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 both thermostats satisfied, there is no live to the boiler and the system is waiting for one of the thermostats to call.

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