How to read the wiring diagram on the back of a programmer or thermostat

The terminal map and the printed screws

Many programmers and timeswitches come in two halves. The BACK-PLATE is screwed to the wall and holds the terminals. The front holds the clock, the buttons and the switches, and plugs onto the plate.

The map is printed on the plate, on the back of the front, or in the installation sheet. It lists every screw by the name or number printed beside it.

Many units share a UNIVERSAL BACK-PLATE. The screws sit in the same places, so one maker’s unit can often replace another’s on the same plate. But the plate only tells you where each screw is. What each screw does depends on the unit fitted to it. Two units on identical plates can switch ON on different screws.

So read the map of the unit you are fitting, every time. A habit from the last job is how a wire lands on the wrong screw.

Before any front comes off on site, the circuit is isolated, locked off and proved dead. See safe isolation in five steps.

L and N, then the numbered outputs

L and N feed the unit. They run its clock and screen, and on most units its switches too.

E is often a parking screw. Many programmers are DOUBLE INSULATED: they need no earth of their own. The screw holds the cable’s earth core, sleeved green-and-yellow, so it stays continuous and safe. Never cut the earth core off.

The numbered outputs are the switches. A dual channel programmer has two channels: hot water (HW) and central heating (CH). On most, each channel is a CHANGEOVER switch. It puts its live on the channel’s ON terminal or its OFF terminal, never both.

A common layout on a universal back-plate is 1 for HW OFF, 2 for CH OFF, 3 for HW ON and 4 for CH ON. Common, not universal: some units use other numbers altogether. An S-Plan uses only the two ON terminals. A Y-Plan uses HW OFF too, where the programmer can select heating only, to tell its valve that hot water is not wanted.

Remember that an OFF terminal is live whenever its channel is off. A wire landed on OFF by mistake runs that zone only while the clock says off.

Mains-switched against volt-free

This is the line on the map that matters most.

• MAINS-SWITCHED: the switches are fed from L inside the unit. An ON terminal gives out the mains live, with nothing more to wire.
• VOLT-FREE: the switches have no supply of their own. Each has a COMMON (COM), and it gives out only what you feed into that common.

A volt-free switch can control a 230 volt (230 V) system, or one whose controls run at EXTRA-LOW VOLTAGE, such as 24 volts (24 V), without mixing the two. If the map shows a COM terminal for a channel, treat that channel as volt-free until the sheet says otherwise.

The installer’s link

On a volt-free unit wired to a 230 V system, the commons need a live. The installer fits a LINK: a short, sleeved conductor from L to the common, as the maker’s sheet shows.

Without the link, the clock runs, the screen says ON, and nothing comes out of the output. With an extra-low voltage system, the maker’s sheet says the opposite: the link must not be fitted. Mains on a low-voltage input can damage what it feeds.

So the link is a decision you read from the map. It is never a habit.

A thermostat is a switch

A room thermostat (stat) or a cylinder thermostat is a switch worked by temperature. Its map uses the same words as any switch:

• COM, the common: the way in.
• NO, normally open: open at rest, closed when the stat CALLS for heat.
• NC, normally closed: closed at rest, open on a call. Some stats call it the satisfied contact.

“Normally” means at rest: the state the switch is in when nothing operates it. The call is always the contact that closes when heat is wanted, whatever number the maker prints beside it.

A mechanical mains room stat often shows L in, a switched live out, and an N. The N feeds a tiny heater inside it, the ACCELERATOR, which makes it switch off a little early so the room does not overshoot.

A digital stat on batteries is usually volt-free. It has no L or N, only COM, NO and perhaps NC. Its common must be fed. On a 230 V system it is fed from the programmer’s heating output, so the clock still decides when.

So a volt-free stat that shows a call with nothing on its output may simply have nothing on its common. Check what feeds the common first.

The same map on the wiring centre

Every screw on a map has a job. Every block on a wiring centre has a job. Reading a diagram is matching the two.

On an S-Plan wired the DrawWarm Method’s way:

• the programmer’s L goes to block 1, the permanent live, because its clock must run all day; N to block 2; E to block 3;
• CH ON goes to block 4, and on to the room stat’s way in;
• the room stat’s call goes to block 5, and on to the heating valve;
• HW ON goes to block 6, and on to the cylinder stat’s common;
• the cylinder stat’s call goes to block 8, and on to the hot water valve.

Name the job of each screw from its map, then land it on the block that does that job. See how the wiring centre’s blocks tell the story.