Replace old Honeywell Thermostat with CM907

6 Aug 2013
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United Kingdom
I have a (very) old Honeywell thermostat that I want to replace with a CM907 thermostat. I have the wiring diagram for the CM907 but not the old one. What I am finding a little confusing is that there appears to be only two wires needed for the CM907 but three wired into the old thermostat.

I would welcome some advice as to what I should do to transfer the wiring.

I have included some picture of the old thermostat and its wiring.


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Red and yellow to A and B - doesn't matter which way round.

Make the blue one safe.

Leave your heating on constant at the timer.

Don't forget to enable the optimisation in the installer settings.
Thanks for that.

I am sure that must be some sort of record, placed post, received reply 4 minutes later, sorted installation within 1/2 hour of placing post.

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