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 would apply live to the white wire of the valve via the room thermostat.  Now that the cylinder temperature has been reached, the cylinder thermostat changes over. The live which was feeding the boiler via terminal 8 is now removed and is applied to the grey wire of the valve via terminal 7.

The boiler shuts down briefly while the valve now drives to the A position. Once this position is reached, the live on the white wire (coming via the room thermostat) is connected to the orange wire by a micro switch inside the valve. This live signal now fires the boiler via terminal 8.

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