Those people are doing this on an OT compatible boiler. Yours isn't. What you are effectively doing is almost like weather compensation. The On-Off trigger is coming via the terminal 4. But since the VR33 is able to vary the requested Flow temperature, you are using a Hybrid system. I still think you should be able to move to a completely OT controlled system. Also the OTB is able to distinguish between CH demand and DHW demand, but unfortunately Evohome doesn't use that. It just assumes 10C is OFF, 90C is Full heat and only 90C is DHW.