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Thread: Y87RF with BDR91 and no Evohome controller

  1. #1
    Automated Home Jr Member
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    Default Y87RF with BDR91 and no Evohome controller

    Having finally given up on Evohome due to constant overshoot with no apparent solution I thought I would try using a Y87RF bound directly to the BDR91 connected to my combi boiler.

    I was amazed to see no overshoot and pretty much perfect temperature control in the room where the Y87RF was placed so with no Evohome controller between these 2 components all appears fine.

    I assume that when I set a target temperature on he Y87RF it transmits both the current room temperature & user set point to the BDR91 which then computes the estimated TPI cycle (I may be wrong here so please educate me if that is the case)

    So for me it looks like the 'smart' bit was not so smart but the BDR91 can control the room temperature very well if given suitable demand.

    An interesting experiment for me and a useful fallback if whatever system I try next fails.

  2. #2
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    What else was Evohome controlling that was causing the overshoots? I bet if you put the Evohome controller as your room sensor in the same place as the Y87RF and used it to control a single BDR91 boiler relay, the result would be exactly the same.

  3. #3
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    Thanks for the your thoughts however the Y87RF was never part of the system when the Evohome controller was part of the system. If it had been your easy fix would have been great.

  4. #4
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    I lot of people don't realise how many variables there are when dealing with a single heat source, multi zone system. That's why I said comparing Evohome to a single zone room thermostat is not a fair comparison.

  5. #5
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    It's all long gone now but if Evohome can't control a single zone in a manner that meets my requirements the system is clearly not for me which I accept. For the record I do not include myself in the large group of people with no knowledge of variables you mention. And was comparing two single zone setups which seemed pretty fair to me.

  6. #6
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    If you had a single Evohome controller using its internal sensor controlling a single BDR91, positioned in the same location as the Y87RF - the two of them should have behaved exactly alike. Or are you saying that even in this condition the Y87RF was more accurately controlling the heating than the Evohome controller was.

  7. #7
    Automated Home Guru
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    Not real sure how you ended up with Evohome to control a single zone and in fact without a valve for that zone. Evohome isn't designed for this and it is not surprising to see overshoots as well as lagging in such a setup. Assuming it was app control that you wanted, what you should have gotten is either Y87RF + RFG100 or a Lyric thermostat.

  8. #8
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    If you visit https://evohome.honeywellhome.com/?src=evohome and complete the system builder for a single zone combi boiler you will find it is a recommended configuration. It did not work to my satisfaction but is at odds with your design limitation viewpoint. So there is a clear conflict between Honeywell & gordonb3 on system requirements I will stick with Honeywell on this. My only requirement was it worked to my satisfaction which it did not that is why it is history. Your 'should have gotten' a Lyric advice explains a lot !

  9. #9
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    Well, allow me to explain to you then why I think the advice was wrong.

    Evohome is designed as a multi zone system where every individual zone must be able to generate a heat request to your heat source. Since the zones operate independent of each other this allows the heat source to be triggered in rapid succession and this is where your problem starts: because this can lead to significant waste of fuel Evohome prevents such events by limiting how fast the boiler may be shut down again and then reactivated. In a multi zone setup this does not lead to overshoots because the zone valve will close at the set temperature but your setup did not feature such a valve and thus you became subject to the boiler logic which has a default setting of six switch points per hour.

  10. #10
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    The Evohome can work as a single zone room stat with a single boiler relay, in fact that is how is arrives when bought new I.e. It can be a direct replacement for a Y87RF and a BDR91, this is why is shows up in the Honeywell configuration. No one should ever do that though because that's a complete waste of buying a multi zone system and operating it as a single zone room stat. However I am not going to debate that.
    What I am curious about is that as a single zone room stat, how can the Y87RF perform better than the Evohome both placed in the same location in the room and both controlling the same single BDR91 for the boiler.

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