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Thread: Evohome - Boiler firing with no demand

  1. #1
    Automated Home Sr Member
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    Default Evohome - Boiler firing with no demand

    Why is it that even after the demand for heat has fallen to zero, minutes later my boiler is asked to fire? At best this is wasting money, and in the worst case scenario there is a danger of the boiler locking out due to overheat. (Purple line demand, green boiler being asked for heat, red is just an indicator of water temperature, and due to the way it is collected, has a small delay)

    This particular plot is taken at a time where I turned off (via override) the last active zone. Not sure if that has any bearing on not.

    Capture.JPG

  2. #2
    Automated Home Guru
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    Was your System Summary in the System Configuration menu showing 0%?

    There can be a delay before 0% is indicated and until then there may still be a call for heat.

  3. #3
    Automated Home Ninja
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    Mine does this, however I usually find the heating valve opens a minute or so later. But yeah it is rather annoying.

  4. #4
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    Funnily enough I got exactly this this morning, looked at controller, zero heat demand from everything, went outside to boiler, BDR came on, boiler fired up, then about a minute later turned off. Went back inside and controller still showing zero demand on all zones ??

  5. #5
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    Quote Originally Posted by lloyd View Post
    Why is it that even after the demand for heat has fallen to zero, minutes later my boiler is asked to fire? At best this is wasting money, and in the worst case scenario there is a danger of the boiler locking out due to overheat. (Purple line demand, green boiler being asked for heat, red is just an indicator of water temperature, and due to the way it is collected, has a small delay)

    This particular plot is taken at a time where I turned off (via override) the last active zone. Not sure if that has any bearing on not.

    Capture.JPG
    Quote Originally Posted by roydonaldson View Post
    Funnily enough I got exactly this this morning, looked at controller, zero heat demand from everything, went outside to boiler, BDR came on, boiler fired up, then about a minute later turned off. Went back inside and controller still showing zero demand on all zones ??
    Welcome to the world of antiquated RF protocols which don't use acknowledgements to confirm that the target device actually received a message...

    In short, the BDR91 continuing to cycle on for a while after the controller head demand changes to zero is because the RF message from the controller to BDR91 setting the heat demand to zero was not heard by the BDR91 for some reason. (Noise/interference, collision etc)

    If that happens the BDR91 will just continue to follow the previous heat demand until the next time the controller sends a heat demand message to it, which could be anywhere from a few minutes to 20 minutes away.

    If the heat demand is not changing - which is typically the case once it goes to zero when all the zones are set back then it will be 20 minutes before another duplicate heat demand message is sent, so the boiler can potentially keep cycling on for up to 20 minutes as a result of a single lost RF message.

    Lost messages like this happen more than you might think but the effect of losing a message occasionally is minimal in most cases and goes unnoticed - for example if a heat demand message changing from 30% to 35% is lost, and 5 minutes later another message is sent that it changes to 40%, the fact that the 35% was skipped makes little difference.

    But a zero heat demand when the heating is turned off being lost may be noticed as the boiler may run on for up to 20 minutes. A BDR91 will only start flashing red (comms error) after 45 minutes of no messages heard, so it would take 3 heat demand messages in a row spaced at 20 minutes before you'd see any sort of overt error, and that is very unlikely to happen. (I've only seen it happen when I've shut down the controller)

    Unless the boiler continues to run on for more than 20 minutes or it happens frequently I wouldn't really worry about it. If it's just occasional and lasts for less than 20 minutes then there's probably not much you can do about it as the system doesn't use acknowledgements to confirm that messages get through (for most message types except binding) so it's inevitable that some will be lost especially when 868Mhz is shared with other devices like smart meters, weather stations, tyre pressure sensors etc.

    I occasionally notice it happen on my system after scheduled shutdown as the boiler relay is visible in the kitchen, but not very often - and I've already done everything I can to attempt to improve the situation.

    Only if Honeywell implements a modern, robust communications protocol which acknowledges all messages will this sort of issue be completely solved, but that would break compatibility with all existing devices, (of which there are many more than just Evohome) it can't be retrofitted into the existing protocol with the existing devices.

  6. #6
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    Thanks. Interestingly enough I've just had a new automatic garage door installed which placed a metal frame within the garage and the boiler and it's controls are the other side of this from the controller inside the house. May run a long cable over the ceiling and move the BDR's to the other side onto a partition wall instead.

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