Why grey is the permanent live on a 2-port zone valve, and brown is not

The five cores

A 2-port motorised ZONE VALVE has two parts. The body sits in the pipe and opens or shuts one route for the water: the radiators, or the coil in the hot water cylinder. The HEAD sits on top. Inside the head are a small motor, a SPRING and an END SWITCH.

The head’s cable usually has five cores. On most UK valves they are:

• Brown: the motor’s live.
• Blue: the motor’s neutral.
• Green-and-yellow: the earth.
• Grey: a live into the end switch.
• Orange: the live out of the end switch.

Brown, blue and green-and-yellow are the colours BS 7671, the IET Wiring Regulations, gives fixed wiring for line, neutral and the protective conductor. The valve’s own lead is part of the product, so its colours are the maker’s. Grey and orange for the switch are a convention most makers follow, not a standard. A few valves use other colours, so read the label on the head before you wire it. Some heads have a sixth core, usually white. An S-Plan does not need it: make it electrically safe as the maker’s instructions say, landed where it can touch nothing.

Brown drives the motor and the spring shuts it

Put 230 volts (230 V) on brown, with blue as its neutral, and the motor drives the valve open. It takes a few seconds, sometimes longer. Take the live away and the spring shuts it again.

So brown must be live only while heat is wanted in that zone. It is a SWITCHED LIVE. On an S-Plan the heating valve’s brown is fed by the room thermostat’s call, and the hot water valve’s brown by the cylinder thermostat’s call.

Why it matters: the valve’s position is the system’s answer to one question, “is heat wanted here?”. A brown that was always live would hold the valve open, and that zone would take heat whenever the boiler ran for anything.

Grey feeds the end switch all the time

The end switch closes only at the very end of the motor’s travel, when the valve is fully open. Grey brings a live to one side of it. On an S-Plan both greys go on the wiring centre’s PERMANENT LIVE block, with the supply. They wait there, live, while the valves are shut.

Why permanent? The end switch is the valve’s message to the boiler: “I am fully open.” It needs a live to send that message with. A permanent live works whichever thermostat opened the valve, and for as long as the valve stays open.

This is the point many people find odd at first. The core that is live all the time is not the one that makes the valve move.

Orange leaves the end switch once the valve is fully open

When the valve reaches fully open, the end switch closes and grey’s live comes out on orange. On an S-Plan both valves’ orange cores land on one block, with the wire to the boiler’s switched live. Either valve fully open fires the boiler. That is the BOILER INTERLOCK: no open valve, no live orange, no boiler.

The end switch opens again as soon as the valve starts to close. So the boiler stops as the zone shuts.

Inside the usual head, the end switch is separate from the motor: orange never meets brown. A live arriving on the shared block from the other valve cannot drive this valve’s motor.

The core easiest to wire wrong

Grey. It looks like one more switched live, so it gets landed with brown, or brown and grey get swapped.

Swap them, and the motor gets the permanent live. The valve is driven open whenever the supply is on, whatever its thermostat says. The end switch now waits on the thermostat’s call instead. The system may still fire, so the mistake can hide.

Commissioning catches it. In the “hot water only” row of the truth table, the heating valve must stay shut. A valve that sits open in a row where nothing asked for it tells you to read its cores again. See what a truth table proves.

On site, a grey or black core in a three-core-and-earth cable often carries a switched live. Sleeving it brown at both ends lets the next engineer read what it carries. In older wiring, from before the colours changed in 2004 to 2006, red is the line and black the neutral, so a black core can be either. A colour can be wrong: you find what a core carries by test, not by its colour.

What the meter reads on a working valve

These are live tests. Regulation 14 of the Electricity at Work Regulations 1989 allows them only where the reading cannot reasonably be taken dead, it is reasonable to take it live, and suitable precautions are taken. Use test leads made to the HSE’s guidance note GS38: finger guards, metal tips of 4 mm or less (2 mm where practicable) and fused leads. Put the black probe on the valve’s own neutral, the blue, so you read the voltage the head actually gets.

• Grey to neutral: about 230 V whenever the supply is on, valve open or shut.
• Brown to neutral: about 230 V only while its thermostat calls and the programmer allows it. Otherwise 0 V.
• Orange to neutral: about 230 V only once the valve is fully open.

One trap: the orange cores share a block. With the other zone running, this valve’s orange can read live even while this valve is shut. Ask for one zone at a time when you read an orange.

Before any work on the head, isolate the circuit, lock it off and prove it dead, the switched lives included. See safe isolation in five steps.