I recently bought a Horstmann C27 Programmer and Digital thermostat. I
installed the C27 and it worked fine. I then installed the thermostat
and stupidly guessed which neutral wire to stick in and guessed the
wrong one - resulting in a pop and untimely death of my C27 programmer.

I then got the correct wiring for the digital thermostat and the
electritian who fixed it used a workaround on the backplate wiring so
that both hot water and heating switch on from the switch with the
expectation that I would move a couple of wires (I'm sure I've got this
right) and plug in my next door neighbour's heating clock temporarily
when I borrowed it.

Here's the problem. It works on his but not on mine, despite everything
else, incluing new thermostat, working from the switch! Anybody got any
ideas?

If I remember correctly then the digital thermostat / programmer I've
got just acts like a switch. there is no + or - and there is not even
the conection for the 3rd wire that you get with the old mechanical
thermostats.

If it's like mine I dont see how you could wire it incorrectly in the
first place. Even if it needs an AC supply then this has no 'polarity '
does it !

Simon

The problem was that there was a third wire in my previous thermostat
(2 neutrals). I put the wrong one in and the Horstmann programmer
literally went pop. I'm not proud of this piece of wiring, but there
you are!