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Thread: Boiler with separate HW and CH max and OpenTherm

  1. #31
    Automated Home Legend paulockenden's Avatar
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    Thing is, Evohome demands 100% most of the time. It’s only when ALL zones are within one degree of setpoint that it'll scale the boiler demand back. And the it just needs one zone to stray outside that (or start oscillating around the setpoint) and it’s back to 100% again.

  2. #32
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    There may be more to it, but from what I can see from the system update - the OT bridge asks for the same percentage as the highest percentage radiator call.

    It doesn't matter if one rad or ten rads are calling for 100% - it still asks for 100%. Or am I missing something?

    If OT takes the control and smart logic away from the boiler, then it should be Evohome that provides it.

  3. #33
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    Quote Originally Posted by dty View Post
    OpenTherm requests specific temperatures from the boiler, not percentages, so it would be even easier!
    You’d just set two maximum temperatures in Evohome - one for hot water, one for heating. This would only work for OpenTherm controlled boilers, of course. Unfortunately, I doubt the OT bridge has this capability right now and I don’t believe it’s field-upgradable. So it would necessitate buying a new OT bridge (unless Honeywell were remarkably generous and offered a swap-out programme. I expect they can reprogram the bridges once they receive them back and send them out to another customer.)
    But... The Evotouch doesn't deal in flow temperatures, it sends a heat demand to the OpenTherm bridge 0-200 as an integer representation of a percentage, just as it does with a BDR91.

    So the OpenTherm bridge must map that 0-200 heat demand range to a flow temperature range of something like 10-90 degrees. (Not sure what the bottom end of the range is, but I think the top is 90) Somebody with the right equipment (both Domoticz and Opentherm logging) would need to measure it to see what the relationship between the two is but I'd put money on it being a fixed, pre-programmed relationship in the OpenTherm bridge, perhaps even a straight line. Therefore the Evotouch could indirectly specify a flow temperature by knowing what heat demand maps to what flow temperature. No hardware upgrade needed.


    Quote Originally Posted by dty View Post
    Actually, I’ve been wondering how some of this affects the learning algorithms. I have one or two particularly cold rooms in my house so at the moment Evohome is requesting the full 90C a lot of the time - even though the boiler modulates down to 6kW to achieve that with only those two rooms asking for any significant heat. However, the boiler limits the flow to 75C. But Evohome thinks it’s getting 90C. Then I see it step down to 85C, but of course nothing changes at the boiler - it’s still producing 75C. Then again at 80C. It’s not until it requests less than 75C that anything useful actually happens. This has surely got to confuse the learning algorithm?
    Yes I've wondered this before and there was a thread a while back where I was discussing it.

    Currently when you limit the maximum flow temperature on an OpenTherm system at the boiler (for boilers that allow it!) you are simply truncating the requested flow temperature. So you end up with dead band in the top end of the control range where increased demand from the Evotouch does nothing. Worse still, the lower you set your maximum flow temperature the more dead band there is.

    This can't be optimal for the control algorithm as it won't be aware of any deadband the boiler has introduced except very indirectly by noticing that when it increases the heat demand from (say) 70% to 100% that nothing happens! It would be better if the boiler's flow temperature maximum scaled the requested temperature instead of truncating it, but that doesn't seem to be the way OpenTherm boilers work.

    Probably beating a dead horse because I've already suggested it before a couple of times but what really needs to happen is for the Evotouch to have a maximum flow temperature setting in the system installer menu (which would only be applicable and visible if an OpenTherm bridge was bound, as the feature couldn't be implemented with a BDR91 - nor does it need to be!) or perhaps even in the normal settings menu. By setting this the maximum heat demand called for from the OpenTherm bridge could be reduced to the value that would map to that flow temperature.

    There's several advantages to this:

    1) Because the maximum flow temperature limit is set in software in the Evotouch it is fully aware of it, and is able to scale in software the calculated heat demand. In other words the control algorithms still work out a 0-100% (0-200) heat demand based on inputs from zones, but at the last step before sending to the OpenTherm bridge this value is then scaled down by a factor such that an original 100% heat demand maps to a lower heat demand and thus the target maximum flow temperature. Because of this there is no dead band in the control loop which should result in better control.

    2) People with boilers which do not allow setting a maximum flow temperature limit with OpenTherm will finally be catered for. The inability of so many boilers to do this right seems very problematic (just look at the posts on here where people have had to go back to a BDR91!) and instead of pointing the finger of blame at boilers where it rightly belongs, just accept the fact that many boilers don't do it right and provide the maximum flow temperature setting in the Evotouch.

