Y-Plan · 8 parts · 27 wires
Y-Plan wiring diagram
In short
A Y-Plan is a fully pumped system with one 3-port mid-position valve. It rests at the hot water port, moves to mid-position when the room thermostat calls, and to the heating-only port when hot water is satisfied or timed off. The white and grey cores drive it; the orange core and the cylinder stat fire the boiler.
A working Y-Plan 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 valve white |
| 6 | HW ON from the programmer, out to the cylinder thermostat C |
| 7 | HW OFF from the programmer and the cylinder thermostat satisfied contact, out to the valve grey |
| 8 | Cylinder thermostat call and the valve orange in, boiler switched live out |
| 9 | Spare |
| 10 | Spare |
| 11 | Spare |
| 12 | Spare |
Every wire, terminal by terminal (27)
| # | From | Terminal | To | Terminal | Colour |
|---|---|---|---|---|---|
| 1 | Fused Spur | L | Wiring Centre | 1 | Brown (live) |
| 2 | Fused Spur | N | Wiring Centre | 2 | Blue (neutral) |
| 3 | Fused Spur | E | Wiring Centre | 3 | Green/yellow (earth) |
| 4 | Circulation Pump | E | Wiring Centre | 3 | Green/yellow (earth) |
| 5 | Dual Channel Programmer | E | Wiring Centre | 3 | Green/yellow (earth) |
| 6 | Circulation Pump | N | Wiring Centre | 2 | Blue (neutral) |
| 7 | Room Thermostat | N | Wiring Centre | 2 | Blue (neutral) |
| 8 | Dual Channel Programmer | N | Wiring Centre | 2 | Blue (neutral) |
| 9 | Wiring Centre | 1 | Dual Channel Programmer | L | Brown (live) |
| 10 | Dual Channel Programmer | 4 | Wiring Centre | 4 | Brown (live) |
| 11 | Wiring Centre | 4 | Room Thermostat | L | Brown (live) |
| 12 | Room Thermostat | SL | Wiring Centre | 5 | Brown (live) |
| 13 | Dual Channel Programmer | 3 | Wiring Centre | 6 | Brown (live) |
| 14 | Cylinder Thermostat B | C | Wiring Centre | 6 | Brown (live) |
| 15 | Dual Channel Programmer | 1 | Wiring Centre | 7 | Brown (live) |
| 16 | Cylinder Thermostat B | 2 | Wiring Centre | 8 | Brown (live) |
| 17 | Heat-Only Boiler | L | Wiring Centre | 1 | Brown (live) |
| 18 | Heat-Only Boiler | N | Wiring Centre | 2 | Blue (neutral) |
| 19 | Heat-Only Boiler | E | Wiring Centre | 3 | Green/yellow (earth) |
| 20 | Wiring Centre | 8 | Heat-Only Boiler | SL | Orange |
| 21 | Heat-Only Boiler | PL | Circulation Pump | L | Brown (live) |
| 22 | 3-Port Valve | E | Wiring Centre | 3 | Green/yellow (earth) |
| 23 | 3-Port Valve | N | Wiring Centre | 2 | Blue (neutral) |
| 24 | 3-Port Valve | W | Wiring Centre | 5 | Grey |
| 25 | 3-Port Valve | G | Wiring Centre | 7 | Grey |
| 26 | 3-Port Valve | O | Wiring Centre | 8 | Orange |
| 27 | Cylinder Thermostat B | 1 | Wiring Centre | 7 | Brown (live) |
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) | Valve rests at B (hot water port); boiler off |
| HW only: programmer HW on (room stat calling, cylinder cold) | Valve stays at B (hot water port); boiler fires |
| CH only, HW timed off: programmer CH on (room stat calling, cylinder cold) | Valve drives to A (heating port); boiler fires |
| CH + HW: programmer HW on and CH on (room stat calling, cylinder cold) | Valve at mid position; boiler fires |
| CH + HW, cylinder now hot (room stat calling) | Valve moves to A (heating port); boiler fires |
| HW on but cylinder hot, CH off (room stat calling) | Boiler off (valve position not asserted by the probe) |
Related
Build it lesson by lesson
- Y-Plan Wiring course
Master the Y-Plan system with hands-on wiring. Connect the 3-port valve and see mid-position operation in action.
Other systems
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.
Your first fault is waiting.
The demo opens with the launch: leave your email and the link lands in your inbox that day.