S-Plan · wiring diagram
S-Plan with a frost thermostat 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.
The wired S-Plan with a frost thermostat added: a permanent live from block 1, its call to block 5, set at 4 °C. There is no pipe stat on this canvas: on site a pipe stat on the return is wired in series with it.
An S-Plan wired in the simulator; this page’s circuit is listed wire by wire below. 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, Frost Thermostat 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, Frost Thermostat 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, Frost Thermostat SL |
| 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) |
| Wiring Centre | 1 | Frost Thermostat | L | Brown (live) |
| Wiring Centre | 2 | Frost Thermostat | N | Blue (neutral) |
| Frost Thermostat | SL | Wiring Centre | 5 | 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 |
| Frost: programmer off, the frost thermostat calling | Heating valve opens; boiler fires and the pump runs |
Where it connects
Lessons that start from it
- Fault Vault jobs: S-Plan
Fault Vault jobs: S-Plan
- Protection and extras
Intermediate · Unit 6
Other systems
- S-Plan
S-Plan
- S-Plan with a digital room stat
S-Plan
- Wireless S-Plan
S-Plan
- S-Plan Plus
S-Plan
- Y-Plan
Y-Plan
- Y-Plan with a frost thermostat
Y-Plan
- Combi boiler with a wireless thermostat
Combi
- Combi boiler with two heating zones
Combi
Back to every wiring diagram.
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.
Practise it in the simulator.
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