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Thread: Evohome DHW overshoots

  1. #11
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    Any luck ? I had my first hot water overshoot in months this morning, here is the CSV output from Domoticz. Keep in mind I use the Web API scripts which means I only get readings every 5 minutes, not at the times when readings are transmitted. Hot water set point is 54 degrees with a 5 degree differential. Notice how the reading rose to 52.56 then "stuck" there for a full 65 minutes before the reading suddenly jumped to 64.75. In reality the temperature would have been rising that whole time, but no updates were being sent. (or received - hard to be sure which)

    Code:
    20/02/2017 04:50	0	20/02/2017 04:50	43.11
    20/02/2017 04:55	0	20/02/2017 04:55	43.11
    20/02/2017 05:00	0	20/02/2017 05:00	43.11
    20/02/2017 05:05	60	20/02/2017 05:05	43.33
    20/02/2017 05:10	60	20/02/2017 05:10	43.33
    20/02/2017 05:15	60	20/02/2017 05:15	50.07
    20/02/2017 05:20	60	20/02/2017 05:20	52.56
    20/02/2017 05:25	60	20/02/2017 05:25	52.56
    20/02/2017 05:30	60	20/02/2017 05:30	52.56
    20/02/2017 05:35	60	20/02/2017 05:35	52.56
    20/02/2017 05:40	60	20/02/2017 05:40	52.56
    20/02/2017 05:45	60	20/02/2017 05:45	52.56
    20/02/2017 05:50	60	20/02/2017 05:50	52.56
    20/02/2017 05:55	60	20/02/2017 05:55	52.56
    20/02/2017 06:00	60	20/02/2017 06:00	52.56
    20/02/2017 06:05	60	20/02/2017 06:05	52.56
    20/02/2017 06:10	60	20/02/2017 06:10	52.56
    20/02/2017 06:15	60	20/02/2017 06:15	52.56
    20/02/2017 06:20	60	20/02/2017 06:20	52.56
    20/02/2017 06:25	60	20/02/2017 06:25	52.56
    20/02/2017 06:30	60	20/02/2017 06:30	64.75
    20/02/2017 06:35	60	20/02/2017 06:35	64.75
    20/02/2017 06:40	60	20/02/2017 06:40	64.75
    As my boiler flow temperature is only set to 70 at the moment 65 degrees is about the hottest the hot water will get even if the hot water zone valve remained open permanently. Good thing my boiler doesn't use an 80 or 90 degree flow temperature boost for hot water or my hot water would have been REALLY (dangerous, scalding) hot this morning after a 65 minute overrun!

    No idea what caused it - no fault logbook entries and no other signs of system misbehaviour, and the hot water behaviour has been flawless for months.

    One question - do you have Alkaline batteries in your CS92A or have you put lithiums in ? I'm considering putting Lithiums in mine because they hold their terminal voltage higher for a lot longer as the battery discharges, and the device does seem to be sensitive to low battery voltage either due to low batteries, or poor battery contact. So a battery that keeps a higher terminal voltage for a longer portion of its life may be beneficial. So far I have only ever used Alkaline's in all my Evohome devices for cost reasons, but I'm starting to reconsider that due to the superior voltage discharge curve of lithium batteries.
    Last edited by DBMandrake; 20th February 2017 at 10:37 AM.

  2. #12
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    I was just thinking yesterday that I should update this thread.

    Having an HGI80 (-equivalent) doesn't help with data resolution. I still get a data-point every 5 minutes in Domoticz. So, as much as the Evohome integration must be capturing that data, Domoticz is somehow quantizing it. Also, because I don't run my Domoticz in debug mode (Raspberry Pi, smashed SD card...) I wasn't able to do any message-level debugging.

    I did, however, disassemble the CS92A and improve the battery connection. Since doing that, I've not had an overrun. And I can understand a bit better about some of the odd behaviour I was seeing. For example, the set point changing at 5:30 in the morning, but the temperature in the cylinder not increasing for 30-45 minutes is cause by the battery-saving heuristics of the sensor (assuming your theory is correct!)

    So I'll be keeping an eye on it, but for now things are definitely improved.

  3. #13
    Automated Home Legend paulockenden's Avatar
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    I've not had an overshoot since mid December.

    chart (2).jpg

    Alkaline batteries, never fiddled with them.

    P.

    p.s. Set point avg being below actual is because the DHW is off overnight, so lowers the average. I wish Domoticz would (optionally) ignore zeroes when averaging.

  4. #14
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    Oh yeah, batteries. I'm using whatever batteries it came with! I didn't pay that much attention.

  5. #15
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    If you're putting lithiums in your HR92s, don't forget to change parameter 9 to 1. Obviously, can't do that with the CS92A.

  6. #16
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    Hmm, another overshoot to 62 degrees tonight, after months of no trouble. Again the reading stayed below 54 for a whole hour then suddenly jumped up to 62. Wonder what's going on...

  7. #17
    Automated Home Legend paulockenden's Avatar
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    Transmission cycle starting to clash with another device?

    I get the impression that Evohome uses fixed time periods, rather than n+random.

  8. #18
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    Quote Originally Posted by paulockenden View Post
    Transmission cycle starting to clash with another device?

    I get the impression that Evohome uses fixed time periods, rather than n+random.
    Not sure what the cause is.

    I've removed and refitted the CS92A batteries (battery voltage measures absolutely fine, >1.5v) to reboot it, rebooted the controller and even power cycled the BDR91's, so if its a timing related problem everything has been randomised again. We'll see if the problem returns...

    Symptoms seem very similar to my original problem, except this time there don't seem to be any updates for about an hour instead of for 20 minutes. The only thing I can think of that I've changed recently is to drop my flow temperature from 75 to 70, which slows the cylinder reheat time considerably, maybe this has affected the timing of transmission of hot water temperature readings in an unfortunate way...
    Last edited by DBMandrake; 21st February 2017 at 10:17 AM.

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