Combination Boiler with Two Heating Zones - Relay

Wiring diagrams for two zone valves with a combination boiler.

This arrangement uses a relay to control the boiler, and can be used with any boiler control voltage including 24V, volt free, 230V and others.

Overview Video

This is a continuation of the previous diagram that covers connections to boilers that do NOT have 230V inputs. View on Youtube.

The video covers two methods of volt free switching for the boiler, the relay version is described below.


Wiring diagram for a combi boiler, two zones, two thermostats and relay switchingAll of the components and most of the wiring is identical to those used for the 230V switching arrangement.

The difference is that the 230V switched output is connected to a relay instead of directly to the boiler.

The relay has a 230V coil, once side of which connects to the switched output, the other side to neutral. When energised, the relay closes a separate contact which is connected to the boiler switch terminals. The relay coil and contacts are totally separate electrically, so can be used with any boiler switching voltage.

All of the other wiring is identical to the 230V switching version described previously.

Heating active

Wiring diagram for a combination boiler with relay switching and two separate heating zones showing Zone A active

When heat is required (zone 1 in this diagram), the thermostat contact closes, which connects 230V to the output terminal 3. This is connected in the wiring centre terminal 7 to the zone valve, so the valve motor opens. (Highlighted in red).

Inside the valve, a switch inside connects grey and orange together. Grey is already connected to 230V (terminal 1), so orange and terminal 9 are also connected to 230V.

A wire from terminal 9 supplies 230V to the relay coil. (Highlighted in red)

The contact in the relay closes and connects the boiler A and B terminals together. (Highlighted in purple)

The boiler terminals are connected together - NOT connected to any outside voltage source.

Additional zones

Operation for zone 2 and both zones together is identical to the 230V system described previously.

The only difference is that the switched output connects to a relay, so that the boiler does NOT have 230V connected to the switched input.