In an earlier contribution to this discussion,
Brownie wrote:

Hi,

I'd just like some help with rewiring a new programmable thermostat.
I think I know what to do but just wnat to be sure if possible.

Presently I have a standard Honeywell (T40) T6360B - the large dial
electro-mechanical type. I am getting a Horstmann Centaur 7, that
has been recommended in the group before. Apparently it's quite
common for installers to use standard 3 core flex to wire up the
original thermostat and these electronic replacements only need two
wires. Presently the Honeywell is:
No wires to Earth terminal, 'the Earth wire' to terminal 2, the
Neutral wire to terminal 3 and the Live wire to terminal 1.

Any idea which goes to which terminal on the Horstmann? I'm led to
believe 'the Neutral wire' now on terminal 2 is redundant and the
'Heat Call' wire (Earth on the flex presently) is likely to be
required along with Live currently on terminal 1?

Hope somebody can make sense of that?

With an electronic stat, you only need live and switched live connections -
no neutral. [On the old stat, a resistor is connected between switched live
and neutral to produce a bit of heat, and increase sensitivity. The new stat
doesn't need this.]

I suspect that you currently have is red=live, black=neutral and
(bare)=switched live. This is how it was often done, and is *diabolical*!
Using the uninsulated earth conductor for switched live is extremely bad
practice. Mine was like that prior to fitting a programmable stat!

What you need to do is to find the *other* end of the cable - which is
probably terminated in a junction box - and change the connections so that
red is live and black is switched live. Then use these same 2 wires at the
stat end.
--
Cheers,
Set Square
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