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Thread: Can an Evohome system be introduced in stages?

  1. #1
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    Question Can an Evohome system be introduced in stages?

    My wife and I are empty-nesters living in a detached 5-bedroom house, with traditional heating and indirect hot water (Potterton Profile 80e balanced flue gas boiler, hot water cylinder and cold water storage tank in the attic). It is a Y plan system using a 3-port motorised Honeywell diverter valve. There are 14 radiators, all of which have TRVs, mostly rather old Draytons. A pump is used to boost hot water pressure for the taps. We also have a separate hot and cold pump for our two bathroom showers. The boiler is old but fully functional.

    Heating control is via a Honeywell CM907 Programmable Thermostat located in our downstairs hall. It is the only room thermostat in the house and allows us to select the required temperature at six times per day for each day of the week. A Honeywell ST6100 Timer controls the on/off times for the hot water. However, the Honeywell CM907 is obsolete and needs replacement due to a faulty LCD panel. Upgrading to a self-installed Evohome system is now under consideration, though I'm currently just starting to climb the learning curve.

    Can an Evohome system be introduced in stages, swapping existing TRVs for HR92s one at a time? If so, what is the minimum working configuration?

    The seating position in our main living room is some distance from a window under which the only radiator is installed. Instead of fitting an HR92, would it be better to use an HR91 and a separate wall-mounted thermostat closer to the sofas?
    Last edited by haggis999; 1st January 2022 at 03:22 PM.

  2. #2
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    Hi,

    Yes you can do it in stages. The first thing you will need to do is buy the controller and associated relays (bdr91) to control your zone valve. You can then add HR92s as and when. Just bear in mind that any radiator not installed with a HR91/92 will heat up when ever one of your zones calls for heat.

    In my experience a wall mounted stat always gives a truer indication of room temp than just the rad valves; however you are then buying extra kit. FYI a Hr91 and 92 have the exact same functionality. The 91 just doesn’t have the lcd screen and I believe the valve stroke settings

  3. #3
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    Quote Originally Posted by MajorDanby View Post
    Yes you can do it in stages. The first thing you will need to do is buy the controller and associated relays (bdr91) to control your zone valve. You can then add HR92s as and when. Just bear in mind that any radiator not installed with a HR91/92 will heat up when ever one of your zones calls for heat.
    Both the Wi-Fi Base Pack ATP921R3100 and the ATF500DHW Hot Water Kit contain a BDR91 Wireless Relay Box. How many BDR91s do I need for my system and what would each of them be wired to?

    I take your point that if any of the Evohome zones call for heat it will turn on the boiler and thus heat up any of my non-Evohome radiators (subject to their TRV setting).

    In my experience a wall mounted stat always gives a truer indication of room temp than just the rad valves; however you are then buying extra kit. FYI a Hr91 and 92 have the exact same functionality. The 91 just doesn’t have the lcd screen and I believe the valve stroke settings
    Assuming the Evohome controller lets you associate a wall-mounted T87RF2033 Single Zone Thermostat with a designated group of HR92s (or HR91s), does this mean I would not be using the intelligence built into each HR92/HR91? If so, would a simpler radiator valve be more appropriate?
    Last edited by haggis999; 2nd January 2022 at 12:55 PM.

  4. #4
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    You will need two BDRs to switch the respective motor lives in your 3 port valve. I am assuming that your boiler is then switched by the switched live from you valve (orange wire).

    If you have a room stat you will still need to have hrs on your radiators so they control the flow through the rads

  5. #5
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    Quote Originally Posted by MajorDanby View Post
    You will need two BDRs to switch the respective motor lives in your 3 port valve. I am assuming that your boiler is then switched by the switched live from you valve (orange wire).
    Thanks for that clarification. I will check out the wiring connections to my 3-port valve.

    I never really thought about it until now, but after a Google search I see that my 3-port diverter valve always gives priority to hot water. I also read that mid-position valves are now more popular than diverter valves.

    If you have a room stat you will still need to have hrs on your radiators so they control the flow through the rads
    I guess you set the required temperature for the room stat via the Evohome controller and the associated HR92s are then configured to follow the guidance of the room stat.
    Last edited by haggis999; 2nd January 2022 at 08:44 PM.

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    You might find that your existing CM907 communicates with a BDR91 that's wired in to the heating side of your 3-port valve, if that's the case then the first stage of your upgrade would be a straight swap of the old controller for the Evohome controller. If the Evohome comes with a pre-paired BDR91 (I think they always do but I may be wrong) then you can just swap the front part out for the new one, leaving the old base part with the wiring connections in place, or if not you can leave the whole BDR91 in place and pair the new controller with it. This would be your absolute minimum working configuration, Evohome would just control the heating in the same way that your existing controller did (no zone control of any radiators and your existing timer still controlling the hot water). Then you can start building the rest up from there.

    This is effectively what I did when I replaced my CM927, which I believe is very similar to the CM907? Only difference for me is that I have a Combi boiler so no DHW control. I then added HR92s to all the rads once my heating engineer had fitted TRV's to the rooms that didn't have them (including the lounge, which is where the Evohome controller is located).

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    Quote Originally Posted by RedEarth View Post
    You might find that your existing CM907 communicates with a BDR91 that's wired in to the heating side of your 3-port valve.....
    It would have been nice to find a way to start my Evohome journey slowly by just replacing my CM907 with the Evohome controller, but there is no BDR91 in my existing central heating system, which was installed in 1997, probably before any Evohome components were available.

  8. #8
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    Quote Originally Posted by haggis999 View Post
    It would have been nice to find a way to start my Evohome journey slowly by just replacing my CM907 with the Evohome controller, but there is no BDR91 in my existing central heating system, which was installed in 1997, probably before any Evohome components were available.
    Is it a wired thermostat? There appear to be multiple wiring configurations, so you should check the wiring first. Simplest version involves a 2-strand telephone wire towards your boiler and if that is the case then all you need to do is find where the wire connects to the boiler. The BDR91 needs to be wired to create a bridge between the two strands of the telephone wire.

    Note: if you detach the original wire you can use that as the power line to the Evohome wall mount. Do NOT lead 230V over that wire though! Instead, disassemble the transformer from the wall mount and place that next to your boiler.

  9. #9
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    Quote Originally Posted by gordonb3 View Post
    Is it a wired thermostat? There appear to be multiple wiring configurations, so you should check the wiring first. Simplest version involves a 2-strand telephone wire towards your boiler and if that is the case then all you need to do is find where the wire connects to the boiler. The BDR91 needs to be wired to create a bridge between the two strands of the telephone wire.

    Note: if you detach the original wire you can use that as the power line to the Evohome wall mount. Do NOT lead 230V over that wire though! Instead, disassemble the transformer from the wall mount and place that next to your boiler.
    Many years ago, I used Adobe Illustrator to make a pretty picture of the mass of multicoloured wires inside a large junction box beside my hot water cylinder. This tells me that a standard 3-core 1.5mmsq mains cable runs to that junction box from my CM907. One of these wires is unused, one supplies a live 240v feed to the CM907, while the third is fed back from the CM907 via the junction box to my 3-port motorised valve.

    To avoid any misunderstandings, I would appreciate a slightly more explicit description of how I would introduce a BDR91 into this scenario.
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    Last edited by haggis999; 3rd January 2022 at 03:08 PM.

  10. #10
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    Quick search for an installation manual gave me the following link: http://www.free-instruction-manuals....f/p4734564.pdf

    Seems like your setup is best described by #4e. Installing the BDR91 is practically the same as described earlier. In your drawing:
    N = N
    L = L/7
    A = L/7
    B = 5

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