    You're literally talking about adding a division or multiplication of the heat demand and a UI element to let you set a maximum flow temperature. Not difficult to implement as far as I can see!

    It might be desirable to set a maximum flow temperature for hot water demand as well. For example if you have your CH max flow set to 60 you may not want or need it asking for 90 degrees to heat hot water on a system without hot water priority! It might be more reasonable and acceptable to ask for say 75 degrees - hot water will heat a little bit slower but your room temperatures wont overshoot much. For those with hot water priority they could leave it set at the default so hot water calls for the full 90 degrees and heats as fast as possible.

    Controlling the maximum flow temperature in the evotouch (and scaling it instead of truncating it!) also opens the door to software based weather compensation where that maximum flow temperature can be configured to depend on the outdoor temperature - which we now have available on the latest firmware. I'm very doubtful that we'll ever see this though as the company line from Honeywell with Evotouch is that weather compensation isn't needed or useful in any way. But as someone who still has to adjust the flow temperature manually with the seasons despite having Evohome I beg to differ.

    The problem with combining OpenTherm and Weather compensation at the moment is that no boilers appear to do this correctly, and even if they do apply the weather compensation max flow temperature to the opentherm requested flow temperature they are almost certainly still truncating instead of scaling, which means the Evotouch is subjected to a constantly varying and unknown deadband! If it was all done in the Evotouch there would be no need for truncation and it could be properly scaled and I believe it would work very well as long as it has both slope and offset adjustments that are set correctly.
    Last edited by DBMandrake; 25th January 2018 at 10:44 AM.

  4. #34
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    Quote Originally Posted by fergie View Post
    There may be more to it, but from what I can see from the system update - the OT bridge asks for the same percentage as the highest percentage radiator call.

    It doesn't matter if one rad or ten rads are calling for 100% - it still asks for 100%. Or am I missing something?
    That's what it does, yes.

  5. #35
    Automated Home Legend paulockenden's Avatar
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    What DBMandrake suggests (I think a few of us have suggested similar, in the past) also has the advantage that without the dead band (great name for it!) there would be finer control at the top end, so the boiler probably wouldn't be running at 100% quite so often. It would allow for better control.

    Providing the extra parameters needed is really no different from exposing the cycle-time values when a boiler relay is configured. So there is precedent here.
    Last edited by paulockenden; 25th January 2018 at 11:07 AM.

  6. #36
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    Is having customisable CH and HW temp limits something anyone has discussed with Honeywell?

    It surely canít be that hard as the HW already has a max temp to switch off at.

  7. #37
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    Quote Originally Posted by DBMandrake View Post
    That's what it does, yes.
    Most of the time, the Opentherm heat demand matches the highest radiator call - BUT NOT ALL OF THE TIME. As I type, my OT Boiler Demand is 27% but the highest zone demand is only 24%. I have see much bigger differences over the past week. The 'variation' seems to get bigger as OAT increases and less zones demand heat. The three zones demanding heat at the moment have 24, 8 and 5% annotated against them.

  8. #38
    Automated Home Legend paulockenden's Avatar
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    Isn't that likely to just be the time delays inherent in the system?

  9. #39
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    Quote Originally Posted by paulockenden View Post
    What DBMandrake suggests (I think a few of us have suggested similar, in the past) also has the advantage that without the dead band (great name for it!) there would be finer control at the top end, so the boiler probably wouldn't be running at 100% quite so often. It would allow for better control.

    Providing the extra parameters needed is really no different from exposing the cycle-time values when a boiler relay is configured. So there is precedent here.
    +1

    This would open up the possibility for Viessmann boilers to be controlled as per the native Viessmann OT controller (and as that manufacturer at least appears to have designed their side of the OT interface to work this way).

    Lack of max CH flow temp control with a Viessmann 111-W is the only reason I had to go back to BDR91 control from OT..

  10. #40
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    Quote Originally Posted by DBMandrake View Post

    ...The problem with combining OpenTherm and Weather compensation at the moment is that no boilers appear to do this correctly, and even if they do apply the weather compensation max flow temperature to the opentherm requested flow temperature they are almost certainly still truncating instead of scaling, which means the Evotouch is subjected to a constantly varying and unknown deadband! If it was all done in the Evotouch there would be no need for truncation and it could be properly scaled and I believe it would work very well as long as it has both slope and offset adjustments that are set correctly.


    Could you expand a little on what you mean by this please?


    Martin.

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