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Thread: Evohome BDR 91 time on/off

  1. #11
    Automated Home Legend paulockenden's Avatar
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    The benefit of OT is that once your house is up to temperature and just needs to be kept warm you'll have a steady flow of warm water running around your heating circuit rather than short pulses of very hot water.

  2. #12
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    Thanks and sounds good.

  3. #13
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    Quote Originally Posted by paulockenden View Post
    The benefit of OT is that once your house is up to temperature and just needs to be kept warm you'll have a steady flow of warm water running around your heating circuit rather than short pulses of very hot water.
    Yes, that is what I had figured. Assuming that I don't have hot water priority, then I'm guessing that some of the benefits would be negated?

  4. #14
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    Quote Originally Posted by smar View Post
    Yes, that is what I had figured. Assuming that I don't have hot water priority, then I'm guessing that some of the benefits would be negated?
    If your system has a heating zone valve you could wire in hot water priority fairly easily.

    Do you have both heating and hot water zone valves and two BDR91's controlling them ? If so just take the feed for the relay contacts for the heating zone valve from the normally closed contact of the hot water zone valve - when the hot water zone valve goes on it will force the heating zone valve off no matter what the heating BDR91 does.

  5. #15
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    Quote Originally Posted by DBMandrake View Post
    If your system has a heating zone valve you could wire in hot water priority fairly easily.

    Do you have both heating and hot water zone valves and two BDR91's controlling them ? If so just take the feed for the relay contacts for the heating zone valve from the normally closed contact of the hot water zone valve - when the hot water zone valve goes on it will force the heating zone valve off no matter what the heating BDR91 does.
    Yes, I do have the separate zone valves/BDR91s, though I had previously disconnected the one for raditators, and left it in the open position (it had started to stick - even though it was only about a year old - possibly due to the constant opening/closing). I'll look at getting this reinstated. Should be easy enough to rewire the way you've described.

    I also have an UFH manifold which has its own water temperature control. Would you recommend taking this offline as well with hot water priority, or leave this as is, given that it has its own water temperature regulation?

    Final question - I'm assuming that the half an hour/40 minutes or so that the hot water may take to heat up shouldn't impact the room temperatures much, given the hot water priority?

  6. #16
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    I'm no expert but I would have thought that hot water priority is only useful if you have a "quick heat" cylinder which has a proper plate heat exchanger in it, so that the heat to the radiators is only disabled for a short time. (10-15 mins at most) Otherwise you might as well have a combi boiler...

    My cylinder is an old one with (presumably) just coils of copper pipe and as such the heat up time is quite slow (about 30 minutes) so for that reason I don't use hot water priority.

    Although I think this is probably non standard, to make this work well I also have a separate ABV in series with the hot water indirect loop set to 0.2 bars so that even when both cylinder indirect loop and radiators are circulating at the same time there is sufficient pressure to supply the radiators enough to keep them hot and even heat up, at a reduced but useful rate. (Albeit at 0.2 bars instead of the 0.5 bars the main ABV regulates to when the cylinder loop is cut off) Without that the very short and easy flow path through the cylinder indirect loop completely starved all the radiators and effectively caused a hydraulic hot water priority...

    How long does a full reheat of your cylinder take if you use all the hot water, and is your system configured with a flow temperature boost during reheat ? (Mine isn't)
    Last edited by DBMandrake; 2nd February 2020 at 09:07 PM.

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