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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 08:07 PM.

  7. #17
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    Quote Originally Posted by DBMandrake View Post
    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)
    Thanks and yes, makes sense. Not really sure if our cylinder would be classifed as a quick re-heat - it is an unvented OSO Powercyl Delta Coil, installed last year. Normally takes about 45 minutes or so to reheat from empty (250 litres). The only way we would be able to reheat in 10-15 minutes is if the reheat started well before the bulk of the hot water is used up.

    I've just installed an OpenTherm bridge, and will see how it goes without the hot-water priority for a few weeks. Interesting to see all the data coming through...

    EDIT: Just checked the cylinder specs:
    • Heating time: 43.3 mins
    • Reheat time (70%): 32.5 mins
    Last edited by smar; 26th February 2020 at 05:22 PM.

  8. #18
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    I would call that slow. My old fashioned (but only 100 litre) cylinder can reheat from fully used in under 30 minutes and I consider that slow. I think a fast reheat cylinder would be anything that can reheat in under about 15 minutes, and to do that requires a plate heat exchanger in the cylinder to get the necessary thermal conductivity - 22mm coiled copper pipes (which is what will be inside mine!) won't do it.

    If you have fast reheat <15 minutes, hot water priority makes sense where the heating zone valve remains closed during the hot water reheat (already hot radiators will stay fairly hot) and the flow temperature is boosted to minimise the reheat time. Once the reheat is finished the flow temperature goes back to normal (controlled by CH) and the heating zone valve opens again.

    For a slow reheat cylinder like mine I allow heating and hot water zone valves to both open at the same time and I don't boost the flow temperature. As a "hack" I have a second automatic bypass valve in series with the indirect loop on the cylinder set to 0.2 bars - this means when hot water is reheating the cylinder doesn't steal all the flow from the radiators (by providing a much lower flow resistance path) because at least 0.2 bars of differential pressure has to build up before anything flows through the cylinder indirect loop. (Conversely 0.2 bars is low enough that with all radiators open the system can still generate >0.2 bars so the hot water indirect loop can flow) Radiators heat a bit slower with 0.2 bars than the 0.5 bars the main automatic bypass valve is set to, but it's still enough for them to warm up at a reasonable pace or stay hot if they were already hot.

    I don't generally notice any difference in radiator performance when hot water is reheating with this configuration unless everything is off and cold and both hot water and heating come on at the same time - then the initial radiator warm up is a little bit slower, but that is to be expected when the cold cylinder is absorbing approximately 6kW of the heat generated by the boiler anyway. With optimal start enabled startup time of each zone and hot water is usually staggered anyway so the only time everything comes on at once is if I have manually disabled the heating and later turned it back on.
    Last edited by DBMandrake; 27th February 2020 at 09:07 AM.

  9. #19
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    Interesting "hack" you are using...

    Yes, the reheat time does seem to be too high for an effective hot water priority solution. I have been running without hot water priority since the system was installed a year or so ago, and it has been working reasonably well (leaving aside CS92 related issues) except for the temperature overshoots in radiator zones which are already at setpoint when hot water heating starts. I'll see how things go with the new OTB, and if things don't noticably improve, may have a look at trying out the hotwater priority even with the slow reheat times, just to see how effective it is, and revert back if it does not work out (via the simple wiring change on the BDR91 C port).

  10. #20
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    Quote Originally Posted by smar View Post
    Interesting "hack" you are using...

    Yes, the reheat time does seem to be too high for an effective hot water priority solution. I have been running without hot water priority since the system was installed a year or so ago, and it has been working reasonably well (leaving aside CS92 related issues) except for the temperature overshoots in radiator zones which are already at setpoint when hot water heating starts. I'll see how things go with the new OTB, and if things don't noticably improve, may have a look at trying out the hotwater priority even with the slow reheat times, just to see how effective it is, and revert back if it does not work out (via the simple wiring change on the BDR91 C port).
    I have HW priority on my system and it works really well. Tbh any larger system it's well worth while IMO whether you've got adapting flow temps or not. You have to take into account the KW rating of your coil, the KW rating of the heat input (Boiler etc.) and the demand from the radiator circuit. For instance the figures OSO you quote will be for inputting the full 16.5kw into the coil, plus you have to remember that you'll never be reheating from cold and thus HW priority can give you a much quicker reheat time. For instance our 20 year old MegaFlo (Coil rated at 14kw) reheats in about 15 minutes with it, when I turn HW priority off and a few radiators are on it'll take 25-30 minutes due to our boiler only being an output of 15kw.

    It's easy enough to do with wiring the BDR91s in a certain way, you don't need any additional equipment. I've added a switch to mine so I can turn the feature on and off.

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