Swapping out old Switchmaster room stat for Digital room stat. Advice?

Discussion in 'Plumbers' Talk' started by SaintH, May 15, 2018.

  1. SaintH

    SaintH New Member

    I'm thinking about swapping out my old dial Switchmaster room stat for a digital one. I have an open vented system with a Baxi Solo 24 HE boiler controlled by a Drayton 241 programmer which controls CH & HW separately.
    I was thinking about going for a Honeywell DT90E which would just have the same function as I have now but have also noticed the CM907 programmable stat which appeals to me as I can set different temperatures at different times.
    My question is as I already have the CH set on the timer, will there be any issue with running a programmable stat as well?
    I would like the stat to change temp at different times but do not want to use the timer function if that makes sense? I.E the programmer CH is set to turn on between 6pm and 11 pm but would like the stat to set the heating at 18 degrees between 6 & 9 and 21 degrees between 9 & 11.
    Could I leave the Drayton Programmer CH function 'on' all the time with the CM907 controlling when the boiler turns on, or would I need to set the programmer and the programmable stat BOTH to the same time to turn the heating on?
  2. :)

    As you sussed, the CH time on the Drayton 241 to 'ON'. That will provide a permanent 'on' signal to the new prog stat (a good choice) and the prog stat will now do everything - times and temps.
  3. Are you going to DIY the swap? If so, make sure you understand what the existing wires are to the old Switchmaster - make notes and take photos.

    If it has a neutral - which it likely has - this won't be needed in the new battery-powered stat. But you MUST correctly ID these wires - or 'bang'.
  4. SaintH

    SaintH New Member

    Thank you! I hoped that would be the case, just needed confirmation.
  5. SaintH

    SaintH New Member

    Yes I intend to. As I understand it I need to wire live to A, switched live to B, leave C alone and make safe neutral and earth. I havn’t looked inside the old one yet but I assume it has 3 cables and an earth.
    Deleted member 33931 likes this.
  6. Yes to the L and SL going to A and B on the new stat. Almost certainly the old wiring will also be as you say, tho' sometimes the neutral has been left disconnected (when folk have used the wrong cable) or even the earth used as a switched live (ditto).

    Chances are the full quota of wires will be there - L, SL, N and earth - but it's worth looking up the wiring for this model to confirm you ID them correctly.

    You clearly know what you are doing :)

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