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Thread: Evohome: BDR91 talks to Opentherm Boiler Relay (R8810)?

  1. #1
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    Default Evohome: BDR91 talks to Opentherm Boiler Relay (R8810)?

    Hi

    Does anyone have experience with this setup:
    BDR91 at the zone valves and OT relay controlling/modulating boiler (instead of another BDR91 at boiler)? Will the boiler modulate or is this an ON/OFF system?

    Regards
    Felix

  2. #2
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    OT Bridge can only talk to an OT boiler. And if used you need to disconnect the zone valve switch to the boiler. The BDR91 will just open the zone valve but not fire the boiler, as that job is now left to the OT Bridge.

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    Yes the boiler is brand new and OT capable. So from what you are saying I will just not wire the zone valves to the boiler (although my builder put wires there, but I dont have to use them). When BDR91 opens the valve, OT bridge fires the boiler (if not already on for another zone), and ranges the boiler temperature to satisfy the heat demand of the most demanding zone.

    The intended result is that the OT bridge changes the temperature of the boiler gradually to keep a constant room temperature, rather than turning boiler (and the valve) on and off at short intervals. Obviously the lower demand zones will need to be fully managed by their zone valves as the boiler will try to meet demand of highest zone. Did I get this more or less right?

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    The Evohome system works best with as few zone valves as possible. So if you are using radiator controls then you shouldn't need another CH zone valve, otherwise that is yet another dependency in the chain.

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    Interesting you say that. I will use one or the other, not both (Rad controls or zone valves), just not sure which one yet Why would Evo have issues with zone valves may I ask?

    I'm concerned about using Rad controls because: 1. they are limited to 12 (I would need more) 2. They are audible in each room 3. They don't look very nice on an elegant radiator (like vasco flatline, jaga strada etc) 4. Battery change bit of a hassle
    Zonevalves give less control but otherwise issue free i thought

  6. #6
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    Why bother with Evohome if you are just looking for large zone control using zone valves. Evohome's strength is in controlling every radiator separately. Also every radiator is not necessarily a separate zone.

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    I agree with Bruce. If you're just working with zone valves you lose a huge amount of what evohome offers, and might as well look for a cheaper option elsewhere.

    Remember zone valves are either open or closed. Evohome opens radiator valves in a proportional manner on order to keep them as just the temperature needed to hold a room at the target temperature.

  8. #8
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    Quote Originally Posted by Felix View Post
    I'm concerned about using Rad controls because: 1. they are limited to 12 (I would need more)
    Evohome is limited to 12 zones, not 12 radiators. There is no hard limit on the number of HR92's or radiators you can use. If you have any rooms with multiple radiators then those would only count towards one zone since you would assign all radiators in the same room to the same zone.

    Also there is a zone type called a "multi-room zone" which can not only have multiple radiators in the same room but multiple radiators in different rooms. In this zone type the different rooms share the same temperature schedule but sense their temperatures separately.

    This kind of zone is useful if you have areas that you always want to schedule together - for example children's bedrooms. In this case all the radiators belonging to the multi-room zone only count towards one zone. In a multi-room zone a temporary override can be made to one of the radiators without affecting the other ones in the zone by turning the knob on the HR92.

    So it's 12 zones of independent scheduling and control you're limited to, depending on the kinds of rooms and usage patterns in your house this may be enough.

    2. They are audible in each room
    About the only time you'll hear them is when there is a large shift in set point scheduled - for example downstairs rooms coming on in the morning or going off at night. Then they can turn for up to about 30 seconds at once.

    But once a room is up to temperature you'll only hear a tiny noise for about a second maybe every 4 minutes at most. In any normal house with household noise you will not hear these small adjustments, and even if you do they are not intrusive.

    The only real issue people might have is in bedrooms - personally our whole family (including 2 year old) sleep through any night time adjustments of bedroom HR92's and have not been woken up once by them, but I know some people on here have complained about bedroom radiator noise.
    3. They don't look very nice on an elegant radiator (like vasco flatline, jaga strada etc)
    I guess that's subjective, but I think the HR92 looks pretty sleek and elegant. You should see some of the competitors!
    4. Battery change bit of a hassle
    In theory yes, in practice no.

    Typical battery life is 18 months, which is long enough that I actually forget they are battery operated as they just do their thing day after day, so when one finally reports low battery it's a surprise and I remember that there are actually batteries in them...

    I keep a large discounted bulk purchase pack of about 40 alkalines in the house - as soon as one starts reporting it's flat I change it immediately, job done in about 30 seconds. The other way you can approach it if you don't mind a bit of waste is to simply change the whole house at the same time and get 18 month free from any low battery warning nagging.

    Or if you want to spend a bit more, fit Lithium batteries, which should last significantly longer, probably well over 24 months.
    Zonevalves give less control but otherwise issue free i thought
    Zone valves can fail. If you get a good brand like Honeywell they last a long time, but some of the cheaper ones are known to fail. When you use zone valves with Evohome they are TPI modulated - which means that when the room is close to the set point the zone valve will be opened and closed every 10 minutes that the heating is on.

    I've found the HR92's to be very reliable. I've only had one fail after about 2 years - but that was physical damage thanks to my 2 year old - he worked out how to remove the screen brace wedge, then once the screen was extended and he had climbed on top of it and smashed some large toy cars into it enough times one of the screen pivot pins broke. It then continued to work for a few months with the screen hanging partly off, then one day he grabbed it and gave it a good tug, ripping the screen right off.

    But none of the ones that hasn't been physically abused have given me any trouble, aside from the battery contacts needing bending to make them a bit tighter on a few of them, which seems to be a known issue with these.
    Last edited by DBMandrake; 27th May 2018 at 10:49 AM.

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