Y-Plan Wiring Guide
What Is Y-Plan?
Y-plan is an older heating system arrangement that uses a single 3-port mid-position motorised valve to control both the central heating and hot water circuits from one unit. Still commonly found on existing properties — important to recognise and wire correctly.
Y-plan wiring: single 3-port mid-position valve, boiler, cylinder thermostat, and room thermostat. Source: flameport.com
The Three-Port Valve — Three Positions
| Position | Condition | What it does |
|---|---|---|
| Port A — heating only | Room stat calling, cylinder stat satisfied | Diverts flow to radiators only |
| Port B — hot water only | Cylinder stat calling, room stat satisfied | Diverts flow to hot water cylinder only |
| AB — mid-position (both) | Both stats calling | Flow to both radiators and cylinder |
| Closed | No demand | Valve closed — boiler and pump off |
Because one valve controls both circuits, a fault with the valve can affect both heating and hot water simultaneously. Unlike S-plan, where two independent valves fail independently.
System Components
| Component | Function |
|---|---|
| Room thermostat | Signals heating demand |
| Cylinder thermostat | Signals hot water demand |
| Programmer / time clock | Sets on/off schedules |
| 3-port mid-position valve | Routes flow to heating, hot water, or both |
| Boiler | Fires when the valve signals demand via the orange or white wire |
Zone Valve Wiring
Standard 3-port mid-position valve wiring (Honeywell V4073 / Drayton HM — most common):
| Terminal | Wire colour | Function |
|---|---|---|
| N | Blue | Neutral |
| 1 | Orange | Heating call — live from room thermostat |
| 2 | White | Boiler/pump call (auxiliary switch output) |
| 3 | Grey | Hot water call — from cylinder thermostat |
| 4 | Green/Yellow | Earth |
| 5 | Brown | Permanent live in |
The white wire (terminal 2) is the boiler call signal — it makes when the valve is not fully in the hot-water-only position, i.e. when any heating demand exists.
Replacing Controls on a Y-Plan System
When fitting a Heatmiser neoStat on a Y-plan system:
- Photograph the existing wiring before disconnecting anything
- Isolate at the consumer unit
- Identify the 3-port valve — do not confuse the orange (heating call) and grey (hot water call) wires
- The room thermostat controls the orange wire (terminal 1 — heating call)
- The new neoStat replaces the room thermostat only — wire its SL output in place of the old thermostat's switched live to terminal 1
- Do not disturb the cylinder thermostat or programmer wiring to terminals 2, 3, or 5
- Power up and confirm the valve moves to mid-position when both heating and hot water call simultaneously
Y-Plan vs S-Plan — Identifying Which System
| Feature | S-Plan | Y-Plan |
|---|---|---|
| Number of zone valves | Two (2-port) | One (3-port mid-position) |
| Valve failure effect | One zone affected | Can affect both zones |
| Typical age | Post-1990s installs | Pre-1990s installs — still widespread |
| Wiring complexity | Simpler | More complex |
If in doubt — count the valves. One valve = Y-plan, two valves = S-plan.
Safe Working
- Isolate at the consumer unit before working
- Photograph existing wiring before disconnecting anything
- The mid-position valve carries mains voltage — always isolate before disconnecting terminals
- Never work on gas connections, the boiler heat exchanger, or burner