I took out a sunvic m.v (crap) and replaced it with a honeywell m.v. The only difference was that the honeywell valve had an earth. I carefully unpicked each wire from sunvic m.v & replaced it with wires from honeywell. Simple not!!! I'm getting heating & hot water on permanently. It doesnt help that the boiler/programmer are on ground floor, room stat on first floor and cylinder stat m. valves & pump on 2nd floor along with 2 spaggetti junction boxes. Aghhh
Have done all this but some wires are double coloured ie brown sleeved with a blue plastic cable cover, some wires are taped up (mainly the brown ones!) and some are just simply cut out ie the blue ones. So UNLESS i SPEND ALL DAY UNPICKING THIS "TIN OF WORMS" I dont think I can see an S plan there at all. Will call a decent electrican to sort it out.