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Thread: Evohome for new installations?

  1. #31
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    Quote Originally Posted by Edinburgh2000 View Post
    I have been thinking about this for a while since DBM suggested the 'three licks of flame' as the icon to show heat demand from a zone. That sounds great and would certainly be very illuminating. However, as I understand it, Evohome remains a binary system for those of us without OpenTherm. BDR91s are either on of off. Thus with the 'licks of flame' you could have, say, three zones showing just one lick, but you would not know which of those zones has triggered its BDR91. A single lick of flame would mean (I think) that that zone would call for heat X minutes out of Y minutes. But you would not know whether it is in the X minutes just from the one lick showing on the display. At the risk of over complicating DBM's excellent suggestion, could I suggest that the licks of flame should change colour when their zone is actually demanding heat (i.e. demanding the BDR91 to be on)?

    (I realise that I am in a minority of about one on this forum in not having ubiquitous HR92s but rather using separate zone valves, each with its own BDR91. I am therefore keen to see on my controller or app whether each BDR91 is 'on' or 'off', without having to camp in my boiler room to watch the BDR91s' little green lights. However, it is possible I am misunderstanding the aggregation of demand by the controller to the single BDR91 controlling the boiler in the full-HR92 world, and my suggestion above might be inappropriate for that - in which case, apologies.)
    Heat demands are always an integer between 0-200, which represents a percentage from 0-100% with a 0.5 resolution.

    The heat demand from an HR92 is just an indication of the valve opening percentage, and for a BDR91 it is a duty cycle - a constant heat demand of 30% sent to a BDR91 would turn it on for 3 minutes out of every 10 minutes.

    The controller is not commanding every individual turn on/off event - it sends a percentage from time to time and the BDR91 does the cycling itself. (The exception is the hot water relay which is always commanded to 0 or 100% to immediately turn it off or on - there is no TPI modulation of the hot water relay)

    So the problem you suggest of the indicators turning on and off constantly doesn't exist - its only the relay that cycles on and off, the heat demand is always a proportional value, which can be reduced down to say 3-4 quadrants for display purposes.
    Last edited by DBMandrake; 8th September 2017 at 04:57 PM.

  2. #32
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    Quote Originally Posted by DBMandrake View Post
    Heat demands are always an integer between 0-200, which represents a percentage from 0-100% with a 0.5 resolution.

    The heat demand from an HR92 is just an indication of the valve opening percentage, and for a BDR91 it is a duty cycle - a constant heat demand of 30% sent to a BDR91 would turn it on for 3 minutes out of every 10 minutes.

    The controller is not commanding every individual turn on/off event - it sends a percentage and the BDR91 does the cycling. (The exception is the hot water relay which is always commanded to 0 or 100% to immediately turn it on or off)

    So the problem you suggest of the indicators turning on and off constantly doesn't exist - its only the relay that cycles on and off, the heat demand is always a proportional value, which can be reduced down to say 3-4 quadrants for display purposes.
    Thanks DBM. Does that mean that the controller sends a "30%" signal to the BDR91, and then the switching logic is done by the BDR91 itself (i.e. the controller does not know whether the BDR91 is on or off)?

  3. #33
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    Quote Originally Posted by Edinburgh2000 View Post
    Thanks DBM. Does that mean that the controller sends a "30%" signal to the BDR91, and then the switching logic is done by the BDR91 itself (i.e. the controller does not know whether the BDR91 is on or off)?
    The controller sends a heat demand update message to the BDR91 every 20 minutes, (as a refresh) or every time the heat demand changes, the 20 minute refresh also re-synchronises the timing of multiple BDR91's so they stay in time with each other and don't drift out of time.

    The BDR91 itself follows the on/off cycle for the duty cycle commanded of it autonomously - if you pull the battery out of the controller you'll notice your BDR91 keeps cycling in the same pattern for over 45 minutes until finally the red error LED appears.

    The controller doesn't care whether the relay is currently in the on or off part of its cycle - although I believe it is able to query the relay if it wants to and does this occasionally to confirm that the BDR91 is actually responding to commands. (If it doesn't respond to these queries for a long time, you get a "comms fault" logged)

  4. #34
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    Quote Originally Posted by DBMandrake View Post
    The controller sends a heat demand update message to the BDR91 every 20 minutes, (as a refresh) or every time the heat demand changes, the 20 minute refresh also re-synchronises the timing of multiple BDR91's so they stay in time with each other and don't drift out of time.

    The BDR91 itself follows the on/off cycle for the duty cycle commanded of it autonomously - if you pull the battery out of the controller you'll notice your BDR91 keeps cycling in the same pattern for over 45 minutes until finally the red error LED appears.

    The controller doesn't care whether the relay is currently in the on or off part of its cycle - although I believe it is able to query the relay if it wants to and does this occasionally to confirm that the BDR91 is actually responding to commands. (If it doesn't respond to these queries for a long time, you get a "comms fault" logged)
    Got it now. Many thanks.

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