I had a thought on my idea about controlling the boiler directly via Pin 4.
Though it should work for old gas boilers and any oil boiler, new gas boilers contain a lot more electronics and may not like being frequently power-cycled; they may go through a lengthy initialisation. (I don't have...
The WT714R (one channel) hub does what you want. It has relay connections that are 'voltage free', so it's just a switch.
Pins 1 and 3 on that hub form a simple switch, that closes when heat is demanded.
The WT724R (two channel) hub has one side of each relay contact internally connected to the...
On that hub, Pin 4 goes LIVE (230VAC) when there is demand for heat. So I would say just PIN 4 and Neutral direct to the Boiler&Pump. (Assuming there are no motorised valves that need to be energised).
I think :)
PIN 1 goes LIVE when HW is OFF
PIN 2 goes LIVE when CH is OFF
PIN 3 goes LIVE...