New to Evohome Smart Thermostat - How to replace a Honeywell st9400c with 2 x BDR91 ?

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  • Mrman365
    Automated Home Lurker
    • Sep 2017
    • 8

    New to Evohome Smart Thermostat - How to replace a Honeywell st9400c with 2 x BDR91 ?

    Hi everyone.

    I have pretty above average skills with DIY, work in IT and love my gadgets.
    I would like to try and fit my new evohome system myself as im struggling to find an installer near me that will actually get back to me with a quote.


    I recently bought the following:

    - Evohome Smart Wireless Controller THR99C3110 kit
    - Evohome Hot Water kit.


    My current setup
    - Honeywell st9400c thermostat hard wired to boiler in kitchen. This controls my heating and hot water.
    - MAIN branded boiler
    - Stored hot Water cylinder in airing cupboard
    - Old Honeywell Sun dial in living room. This controls my heating on or off as long as the st9400c’s heating function is switch on too. I wouldnt mind removing this altogether or leaving it set to maximum its more trouble to remove.

    Can someone please tell me after I remove my old Honeywell st9400c thermostat, which wires go where to the BDR91 devices?
    Im struggling to decipher the diagrams Honeywell provide.

    I've attached some pictures.

    Thanks all!
    Attached Files
  • DBMandrake
    Automated Home Legend
    • Sep 2014
    • 2361

    #2
    You will presumably have two BDR91 relays - one you will configure for hot water the other for heating demand. I also presume you have your boiler wired to fire via the limit switches in your zone valves ?

    If so it's pretty straight forward - each BDR91 replaces one of the two relays in your original timer indicated on the diagram on the back of the old timer.

    So Neutral (wires currently going to N) go to Neutral on both BDR's, your old incoming Live (L) to Live on both BDR's, also inside each BDR connect a link wire between the second L and A to provide a live feed to the common contact of the relay.

    Pin 1 wires go to C on the HW BDR
    Pin 2 wires go to C on the CH BDR (looks like you don't have a wire here so ignore that one)
    Pin 3 wires go to B on the HW BDR
    Pin 4 wires go to B on the CH BDR

    That's all it should take - you are literally just duplicating the wiring shown on the original timer. Make sure the two relays are at least 30cm apart for wireless signal reasons. You'll probably need to run some multicore flex between the two if you just have wires coming out of the wall in one spot - or consider tracking the wires back to their source and installing the relays near your wiring centre, if you have one.

    It's up to you to decide which BDR is for hot water and which is for heating - make a note of it so that when you bind them you assign them to the correct function. Also if you have bought them in a pack I would advise following the procedure to clear bindings on both BDR's before you attempt to bind them, as they come pre-bound in some packs and that can confuse things if you mix them up once you have unpacked them.

    To clear the binding hold the button for 15-20 seconds until there is a rapid flash of the red light, then it is cleared and ready to be bound.
    Last edited by DBMandrake; 19 September 2017, 02:39 PM.

    Comment

    • Mrman365
      Automated Home Lurker
      • Sep 2017
      • 8

      #3
      You legend!

      Originally posted by DBMandrake View Post
      You will presumably have two BDR91 relays - one you will configure for hot water the other for heating demand. I also presume you have your boiler wired to fire via the limit switches in your zone valves ?

      If so it's pretty straight forward - each BDR91 replaces one of the two relays in your original timer indicated on the diagram on the back of the old timer.

      So Neutral (wires currently going to N) go to Neutral on both BDR's, your old incoming Live (L) to Live on both BDR's, also inside each BDR connect a link wire between the second L and A to provide a live feed to the common contact of the relay.

      Pin 1 wires go to C on the HW BDR
      Pin 2 wires go to C on the CH BDR (looks like you don't have a wire here so ignore that one)
      Pin 3 wires go to B on the HW BDR
      Pin 4 wires go to B on the CH BDR

      That's all it should take - you are literally just duplicating the wiring shown on the original timer. Make sure the two relays are at least 30cm apart for wireless signal reasons. You'll probably need to run some multicore flex between the two if you just have wires coming out of the wall in one spot - or consider tracking the wires back to their source and installing the relays near your wiring centre, if you have one.

      It's up to you to decide which BDR is for hot water and which is for heating - make a note of it so that when you bind them you assign them to the correct function. Also if you have bought them in a pack I would advise following the procedure to clear bindings on both BDR's before you attempt to bind them, as they come pre-bound in some packs and that can confuse things if you mix them up once you have unpacked them.

      To clear the binding hold the button for 15-20 seconds until there is a rapid flash of the red light, then it is cleared and ready to be bound.
      You absolute Legend. Thanks very much!
      This is exactly the clear info i was hoping for.
      I will give this a go.


      Is there a tidy way of looping the Live and Neutral wires to both BDRs? I was thinking of mounting the BDRs on back plates a few inches apart then have the Live and Neutral wires trunked between them..

      Comment

      • DBMandrake
        Automated Home Legend
        • Sep 2014
        • 2361

        #4
        Originally posted by Mrman365 View Post
        You absolute Legend. Thanks very much!
        This is exactly the clear info i was hoping for.
        I will give this a go.


        Is there a tidy way of looping the Live and Neutral wires to both BDRs? I was thinking of mounting the BDRs on back plates a few inches apart then have the Live and Neutral wires trunked between them..
        The problem is you need the BDR's separated by 30cm or more otherwise wireless interference between them may cause you problems with comms errors.

        When you say back plates do you mean raised back boxes like you would find on some plastic fronted power switches designed to mount on concrete walls ? If you did have raised back boxes you would have enough room for some terminal blocks behind the one that covers the hole in the wall to use some terminal blocks to join the wires that need to go across to the other BDR - and some plastic trunking to hide the cable is probably the neatest way, other than hiding the whole lot away in the boiler cupboard as I ended up doing.

        Comment

        • Mrman365
          Automated Home Lurker
          • Sep 2017
          • 8

          #5
          Oh I see, glad you told me about the distance between them, thanks.

          Yep, the plastic raised back boxes normally used for light switches / plug sockets.

          Comment

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