Swopping Room Thermostat for Programmable type (Screwfix/Horstmman Centaur 7)

[Brownie]

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?

[Set Square]

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.

[Ed Sirett]

Currently the three wires you have are live from CH timer, neutral and switched live to the zone valve (or boiler directly sometimes).

Whatever the earth wire is begin used for should be disconnected both ends and ideally reconnected to earth at the far end. There is a spare "no connection" terminal in the CentaurStat 7 (#4 IIRC) which can be used to park the earth at the thermostat end.

The remaining two wires go into terminals 1 and 2 on the centaurstat and are connected CH timed live and the switched live at the other end.

The wire connected to neutral (whichever it is) is no longer required at all.