Wiring a Replacement room thermostat for central heating

[ericmark]

Have the pictures changed or some thing? I see it marked 95 to 240 volt, and it has volt free contacts.

The old thermostat has a small heater to reduce the hysteresis, but will still work without it, so step one is ensure there is a neutral to it, live should be red, neutral blue, and switches live yellow. So 1 + 4 are Red, 2 yellow, and 3 blue.

One assumes not time switch, and it is 230 volt, but I would always measure to be sure, the old thermostat would work with 24 volt, although the hysteresis would be high, so I always verify before connecting.

 

[robinbanks]

Use a multimeter to determine whether the red wire is live all the time or only sometimes. If only sometimes, it may be simplest to find the other end of it and wire it so it's live all the time (presumably the device it is connected to/that activates it has a permanent live somewhere accessible)

 

[mcprinter]

OK, I should maybe have asked, what controls the boiler just now? I.e. is there a programmer that calls for the CH or is it just the boiler. Only when there is a call for central heating either from the boiler or the programmer is there 240v sent down the red live (L), then if the temp is too cold does the stat then switch that down the yellow (SL) to the boiler telling it to fire up.

Understood.
Whatever supplies the live to the current stat would need making permanent or the stat won't power/operate.
I have my timer set to constant for CH

OP would probably be better with a dial thermostat or a battery powered one.

 

[Deleted member 267285]

OK, I should maybe have asked, what controls the boiler just now? I.e. is there a programmer that calls for the CH or is it just the boiler. Only when there is a call for central heating either from the boiler or the programmer is there 240v sent down the red live (L), then if the temp is too cold does the stat then switch that down the yellow (SL) to the boiler telling it to fire up.

I thought on the Vaillants terminal 3 (output) was a permanent live out, terminal 4 is switching? I might check this today as no doubt will be working on one.

 

[Madrab]

I thought on the Vaillants terminal 3 (output) was a permanent live out, terminal 4 is switching? I might check this today as no doubt will be working on one.

You could be right, I know some boilers do have a permanent live out others only energise that side when calling. Certainly the case with the programmer unless always on of course.