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Andrew Smith
Andrew Smith, HVAC Technician
Category: HVAC
Satisfied Customers: 3767
Experience:  Heating and Controls engineer
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I want to replace an old (20 years+) honey mechanical rotary

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I want to replace an old (20 years+) honeywell mechanical rotary room thermostat with a programmable version, CM907. The old thermostat has 4 wires; yellow connected to terminal 1, blue to terminal 2 and red to terminal 3 plus earth. The cm907 only requires 2 connections and from the wiring diagram it looks like red to A and yellow to B. When I then insert the batteries and before switching on the power the display lights and asks for date change but pressing the up or down date buttons turns the display off. The date can be changed with the batteries inserted without any wires connected. Any help would be appreciated, thanks, Gareth
***** ***** : You should only need your stat to link the red and yellow wires, terminate the other 2 safely and connect them in
***** ***** : Just the same if you link the two together the heating should fire
***** ***** : Let me know how you get on

Andrew, I tried this with red in terminal A and yellow in B but then had the problem described in my original post of the thermostat screen blanking out when trying to set the date.


Andrew, I posted a supplementary reply last night but then have continued to receive an email request to rate your answer every hour since, a total of 12 so far, not the best way to get a positive response.

***** ***** : I am back on now
***** ***** : Will it let you set the date and time if it is not connected to the wiring?




***** ***** : and then it starts flashing when you connect the wiring?
***** ***** : I take it that the power is off when you are connecting the stat?
***** ***** : Can you remember how it was wired originally?
***** ***** : There would have been a live (red)
***** ***** : a switched live (yellow)
***** ***** : a neutral (blue)

I connected the wiring and then inserted the batteries, date screen flashed but as soon as I tried to amend the date with the up/down arrows the screen just went blank. I had not restored power to the thermostat. I removed the thermostat and replaced the old one, then inserted the batteries and it allowed me to adjust date as necessary.


Original wiring as in first post.

***** ***** : ok, lets just check the wiring is correct
***** ***** : can you link the red and yellow with some choc block and turn the power back on carefully and see if the heating comes on?

OK, can you wait?

***** ***** : no problem, i am in now
***** ***** : i cannot see all the original text
***** ***** : can you confirm the make and model?
***** ***** : Found it
***** ***** : don't worry
***** ***** : I will dig out their instructions again while you do that
***** ***** : Live to A
***** ***** : Load to C
***** ***** : That is all it says

Heating did come on with red and yellow connected, which suggests my thoughts on the wiring were correct.

***** ***** : Good
***** ***** : Lets try from scratch again one step at a time
***** ***** : Red to A
***** ***** : Yellow to C
***** ***** : Terminate the blue
***** ***** : Terminate the green/yellow

Ok give me a few minutes to connect the new one

***** ***** : Looking at the original stat live is yellow as 1 and 2 are live and neutral
***** ***** : It should not make any difference as this stat just links the two together?

Same thing is happening, screen lights up and asks to set date, slider is set to date but as soon as the up button is pressed the screen goes blank and reverts to the factory set dye. The screen then lights up again a few seconds later and asks for the date to be set.


I have not put power back to the unit.

***** ***** : It sounds like a faulty programmer
***** ***** : We can prove the system works by linking the wires together
***** ***** : Are you following the programming instructions to get into the installer menu?

Haven't tried to go to installers menu, the instructions are to set date as the first action

***** ***** : You have done everything right - I suggest you swap the thermostat

OK thanks for the help.

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