How to wire Honeywell CM907 thermastat

19 Oct 2011
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United Kingdom
I am presently in Italy and I am trying to install a CM907 to replace an existing thermostat. I have removed the cover of the old thermostat and there are 3 wires, brown, blue and grey.

Can anyone advise me in what order I connect these to the A,B & C terminals on the CM907. The boiler is a combi Ferroli New Elite boiler if that makes any difference.

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I connected the grey wire ( which was live when I tested it) to the A terminal on the CM907 and the blue wire to terminal B. I isolated the brown cable which was not live when I tested it. Nothing happened until I increased the temperature on the on the thermostat. As soon as I did this the fuse tripped.

I then connected the brown to terminal A and left the blue in B ( I isolated the grey wire) but nothing has happened. The fuse did not trip.

Can anyone offer any advice on the correct way to wire the CM907.

Can anyone expalin why there is a C terminal as it does not appear to be required!
I think you will find that the third combination will work. That is grey to A and brown to B, although it doesn't matter which way round they are connected. The blue will be neutral, which is why you blew the fuse.
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Mikely - thanks for that. I have done as you have suggested and the boiler has fired up without tripping the fuse and seems to be working a treat.

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