Hi Croyddon corgi, i need some advice plese, i have a honeywell sundial y system in my house that i plumbed in in 1986, still go the original pump and boiler great, however i have just decorated my lounge out and wanted to replace the outdated honeywell roomstat, with one that i purchased the horstman 7 programmable stat, (battery operated) i know they work on live and switched live, but in my original roomstat (photo can be supplied ) i have four wires coming in the top one has nothing on it the next two both have power on the them and the bottom one when i turn up the room stat also has power, inidcating to me that this is the switched live, other persons have said i should not have four wires, but i read one of your articles and you mention a four wire roomstat, unfortunately my cable is just numbered and not coloured as when i got it , it was heat resistant butal cable very expensive, but better still ''free '' question is why would i have two live terminals, in the stat, as i said i could provide or e mail a picture to you, i know you cannot say definatively what the problem is , but the electrics when i installed all the plumbing was undertaken by a fully qualifield electrician who now works off shore on the rigs, and i have lost touch, kindest regards David