Replacing a Switchmaster thermostat problems...

[lister01]

Hi there. I'm trying to replace my ancient Switchmaster thermostat with no luck.

Currently it is wired so:
Live (Red) into terminal 3
Neutral (Black) into terminal 1
Earth (Yellow/green) into the Earth terminal.
Terminal 2 is empty.

I've now bought a cheap Sunvic stat because it was the only one I could find for two wire installation. The last one I bought was asking for Live, neutral and switched live. As I only have Live and neutral coming out of the wall (discounting earth) I figured this was wrong and went to buy the Sunvic instead.

So, the Sunvic has three terminals; 1, 2 and 3. The instructions state:

COM 2, DEM 1, SAT 3, NEUT n/a. If neutral isn't required, what do I do with the black wire? Basically I just need to know where I put the red, black and yellow wires! I assume red goes into COM(2) so should black go into DEM(1) or SAT(3)? And where should the earth go? Any help greatly appreciated! I thought it would be a case of taking the old one off and replacing like for like (as you would with sockets and switches) but not so! Each stat appears to be totally different.

 

[stem]

Assuming that the existing thermostat works, ie it switches the heating on and off as it should, the two wires you have (red & black) will be "live" and "switched live". Really, the black should have a red sleeve on it to identify this. When the thermostat switches on, the two wires are connected together, completing the circuit.

If the existing stat is working and no other wires have been removed from it, on your new thermostat, connect the red wire to COM 2 (common) and the black wire to DEM 1 (demand)

If you have a wiring diagram for the switchmaster then you could cross check that there is no neutral connection presently. I can't, because you don't say what model of switchmaster you have. (my guess is neutral would be terminal 2 on the switchmaster and as you don't have one, nothing goes there)