How to wire an interlock?

[mikely]

Hello Ned.
Looking at the Drayton HTS3, the terminal connections to tie up with my diagram, from left to right, would be 1(call for heat), Common, 2 (satisfied). It sounds like you might have Common and terminal 1 swapped round. Check wire colours at both ends.

 

[nedflanders]

Hi, Mikely. Thanks for your reply. I've just checked again, and it's definitely wired ok. The terminals are marked inside the unit, 1, 2, C. I used ordinary 3 core cable. I have Blue from HW to Common on the stat, Brown from 1, through a connector to Boiler live, and Y/G connected to Y/G from 3 on roomstat to pump live.
Room stat=Blue from N to N, Brown from CH to L, Y/G from 3 (in the stat) to connector to pump.
How is the pump powered when the tankstat is satisfied? (via 3 on the roomstat/G/Y wires/ 'hot' connection in tankstat/ then down the brown?)
That seems to be the problem; no power to the pump when the water's hot.
Aaaaggghhhh!!

 

[mikely]

Ned,
The feed from programmer HW should go to terminal 1 of the stat, then Common (C) to the boiler. Your description has these swapped.

When the tank is satisfied, the pump and the boiler are fed from the roomstat.

I would recommend, if you can, replacing the three core flex to the stats with flat three core and earth (brown, black, grey) so you are not using g/y for live.

 

[Lectrician]

Dont take flat three core and earth directly to the cyl stat (room stat ok). Take it to an outlet plate first, and the final connection in some 4 core flex, with the earth isolated in a term block at the cyl stat end.

You are not meant to use an earth core for anything other that earth

 

[nedflanders]

Hi, Mikely, That's fixed it! Brilliant. I had it wired the way Drayton told me though. I phoned them again yesterday to check, and they confirmed it. Sadly I have to revise my high opinion of their Tech line, cos this guy (who kept calling me SIR all the time in that sarcastic way that call centre people have) insisted that if I did it your way the tank would get hotter and hotter to the point where it would explode. I guess he was trying to make the point that with no valves in the system I have no real control over the water while the heating's on. But I never have, and it's not been a problem thus far.
Anyway, it was perfect last evening and this morning, and I thank you again for your patience and perseverance. I really appreciate your help.
Thanks too, to Lectrician. I take your point regarding the wiring colours. I've taped over the Y/Gs for the moment. (Okay, not exactly building regs!!!).
Thanks again.