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Thread: OpenTherm issue

  1. #1
    Automated Home Jr Member
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    Default OpenTherm issue

    Hi folks,

    Iíve got a bit of an issue. I have an UFH zone which is controlled by the EvoHome internal temperature sensor, and the valve / pump are connected to a BDR.

    Boiler is an Intergas Eco RF.

    The issue is that as the zone gets close to temperature, the evohome gradually reduces the demand to both the opentherm and BDR as observed on the status page.

    This is all well and good except it leads to a scenario where the boiler is still running at, for example, 48% but the BDR has shut off at this point so the boiler is doing nothing other than circulating via the ABV.

    This canít be normal, nor desirable?

    Cheers,

    Steve

  2. #2
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    Default

    I assume you have other zones as well as the UFH ? And that the UFH is just configured as a "zone valve" zone type ?

    If so yes, it is behaving as expected. Individual zones that use a BDR91 as the controller for the zone - like the "zone valve" type, use TPI modulation, so when it gets close to the set point it will open and close on a (default) 10 minute cycle in proportion to the heat demand.

    This is necessary in the event that some other zone has a higher (or maximum) heat demand that would cause the boiler to be running at a higher or maximum flow temperature - in that situation the only way this individual zone could throttle back it's heat demand independently of others is to TPI modulate the zone valve.

    As you point out, if no other zones have any heat demand (all HR92's etc are closed) then the flow during the period of the TPI cycle where the zone valve is closed will have to go through the automatic bypass valve - and that's part of the job of the ABV.

    Is it undesirable ? A better question would be, is there an alternative which still cooperates properly with other zones in the system without excessive interaction between zones ? To which I'd say, probably not.

    It doesn't waste any significant amount of gas because with only the ABV flowing the boiler will very quickly reach the requested temperature and turn off the burner. Only the pump will keep running during that zone valve closed phase, and that is a good thing to avoid heat exchanger hot spots.

    The only concern really is whether it would trigger one of the over zealous cutouts that some boilers have that can trigger on things like excessive temperature rise rate or insufficient flow/return differential temperature, but I'd say that's more of a boiler control system design flaw than a real problem, as those kind of limits can sometimes be triggered even on a system with all HR92's.
    Last edited by DBMandrake; 29th July 2018 at 06:24 PM.

  3. #3
    Automated Home Guru
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    Default

    Quote Originally Posted by stevelup View Post
    The issue is that as the zone gets close to temperature, the evohome gradually reduces the demand to both the opentherm and BDR as observed on the status page.

    This is all well and good except it leads to a scenario where the boiler is still running at, for example, 48% but the BDR has shut off at this point so the boiler is doing nothing other than circulating via the ABV.

    This can’t be normal, nor desirable?

    I don't have UFH but I am guessing that your UFH is supplied through a spring-return valve which closes whenever the BDR shuts off. That is a hobby-horse of mine on this forum. I am in a minority of one but I do not like the spring-return valves for exactly that reason - that they inhibit the pump running on after the boiler has shut down - and I use motor open / motor-close (MOMO) valves instead and wire those such that they will only close when there is boiler demand. Thus, when there is no boiler demand (all BDRs are off) the valves stay where they are and let the pump circulate the water round whichever was the last zone to be open. That was discussed on this thread.

    I realise your situation is different, in that you use Opentherm and so the boiler is still firing even though all BDRs are off, but the principle is still the same, that a MOMO valve could help.
    Last edited by Edinburgh2000; 30th July 2018 at 08:37 AM.

  4. #4
    Automated Home Jr Member
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    Hi

    The arrangement is as follows:-

    UFH zone with BDR91 - this has a two-port valve and pump connected to it
    DHW with BDR91 - again, two-port valve here
    All other rads have HR92s except for two towel radiators which have conventional TRVs
    ABV is a Honeywell DU144 installed adjacent to the boiler and set at the factory default

    Glad to hear it's working as designed. I still think it's a bit clumsy. If there's only one zone demanding heat (or indeed if there's only one zone valve full stop), then it would be logical to -not- use TPI on that zone. Seems like it would only be a handful of lines of code to handle this scenario!

  5. #5
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    I don't agree. What would be clumsy though with your proposed solution would be the transition from just the one zone demanding heat to another zone demanding a very small amount of heat - the transition from the zone not doing TPI and relying entirely on flow temperature control to having the flow temperature forced higher by another zone and having to guess how much to TPI modulate the first zone to maintain the same heat output. Chances of that being anything remotely smooth in operation is, well, remote.

    Zone's aren't just on or off, as the temperature falls in a zone that was off it initially opens an HR92 a very tiny amount and gradually increases the opening and heat demand as required. So any sudden transition in mode of operation at this point is going to cause temperature instability in the zone that was previously relying on only flow temperature control.

  6. #6
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    I can see that argument.

  7. #7
    Automated Home Legend paulockenden's Avatar
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    What's needed is UFH that opens gradually, like an HR92, rather than TPI on/off. I’m sure that must be possible. Isn’t there a suitable actuator?

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