Thanks Plumbob - it's good to know I wasn't being stupid!
My C-plan is wired sensibly: the HW and CH are truly independent -so I should have no problems. Thanks again.
I have a conventional oil-fired boiler with gravity DHW and pumped rads (Sundial C-plan - motorised valve in gravity circuit). There is a mechanical 3-wire room-stat in one room and TRVs in all other rooms. I want a better stat (e.g. Drayton Digistat 3+) to give accurate control and 5/2 programmabil...
