Explained · updated 5 November 2026
Y-Plan explained: what white, grey and orange do on a mid-position valve
In short
A Y-Plan’s 3-port valve rests at hot water. A live on white drives it to the mid position, both ports open. White and grey together drive it on to heating only. Grey means hot water is not wanted. Orange is live only at heating only, where it fires the boiler. None of its cores is a permanent live.
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.
Live tests. On site, any reading taken with the supply on is work on or near live parts. Regulation 14 of the Electricity at Work Regulations 1989 allows it only when working dead is unreasonable, as when a fault shows only while the controls work, and only with suitable precautions: test leads made to the HSE’s guidance note GS38, fingers behind the guards. Regulation 16 allows it only for a competent person, or a learner under the supervision of one. Wiring and repairs are always done dead. In the simulator, nothing is live.
One valve, three positions
A Y-PLAN is a fully pumped system with one 3-PORT MID-POSITION VALVE, where an S-Plan has two zone valves. The water from the boiler comes in at one port. It leaves by port A to the radiators, by port B to the coil in the hot water cylinder, or by both at once.
The valve can sit in three places:
- B: hot water only. With white and grey both dead, a SPRING returns it here.
- MID: the mid position, both ports open: heating and hot water together.
- A: heating only.
Its head holds a motor, the spring and a switch. Its cable usually has five cores: white, grey, orange, blue for the neutral and green-and-yellow for the earth. White, grey and orange are a convention most makers follow, not a standard. Some valves use brown for the heating call or red for the boiler’s live, and some older heads have a sixth core, so read the label on the head.
White calls for heating
White is the heating call. On a Y-Plan the room thermostat’s call goes on to the valve’s white core.
A live on white alone drives the valve from B to MID and stops there. Why stop halfway? Because grey is dead, and a dead grey means hot water is still wanted. So port B must stay open, and the valve shares the water between the radiators and the cylinder.
Grey means hot water not wanted
Grey is the core people get backwards. It is not a hot water call. It is the opposite: a live on grey means hot water is NOT wanted right now.
Two things can say that, and either one is enough:
- the programmer’s hot water OFF terminal, live while the hot water is timed off (wired only where the programmer can select heating only, as the valve maker’s sheet says);
- the cylinder thermostat’s satisfied contact, live once the cylinder is hot.
With white and grey both live, the valve drives on past MID to A: heating only, port B shut. Grey on its own calls for nothing. It only changes where a heating call takes the valve. On the common valve of this type, grey alone holds the valve where it last stopped, its motor warm, while the boiler stays off. Its maker’s instructions describe this as normal.
A warning for the meter. With the hot water timed off and the cylinder hot, the programmer’s hot water OFF live sits on the satisfied contact. Through the closed contact it reaches the thermostat’s common, and runs back along the hot water ON wire towards the programmer. That is a BACKFEED. So 230 volts (230 V) on that wire does not prove the hot water is timed on.
Orange fires the boiler
On the usual 3-port valve, orange comes from a switch in the head that makes only once the valve reaches A, driven there by white and grey together. So when the heating is the only thing wanted, the heating’s own call fires the boiler, through orange, once the valve has reached A.
At B and at MID, hot water is wanted, so the cylinder thermostat is calling. Its call fires the boiler instead. That is why the cylinder thermostat’s call and the valve’s orange share one block, with the wire on to the boiler’s switched live.
| The demand | Where the valve sits | What fires the boiler |
|---|---|---|
| Hot water wanted, no heating call | B, at rest | The cylinder thermostat’s call |
| Hot water wanted and a heating call | MID | The cylinder thermostat’s call |
| A heating call, hot water not wanted | A | The valve’s orange |
| Nothing wanted | B, or held where it last stopped | Nothing: the boiler stays off |
That last row is the BOILER INTERLOCK: with no call from either thermostat, nothing can fire the boiler, wherever the valve sits.
Nothing on a 3-port valve is a permanent live
On an S-Plan, each zone valve’s grey is a permanent live, waiting at its end switch. See why grey is permanent on a 2-port valve. On a Y-Plan, nothing on the valve is.
- White is switched by the room thermostat.
- Grey is switched by the programmer and the cylinder thermostat.
- Orange is live only at A.
So the permanent live block of a Y-Plan holds only the supply, the programmer’s L and, where it needs one, the boiler’s permanent live. Never land white, grey or orange there. Each of them has to switch off, and on the permanent live it never would. Moving to a Y-Plan with S-Plan habits is a common way this goes wrong.
Blocks 7 and 8
On a wiring centre numbered the common way (a convention in makers’ diagrams, not a standard), a Y-Plan shares blocks 1 to 6 with an S-Plan: the supply on 1 to 3, the heating on 4 and 5, the hot water demand on 6. Then it adds two blocks with more than a pair on them.
- Block 7 is “hot water not wanted”. The programmer’s hot water OFF, where it is wired, and the cylinder thermostat’s satisfied contact come in. The valve’s grey goes on.
- Block 8 is the boiler’s block. The cylinder thermostat’s call and the valve’s orange come in. The boiler’s switched live goes on, with the pump’s live where the boiler has no pump overrun.
On an S-Plan, block 7 is left empty, so the numbers the two systems share keep the same jobs. Some makers’ guides number the blocks differently, so on site, read the label in the box. Then commission every row of the table above before you leave. See what a truth table proves.
Where it connects
- Y-Plan wiring diagram
Wiring diagram
- 3-Port Mid-Position Valve
The part
- The Draw Warm Method: Y-Plan
Foundation · Unit 6
Read next
- How the wiring centre’s numbered blocks tell the story of an S-Plan
An S-Plan wired in the order it works: the supply on blocks 1 to 3, heating on 4 and 5, hot water on 6 and 8, the valves’ orange cores on 10.
- Why grey is the permanent live on a 2-port zone valve, and brown is not
A 2-port zone valve: brown drives the motor, grey feeds the end switch all the time, orange fires the boiler. Why grey, not brown, is the permanent live.
- Wiring a wireless thermostat to a combi boiler: the receiver, the link and the room-stat terminals
A combi needs only its supply and a call on its room-stat terminals. The receiver’s relay replaces the link; check volt-free against mains-switched first.
Practise it in the simulator.
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