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

 
The CH ON signal from the programmer applies live to the white wire of the valve via the room thermostat. As there is no signal on the grey wire, the valve to drives to the mid position.

The HW ON signal from the programmer fires the boiler via the cylinder thermostat and terminal 8.

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