S-Plan or Y-Plan? The wiring difference, block by block

Two systems, one job

Both systems heat the radiators and a hot water cylinder from one boiler. Both have a programmer, a room thermostat (stat) and a cylinder thermostat. Both are FULLY PUMPED: the pump drives the water to the radiators and to the coil inside the cylinder.

What differs is the valve that shares the boiler’s water out.

• An S-PLAN has two 2-port ZONE VALVES. Each one opens or shuts one pipe: one feeds the radiators, the other the cylinder’s coil. Each valve answers its own thermostat.
• A Y-PLAN has one 3-PORT MID-POSITION VALVE. The water comes in at one port. It leaves for the radiators, for the cylinder, or for both at once from its MID position.

So an S-Plan’s two valves each work alone. A Y-Plan’s one valve has to decide between the heating and the hot water. Most of the wiring differences come from that one decision.

The blocks they share

The DrawWarm Method fills the wiring centre in the order of its numbers, which is the order the system works. Both systems start the same way:

• Blocks 1, 2 and 3: the supply. The permanent live, the neutral and the earth.
• Block 4: the programmer’s central heating ON (CH ON), on to the room stat’s way in.
• Block 5: the room stat’s call, on to the valve. On an S-Plan it drives the heating valve’s brown core. On a Y-Plan it drives the 3-port valve’s white core.
• Block 6: the programmer’s hot water ON (HW ON), on to the cylinder stat’s common.

So a heating call means the same thing on both systems: a live on block 5. Only the core it drives changes colour.

These numbers are a CONVENTION found in widely used makers’ diagrams, not a standard. On site, read the label in the box and the diagram for the job.

Where they part: blocks 7, 8 and 10

On an S-Plan:

• Block 7 stays empty.
• Block 8 takes the cylinder stat’s call on to the hot water valve’s brown.
• Block 10 takes both valves’ orange cores, with the wire on to the boiler’s switched live.

On a Y-Plan:

• Block 7 means hot water is NOT wanted. The programmer’s hot water OFF and the cylinder stat’s satisfied contact come in. The valve’s grey goes on.
• Block 8 means hot water is wanted, and it is the boiler’s block. The cylinder stat’s call and the valve’s orange come in. The boiler’s switched live goes on.
• Block 10 stays empty.

So the cylinder stat uses two of its terminals on an S-Plan, its common and its call, and all three on a Y-Plan. The programmer uses HW ON and CH ON on an S-Plan. A Y-Plan adds HW OFF, where the programmer can select heating only.

On either system, a boiler with a pump overrun may have its pump live on block 9, and nothing else there. See pump overrun.

What fires the boiler

On an S-Plan, only the valves fire the boiler. Each valve’s END SWITCH closes once the valve is fully open. Then the permanent live on its grey core comes out on its orange. Either orange on block 10 fires the boiler.

On a Y-Plan, the cylinder stat’s call fires the boiler whenever hot water is wanted, with the valve at B or at MID. The valve’s orange fires it only at A, when heating is the only thing wanted.

On both systems, with nothing calling, nothing fires the boiler: the BOILER INTERLOCK. See boiler interlock. Prove every row before you hand the system over. See what a truth table proves.

The habit that does not carry over

On an S-Plan, both valves’ grey cores go on block 1, the permanent live. Each one waits there at its end switch. See why grey is the permanent live.

On a Y-Plan, nothing on the valve is a permanent live. White is switched by the room stat. Grey is switched by the programmer and the cylinder stat. Orange is live only at A. Each of them has to switch off, and on block 1 it never would.

Wiring a Y-Plan with S-Plan habits is a common way this goes wrong. See what white, grey and orange do.

Which one is in front of you

Start with the motorised valves, often near the cylinder. Look only: nothing needs opening for this.

• Two valves, each on one pipe, usually mean an S-Plan. A third valve with its own room stat usually means an S-PLAN PLUS: a second heating zone.
• One valve with three pipes into one body usually means a Y-Plan.

Two cautions. A W-Plan also has one 3-port valve, a DIVERTER, which sends the water one way or the other, never both. A C-Plan has a single 2-port valve, on the hot water only. So read the label on the valve’s head as well.

The wiring centre confirms it, once the system is isolated and proved dead. In the common numbering, a Y-Plan’s boiler switched live sits on block 8 with an orange, and block 7 holds a grey. An S-Plan’s sits on block 10 with two oranges, and block 7 is empty.

One order for both

Learn one system block by block, and the other changes only at blocks 7, 8 and 10. The DrawWarm Method teaches the S-Plan first, then the Y-Plan in the same order.

The same order finds a fault. Follow the demand from block 4 onwards with the meter. The fault lies between the last point with 230 volts (230 V) and the first without. See last 230 V, first 0 V.