Replace Honeywell T40 with CM907 - is it possible?

Discussion in 'Electricians' Talk' started by Porkpie2310, Mar 25, 2014.

  1. Porkpie2310

    Porkpie2310 New Member

    So, I have a Honeywell T40 which has had the dial broken off and have decided to upgrade it at the same with a programable (7 day ideally) one and have been looking at the CM907 and have questions about the wiring.

    The T40 has 3 wires and an earth (see pic) and I want to know if it possible to replace with the CM907?

    Details can be found at

    The system it will connect to is A A-Class Boilermate SP in the cupboard and an Ideal Icos in the kitchen, not sure if this is needed, I'm no expert....

    Any help on this would be very much appreciated

    Attached Files:

  2. Lectrician

    Lectrician Screwfix Select


    On your old one, take the grey (sleeved blue) and put it in a terminal block on its own.

    The brown and black can go either way around to terminals A and B.

    Your current time clock would need to be turned on to 24hr, or bypassed/removed.

    DO NOT be tempted to put the grey (sleeved blue) cable into terminal C!
  3. How come I'd have taken two pages to say that... :(

    Porkpie, since you do have a Neutral wire available (the blue-sleeved grey wire), you can consider a mains-powered model of Prog Stat instead of that battery one. Does Honeywell make a mains equivalent to the CM907?

    The difference is, one is battery powered and t'other, y'know, doesn't need batteries. That's all...
  4. unphased

    unphased Screwfix Select

    There is something odd about that wiring on the photo. I would have thought that 2 was neutral and 1 and 3 live feed and switched live respectively. It could be that the sleeved blue, whilst expecting it to be a neutral has been incorrectly sleeved blue and should be brown, and the neutral is black. This is why people should be careful to verify, and not just trust, colour coding. Anyone confirm if I'm right?
    Last edited: Mar 26, 2014
  5. You are SO right :eek:.

    Something amiss indeed.

    In a big bang way.

    Porkie - you there? Porkie! PORKIE!! Talk to us... :oops:
  6. Sean_ork

    Sean_ork Screwfix Select

    the old stat is a single pole double throw action - all a boiler usually requires is a simple open/close on heat rise/fall - so those wires could just be a live, switched by the stat action - giving a simple switched live (plus neutral for the inbuilt heater) - or could be using both sides of the SPDT switch to switch a live to open and then close a valve - so testing is essential before connection
  7. unphased

    unphased Screwfix Select

    Yes, that is true, but on the older analogue type stats, like the T40, a resistor/heater was placed in the neutral contact to give a more accurate temperature control. I am pretty sure that terminal 2 is the neutral in which case it needs to be checked at the other end to make sure the wires are correct.
  8. Sean_ork

    Sean_ork Screwfix Select

    you call it a resistor I call it a heater, same purpose - but yes the wiring needs confirming
  9. Oh, you two talkin' aboot the accelerator...?
  10. unphased

    unphased Screwfix Select

  11. Sean_ork

    Sean_ork Screwfix Select

    that's what the marketing department like calling it
  12. Well, if you don't use the correct terms, no wonder yous two's gettin' confused...
  13. unphased

    unphased Screwfix Select

    Please accept my apologies. :)
  14. Sean_ork

    Sean_ork Screwfix Select

    sorry Dad
  15. Wait - you two gettin' sarcy... :(
  16. Porkpie2310

    Porkpie2310 New Member

    After the first two posts I was only thing about battery vs mains but now... o_O

    So its not as simple as
  17. unphased

    unphased Screwfix Select

    Porkpie :) (Love the name). If you can get a pic of the wiring in the programmer maybe we can identify the wires and confirm.
  18. Lectrician

    Lectrician Screwfix Select

    It does indeed look to wired incorrectly. The other end needs to be confirmed.

  19. Sean_ork

    Sean_ork Screwfix Select

    here's my best assumption - but don't do this until you have confirmed the wiring

    1 = phase
    2 = neutral
    3= switched live (ignore the blue sleeve)

    so if the battery powered version of the new timer is used terminals A & B use the wires currently going to 1 & 3 as all the boiler appears to want is open/close across the L and Ls terminals

    as an aside, if anyone gets asked to wire anything outside of the usual Combi or system boiler with any of the simple X,Y plans here is a very comprehensive document that illustrates (with pictorial circuits for those not used to proper circuit diagrams) how to wire all manner of heating controls system, including that old favourite of zoning a combi using valves, UFH and a combi, cascading a number of boilers .......... all good stuff that many electricians (and most plumbers) would baulk at taking on such a job
    Last edited: Mar 26, 2014
  20. unphased

    unphased Screwfix Select

    I don't know how many times I still see electricians using the black as neutral on new colours. Convention is grey sleeved blue and black for switched live. That's clearly a 3-core and earth (6243Y 1.5mm2 probably) and it could be labelled correctly and wired wrongly!

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