Wiring a wireless thermostat to a combi boiler: the receiver, the link and the room-stat terminals

No cylinder, no valves, no wiring centre

A COMBI boiler heats the radiators and makes hot water at the tap. Its pump and the valve that sends heat to the radiators or to the taps are inside it. Hot water usually needs no wiring from the controls: a flow sensor inside starts the boiler when a tap runs.

So the wiring outside a combi is short:

• a supply: a permanent live, a neutral and an earth, usually from a fused spur;
• a heating call, on the boiler’s room-thermostat terminals.

That is the whole job. Everything below is about getting the call right.

The room-thermostat terminals and the link

Every combi has somewhere for a room thermostat (stat) to land. Makers name the terminals in their own ways. Some print a live out and a live return; some a pair marked for the room stat; some a single input for a switched live. The installation manual is the only map that counts.

From the factory, the room-stat terminals are often joined by a LINK, a short wire. With the link in, the boiler heats the radiators whenever its own controls allow. To add a thermostat, the link comes out and the thermostat’s switch goes in its place: the boiler’s feed out to the switch’s common, the call back to the boiler.

Leave the link in, and the thermostat switches nothing. The boiler ignores it.

In England, a gas boiler fitted in an existing home needs time and temperature controls: the Boiler Plus standards of April 2018, now in Approved Document L. A gas combi also needs at least one extra energy-saving measure: flue gas heat recovery, weather or load compensation, or a smart thermostat with automation and optimisation. The timer says when the heating may run; the thermostat says whether it is needed. Often one programmable thermostat does both.

The receiver’s relay replaces the link

A wireless thermostat comes as a pair. The thermostat runs on batteries and sits anywhere in the room. The RECEIVER is a mains-powered box on the wall, usually near the boiler.

The thermostat sends its call by radio. The receiver’s RELAY, a switch worked by a small coil, closes. Those relay contacts are what take the place of the link. The thermostat itself has no wires at all: the receiver is the part you wire, and the part you test.

Volt-free against mains-switched receivers

Receivers come in two kinds, and the boiler decides which one you can use.

• VOLT-FREE: the relay’s contacts, usually COM and NO, are not joined to the receiver’s supply. They give out only what you feed into them. Across a boiler’s room-stat terminals, the boiler feeds its own contact.
• MAINS-SWITCHED: the relay is fed from the receiver’s live inside. Its output is a switched live at 230 volts (230 V).

Read the boiler’s manual before you choose. Some combis expect a 230 V switched live on their room-stat input. Others run that input at an EXTRA-LOW VOLTAGE, such as 24 volts (24 V), and accept only a volt-free contact. Mains on a low-voltage input can damage the boiler’s control board.

Where the boiler gives a pair, a feed out and a return, a volt-free contact across that pair carries only the boiler’s own voltage, whatever it is. Check the contact is rated for it. Some receivers can be set either way, by a link or a switch inside: read its sheet too.

Where the receiver’s L and N come from

The receiver needs a live and a neutral to run its radio and its relay. Without them it cannot switch, however hard the thermostat calls.

Take its supply from the same place as the boiler’s: the boiler’s own supply terminals where its manual allows, or the same fused spur. Why: then one isolator makes the boiler and its controls dead together. A receiver fed from a different circuit could put a live on the boiler’s terminals that the boiler’s own isolator does not cut off.

Many receivers are DOUBLE INSULATED. Their earth screw only parks the cable’s earth core. Land it there and keep it continuous; never cut it off.

Proving it with the meter

Isolate, lock off and prove dead before the boiler’s cover or the receiver’s front comes off. See safe isolation in five steps. When the wiring is done, make the dead tests first: earth continuity to everything you worked on, and the link gone from the room-stat terminals.

Then, with the supply on, prove it works:

• The receiver’s supply: its L to N reads about 230 V.
• The call: set the thermostat above the room temperature. The receiver switches, usually with a light and a click, and the boiler starts to heat the radiators.
• Satisfied: set the thermostat below the room. The call drops and the boiler stops heating. Its pump may run on for a short time to carry the last heat away.

On a mains-switched receiver, its output reads about 230 V to neutral while it calls. On a volt-free receiver, the contact carries only the boiler’s own feed. Read it where the boiler’s manual says: on an extra-low voltage input, a reading to the mains neutral tells you nothing. Never test continuity on a live circuit.

If the thermostat shows a call and the receiver never switches, look at the radio side first: the batteries, the distance, and the PAIRING, the link that lets the thermostat and its receiver know each other.