Replacing a room thermostat

[GeorgeK]

Hello all - first post so be gentle!

Our old thermostat is on the way out and so, combined with the fact that it's very basic, I've purchased a new one. The old one is a Drayton RTS1 and I will be replacing it with a Honeywell CM907. It's connected to a combi boiler. Before I go ahead and swap them over I have a couple of questions

1) The old thermostat has 3 (well 4) wires attached. One blue to N, one red to L, one yellow to 3 and one yellow/green to nothing


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and the wiring diagram for the new thermostat looks like this:

Am I right in thinking that I will not use the blue and yellow/green wires for the Honeywell and just attach the red to A and the yellow to B?

2) Am I right in thinking that I will need to flip off the power at the fuse box before I start disconnecting the old thermostat? If so, any ideas as to which circuit it would be on:


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Annoyingly the label was damaged when we moved in a few years ago I would like to avoid switching off the power to the whole house for a number of reasons - primarily because we need our house alarm backup battery changed so that it doesn't go off if mains power is lost...

3) Following on from above, there is a switch by the boiler which turns power to it on and off but I'm guessing that wouldn't cut power to the wires behind the thermostat? If it would then that would definitely be preferable...

Thanks in advance

GK

 

[45yearsagasman]

You are correct in using only red(power in) and yellow switched live out.
As for which circuit it's any ones guess, but it should be on one of the socket circuits. Best to have a meter to test for positive isolation.

 

[St0rmer66]

The switch by the boiler is extremely likely to isolate the thermostat too, it would be strange if it didn't!

 

[D_Hailsham]

there is a switch by the boiler which turns power to it on and off but I'm guessing that wouldn't cut power to the wires behind the thermostat?

If the system has been wired correctly, the switch by the boiler should also isolate the thermostat.

Do you have an existing timer?

If Yes: does it stop working when you turn the boiler switch off?

If Yes: the thermostat will be isolated.

The existing timer will have to be set for heating always on, so control is passed to the CM907.

 

[GeorgeK]

thanks for the replies

Firstly, glad that I managed to get the wiring right

As for the thermostat / boiler / timer - there is an existing timer by the boiler. When I turn the switch off the LCD display of the timer stays on but it no longer controls the boiler which I presume means that it's running the display from a battery of some kind and so is now isolated from the mains.

Sounds like getting myself a multimeter would be a good idea... Any suggestions? Maplin or Argos would be the most convenient places (at least they're the two that spring to mind when I think of places that will sell a multimeter on a Saturday...)

Thanks again

GK

 

[BigSnoopy01]

multimeters are not what you want to test for isolation. get an approved voltage tester. It will protect you better.

test the tester on a known source after testing the circuit to prove the tester is working.

 

[GeorgeK]

multimeters are not what you want to test for isolation. get an approved voltage tester. It will protect you better.

test the tester on a known source after testing the circuit to prove the tester is working.

Good point

Any suggestions for one of those?