How the wiring centre’s numbered blocks tell the story of an S-Plan

What a wiring centre is

A WIRING CENTRE is a box of numbered connector BLOCKS. Every cable in the system runs to it: the supply from the fused spur, the programmer, both thermostats, both zone valves, the boiler and the pump. Each part needs one cable back to the box, and nothing else.

Each block is a plain connector. Every wire landed on a block is joined to every other wire on that block. In a plain wiring centre, no block is joined to another and nothing switches; some have links inside, cut to suit the plan.

A block’s number does nothing by itself. The numbers are a CONVENTION, not a standard: the place you agree to land each job, so the next engineer can open the lid and read the system. The numbering below matches the 10-way junction box in widely used makers’ S-Plan diagrams, and the DrawWarm Method uses it. Some guides number the blocks another way. On site, read the label in the box and the diagram for the job.

Blocks 1 to 3: the supply

Wire the supply first, out of the way.

• Block 1 is the PERMANENT LIVE: 230 volts (230 V) all the time the spur is on. The fused spur’s live comes in here. Then every part that needs a live all day goes on it: the programmer, so its clock keeps time; the boiler, where it needs a permanent live; and each zone valve’s grey core, which waits at the valve’s end switch.
• Block 2 takes every neutral. Every part that uses electricity needs a way back to the supply.
• Block 3 takes every earth core, including those that run to double-insulated parts and are only parked there.

Never land anything on block 1 that must switch off. A thermostat’s feed or a valve’s brown on block 1 would run all the time, whatever the programmer says.

Blocks 4 and 5: the heating

The heating happens in a set order, and the blocks follow it.

The programmer’s central heating ON (CH ON) terminal goes live when the heating is timed on. That live is the heating DEMAND. It lands on block 4, and block 4 takes it on to the room thermostat’s live in.

When the room is colder than its setting, the room thermostat (stat) CALLS for heat. Its switched live goes live, and lands on block 5. Block 5 takes it on to the heating valve’s brown core, which drives the valve’s motor. The valve opens.

The thermostat is fed from block 4, never from block 1. Why: the programmer is the system’s clock. A thermostat fed from the permanent live would ignore the clock, and the heating could never be timed off.

Blocks 6 and 8: the hot water

The hot water tells the same story two blocks along. The programmer’s hot water ON (HW ON) terminal lands on block 6, and block 6 feeds the cylinder thermostat’s common. While the stored water is cool, the cylinder thermostat calls. Its call lands on block 8, and block 8 drives the hot water valve’s brown.

Block 7 stays empty on an S-Plan. A Y-Plan uses block 7 for its “hot water not wanted” signal, so with it empty here, the numbers both systems share keep the same jobs. Block 9 is spare: the DrawWarm Method keeps it for a second heating zone, and some makers’ diagrams put a pump-overrun boiler’s pump live there.

Block 10: the oranges fire the boiler

Each zone valve has a small END SWITCH in its head. It closes only when the valve is fully open. Then the grey core’s permanent live comes out on the orange core.

Both valves’ orange cores land on block 10, together with the wire to the boiler’s switched live. Whichever valve opens fully, the boiler fires. With no valve open, block 10 is dead and the boiler stays off. That is the BOILER INTERLOCK: the boiler runs only when its heat has somewhere to go.

On a boiler with a pump live terminal, the pump takes its live from the boiler. So the boiler can keep it running after the burner stops, to carry the last heat away: the PUMP OVERRUN. If that pump live runs through a spare block, the block carries the pump live and nothing else. On a boiler without a pump live terminal, the pump’s live usually joins block 10, so it runs whenever a valve is fully open.

The pair rule: one wire in, one wire on

From block 4 on, most blocks hold a PAIR. One wire brings the power in. One wire takes it on to the next part.

• Block 4: the programmer’s demand in, on to the room thermostat.
• Block 5: the thermostat’s call in, on to the heating valve.
• Block 6: the programmer’s hot water demand in, on to the cylinder thermostat.
• Block 8: the cylinder thermostat’s call in, on to the hot water valve.

Block 10 is the exception: two oranges in, one wire on to the boiler, and the pump’s too where the boiler has no pump overrun.

The pair rule turns a crowded box into something you can check by eye. Three wires where you expect two, or one where you expect two, is worth a second look before the power goes on.

Why the order matters on site

Fill the blocks in the order of their numbers and you remember a story, not a list of wires. Each block follows the electricity one step further. That is why the order can be wired from memory, then checked against the maker’s diagram.

The same order does three more jobs:

• Commissioning. You prove the system in the order it works. See what a truth table proves.
• Fault finding. A heating demand travels block 4, block 5, the valve’s brown, its orange, block 10, the boiler. Follow it with the meter, and the fault lies between the last point with 230 V and the first without. See last 230 V, first 0 V.
• Safety. Most of the system’s lives meet in this one box. Isolate, lock off and prove it dead before the lid comes off. See safe isolation in five steps.