Honeywell V4073A Mid-Position Zone valve
  

White
(input)		 Grey
(input)		 Valve Position Orange
(output)
O O B (idle)
Hot Water		 O
Live O A + B
CH and HW		 O
O Live Hold in last position  O
Live Live A
Radiators		 Live

 
With the room thermostat now open, the live is removed from the white wire to the valve. The valve now returns to its idle position, B. The valve does this under spring tension, no electrical power is used. Live from the programmer (HW ON) now fires the boiler via the cylinder thermostat (cold) and terminal 8.

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.