valve (heating) | E | Wiring Centre | 3 | Green/yellow (earth) |
| 2-Port Valve (hot water) | E | Wiring Centre | 3 | Green/yellow (earth) |
| Dual Channel Programmer | E | Wiring Centre | 3 | Green/yellow (earth) |
| Fused Spur | E | Wiring Centre | 3 | Green/yellow (earth) |
| Fused Spur | N | Wiring Centre | 2 | Blue (neutral) |
| Circulation Pump | N | Wiring Centre | 2 | Blue (neutral) |
| 2-Port Valve (heating) | N | Wiring Centre | 2 | Blue (neutral) |
| Heat-Only Boiler | N | Wiring Centre | 2 | Blue (neutral) |
| Room Thermostat | N | Wiring Centre | 2 | Blue (neutral) |
| 2-Port Valve (hot water) | N | Wiring Centre | 2 | Blue (neutral) |
| Dual Channel Programmer | N | Wiring Centre | 2 | Blue (neutral) |
| Fused Spur | L | Wiring Centre | 1 | Brown (live) |
| Heat-Only Boiler | L | Wiring Centre | 1 | Brown (live) |
| 2-Port Valve (heating) | O | Wiring Centre | 10 | Orange |
| 2-Port Valve (hot water) | O | Wiring Centre | 10 | Orange |
| Room Thermostat | L | Wiring Centre | 4 | Black |
| Room Thermostat | SL | Wiring Centre | 5 | Brown (live) |
| Wiring Centre | 4 | Dual Channel Programmer | 4 | Black |
| Cylinder Thermostat | C | Wiring Centre | 6 | Grey |
| Wiring Centre | 6 | Dual Channel Programmer | 3 | Grey |
| Wiring Centre | 8 | Cylinder Thermostat | 1 | Brown (live) |
| 2-Port Valve (hot water) | B | Wiring Centre | 8 | Brown (live) |
| 2-Port Valve (heating) | B | Wiring Centre | 5 | Brown (live) |
| 2-Port Valve (heating) | G | Wiring Centre | 1 | Grey |
| 2-Port Valve (hot water) | G | Wiring Centre | 1 | Grey |
| Circulation Pump | E | Wiring Centre | 3 | Green/yellow (earth) |
| Heat-Only Boiler | SL | Wiring Centre | 10 | Orange |
| Dual Channel Programmer | L | Wiring Centre | 1 | Brown (live) |
The truth table
Commissioning proves the system does what each demand asks, row by row, before it is handed over. Polarity and earth continuity come first.
| Demand | What the system does |
|---|---|
| No demand: programmer hot water off and heating off (room stat calling, cylinder cold) | Both valves stay closed; boiler off |
| Heating only: programmer heating on (room stat calling, cylinder cold) | Heating valve opens, hot water valve stays closed; boiler fires and the pump runs |
| Hot water only: programmer hot water on (room stat calling, cylinder cold) | Hot water valve opens, heating valve stays closed; boiler fires and the pump runs |
| Heating and hot water: programmer both on (room stat calling, cylinder cold) | Both valves open; boiler fires and the pump runs |
| Programmer both on, both stats satisfied | Both valves closed; boiler off |
Where it connects
Lessons that start from it
- The Draw Warm Method: S-Plan
Foundation · Unit 5
- Quick Jobs
Quick Jobs
- Fault Vault jobs: S-Plan
Fault Vault jobs: S-Plan
- Using the simulator
Foundation · Unit 0
- Reading a circuit
Foundation · Unit 1
- Testing
Foundation · Unit 7
Other systems
Back to every wiring diagram.
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.
Read the explanation
- How the wiring centre’s numbered blocks tell the story of an S-Plan
An 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.
- 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.
- 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.
- What a truth table proves when you commission a heating system
Commissioning 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.
Practise it in the simulator.
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