Honeywell V4073A Mid-Position Zone valve

Valve Position Orange
O O B (idle)
Hot Water
Live O A + B
CH and HW
O Live Hold in last position  O
Live Live A
The 'CH ON' output from the programmer applies live, via the room thermostat, to the white wire of the valve. The 'HW OFF' output is also live as the hot water is set to off. This applies live to the grey wire. As can be seen from the table (above), live on both white and grey will cause the valve to take up the 'A' position. When the valve reaches this position, a micro switch inside the valve will switch the live coming in on the white wire to the orange wire. This is used to feed the boiler and fire the system.

When the room thermostat is satisfied, the contact will open thus removing the live from the white wire. This, in turn, will remove the live coming back down the orange wire and the boiler will shut down. 

In the diagram above, the pump will continue to run via the 'overrun' thermostat in the boiler. This is to disperse latent heat in the boiler and prevent possible damage. If the boiler does not have this facility, the pump will be fed from terminal 8 (same as the boiler) and will shut down at the same time as the boiler.

Please note that all information on this web site is provided 'as is' . Whilst every effort has been made to ensure its accuracy, no responsibility can be accepted for any damage, loss or other misadventure as a result of its use.