🛠 How to Wire an S-Plan Heating System — Full Technical Guide (With Diagram)
Version 1.1 (Must have Electrical qualification)
If you're installing a heating system with separate control for hot water and central heating, then the S-Plan system is likely the configuration you need. This post walks you through how to wire an S-Plan setup using the diagram shown above — with two 2-port zone valves, room and cylinder thermostats, a programmer, and a conventional boiler and pump.
🔧 What Is an S-Plan System?
An S-Plan system uses two motorised zone valves — one for heating and one for hot water — allowing independent control. Each zone valve contains an internal end switch that signals the boiler and pump to operate only when there’s demand. It’s one of the most popular configurations in UK heating systems, especially where a hot water cylinder is involved.
🔍 Understanding the Components
| Component | Role |
|---|---|
| Programmer (2-Channel) | Controls ON/OFF times for HW and CH |
| Room Thermostat | Detects air temperature and calls for heat |
| Cylinder Thermostat | Monitors hot water temperature in the cylinder |
| Zone Valves (x2) | One for CH, one for HW — opens when thermostat calls |
| Pump | Circulates water through heating and hot water pipework |
| Boiler | Heats the water when signalled via the zone valve(s) |
| Wiring Centre | Central connection block for system wiring |
| 3A Fused Spur | Power supply for the whole system |
⚙️ How It Works (Overview)
- The programmer sends power to the relevant thermostat based on time settings.
- If there's a demand for heat, the thermostat sends power to the appropriate zone valve.
- When the valve fully opens, its end switch closes, sending 230V to the boiler and pump.
- Boiler fires up and the pump starts circulating water.
🔌 Terminal Breakdown
| Terminal No. | Function |
|---|---|
| 1 | Neutral (N) – common to all components |
| 2 | Earth (G/Y) |
| 3 | Permanent Live (L) |
| 4 | HW ON (from programmer) |
| 5 | CH ON (from programmer) |
| 6 | Common (C) from cylinder stat |
| 7 | HW call from cylinder stat |
| 8 | CH call from room stat |
| 9 | Switched Live from valves (orange) |
| 10 | Output to boiler/pump live |
🔌 Step-by-Step Wiring Instructions
🔋 1. Power Supply
- Connect a 3A fused spur.
- Live (brown) → Terminal 3
- Neutral (blue) → Terminal 1
- Earth (G/Y) → Earth block
📅 2. Programmer Wiring
- Neutral → Terminal 1
- Live → Terminal 3
- HW ON → Terminal 4
- CH ON → Terminal 5
This allows time-controlled output to HW and CH channels independently.
🌡 3. Room Thermostat Wiring
(Check your Programmer/Thermostats installation guide to confirm)
- Terminal 1 → From Terminal 5 (CH ON from programmer)
- Terminal 3 → To Terminal 8 (CH call for heat)
- Terminal 2 → Neutral (if used for anticipator)
- Earth → Earth bar
The stat closes the circuit between terminals 1 and 3 when below set temp.
💧 4. Cylinder Thermostat Wiring
- Common (C) → Terminal 6 (linked to Terminal 4 – HW ON)
- Call (1) → Terminal 7
- Earth → Earth bar
When HW is below setpoint, it closes between C and 1 and sends live to the 2-port HW Valve.
🔁 5. Motorised Zone Valves
🟠 Hot Water Valve (2-port)
- Brown → Terminal 7 (HW call for heat)
- Grey → Terminal 3 (Permanent Live)
- Orange → Terminal 9 (Switch Live output to boiler)
- Blue → Terminal 1 (Neutral)
- Earth → Earth bar
🔵 Heating Valve (2-port)
- Brown → Terminal 8 (CH call for heat)
- Grey → Terminal 3
- Orange → Terminal 9
- Blue → Terminal 1
- Earth → Earth bar
Both valves operate the same: Brown opens valve → when fully open, orange goes live via internal switch.
💨 6. Pump Wiring
- Live → Terminal 10
- Neutral → Terminal 1
- Earth → Earth bar
Pump runs only when terminal 10 is live (via valve end switches).
🔥 7. Boiler Wiring
- Live (Switched) → Terminal 10
- Neutral → Terminal 1
- Earth → Earth bar
- Optional: Some boilers use volt-free contacts — in this case, terminal 10 can be connected to the boiler's volt-free demand input (consult manufacturer)
🧠 How It All Comes Together
🟡 When Hot Water Is Demanded:
- Programmer sends power to Terminal 4 (HW ON).
- This powers the cylinder stat’s Common (C).
- If the tank is below set temp, power flows to Terminal 7.
- Terminal 7 feeds the brown wire of the HW zone valve.
- Valve opens, internal switch closes, orange wire becomes live.
- Terminal 9 → Terminal 10 → Boiler and pump start.
🔴 When Central Heating Is Demanded:
- Programmer sends power to Terminal 5 (CH ON).
- Power goes to Room Stat terminal 1.
- If room is cold, stat sends power from terminal 3 → Terminal 8.
- Terminal 8 powers CH zone valve.
- Valve opens → orange goes live → boiler and pump start.
🧪 Testing & Commissioning Tips
- Use a multimeter to verify live voltages at key terminals: 4, 5, 7, 8, 9, and 10.
- Test each circuit (CH and HW) separately before testing both together.
- Check valves open and return smoothly.
- Ensure boiler fires and pump runs only when needed.
- Confirm earth continuity and neutral connections are correct.
⚠️ Common Faults & Fixes
| Problem | Cause | Fix |
|---|---|---|
| Boiler always running | Orange wire live all the time | Stuck valve or faulty end switch |
| Heating or HW not coming on | No live to brown wire on valve | Faulty thermostat or programmer |
| Boiler not firing even though valve is open | Faulty end switch | Replace valve head or microswitch |
| No power to pump | Check Terminal 10 output | Verify orange and grey connections |
✅ Final Notes
This diagram represents a classic, fully compliant S-Plan wiring system. It's simple, scalable, and supports future upgrades such as:
- Smart thermostats (Nest, Hive, etc.)
- Additional zones (S-Plan+)
- Weather compensation or load compensation
If you follow this wiring step-by-step and use a good wiring centre, fault-finding and commissioning will be straightforward. Remember to label wires clearly and always test with the circuit isolated.