S-Plan · 9 parts · 29 wires
S-Plan with a digital room stat 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 working S-Plan with a digital room stat in the simulator. Wire colours follow BS 7671; every terminal is live, neutral, earth or dead, and the meter reads what the circuit is doing.
Wiring centre numbering
The numbering is a convention so that the next engineer can read your work. A block carries whatever you land on it; the test harness proves the same system wired on scrambled blocks.
| Block | Carries |
|---|---|
| 1 | Permanent live from the fused spur (brown) |
| 2 | Neutral (blue) |
| 3 | Earth (green/yellow) |
| 4 | CH ON from the programmer, out to the room thermostat L |
| 5 | Room thermostat call, out to the heating valve brown |
| 6 | HW ON from the programmer, out to the cylinder thermostat C |
| 7 | Spare on an S-Plan |
| 8 | Cylinder thermostat call, out to the hot water valve brown |
| 9 | Spare |
| 10 | Both valve oranges in, boiler switched live out |
| 11 | Spare |
| 12 | Spare |
Every wire, terminal by terminal (29)
| # | From | Terminal | To | Terminal | Colour |
|---|---|---|---|---|---|
| 1 | Heat-Only Boiler | PL | Circulation Pump | L | Grey |
| 2 | Heat-Only Boiler | E | Wiring Centre | 3 | Green/yellow (earth) |
| 3 | 2-Port Valve (heating) | E | Wiring Centre | 3 | Green/yellow (earth) |
| 4 | 2-Port Valve (hot water) | E | Wiring Centre | 3 | Green/yellow (earth) |
| 5 | Dual Channel Programmer | E | Wiring Centre | 3 | Green/yellow (earth) |
| 6 | Fused Spur | E | Wiring Centre | 3 | Green/yellow (earth) |
| 7 | Fused Spur | N | Wiring Centre | 2 | Blue (neutral) |
| 8 | Circulation Pump | N | Wiring Centre | 2 | Blue (neutral) |
| 9 | 2-Port Valve (heating) | N | Wiring Centre | 2 | Blue (neutral) |
| 10 | Heat-Only Boiler | N | Wiring Centre | 2 | Blue (neutral) |
| 11 | 2-Port Valve (hot water) | N | Wiring Centre | 2 | Blue (neutral) |
| 12 | Dual Channel Programmer | N | Wiring Centre | 2 | Blue (neutral) |
| 13 | Fused Spur | L | Wiring Centre | 1 | Brown (live) |
| 14 | Heat-Only Boiler | L | Wiring Centre | 1 | Brown (live) |
| 15 | 2-Port Valve (heating) | O | Wiring Centre | 10 | Orange |
| 16 | 2-Port Valve (hot water) | O | Wiring Centre | 10 | Orange |
| 17 | Wiring Centre | 4 | Dual Channel Programmer | 4 | Grey |
| 18 | Cylinder Thermostat | C | Wiring Centre | 6 | Brown (live) |
| 19 | Wiring Centre | 6 | Dual Channel Programmer | 3 | Grey |
| 20 | 2-Port Valve (hot water) | B | Wiring Centre | 8 | Brown (live) |
| 21 | 2-Port Valve (heating) | B | Wiring Centre | 5 | Brown (live) |
| 22 | 2-Port Valve (heating) | G | Wiring Centre | 1 | Grey |
| 23 | 2-Port Valve (hot water) | G | Wiring Centre | 1 | Grey |
| 24 | Circulation Pump | E | Wiring Centre | 3 | Green/yellow (earth) |
| 25 | Heat-Only Boiler | SL | Wiring Centre | 10 | Orange |
| 26 | Dual Channel Programmer | L | Wiring Centre | 1 | Brown (live) |
| 27 | Wiring Centre | 8 | Cylinder Thermostat | 1 | Grey |
| 28 | Wiring Centre | 4 | Digital Room Stat | 1 | Brown (live) |
| 29 | Digital Room Stat | 2 | Wiring Centre | 5 | Grey |
The truth table it must pass
Commissioning is four demand states and four results. The test harness runs this table against the circuit before every release.
| Demand | Result |
|---|---|
| No demand: programmer HW off and CH off (room stat calling, cylinder cold) | Both valves stay closed; boiler off |
| CH only: programmer CH on (room stat calling, cylinder cold) | Heating valve opens, hot water valve stays closed; boiler fires and the pump runs |
| HW only: programmer HW on (room stat calling, cylinder cold) | Hot water valve opens, heating valve stays closed; boiler fires and the pump runs |
| CH + HW: programmer HW on and CH on (room stat calling, cylinder cold) | Both valves open; boiler fires and the pump runs |
| Programmer HW on and CH on, both stats satisfied | Both valves closed; boiler off |
Related
Build it lesson by lesson
- S-Plan Wiring course
Wire a complete S-Plan heating system step-by-step. Each connection is validated as you build a fully working system.
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- Combi boiler with a wireless thermostat
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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.
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