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Thread: S-Plan vs Y-Plan for Evohome ?

  1. #31
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    I also think you have uncovered a firmware issue because my DHW is set to 50, but it never reads more than 49 (by the CS92), i.e. the boiler is switched off at 49 so it never stops at 50C the set-point. It would be really easy to check the Evohome source code and see what values are being compared see if (most likely) one is being rounded and the other not.

  2. #32
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    Quote Originally Posted by g6ejd View Post
    I also think you have uncovered a firmware issue because my DHW is set to 50, but it never reads more than 49 (by the CS92), i.e. the boiler is switched off at 49 so it never stops at 50C the set-point. It would be really easy to check the Evohome source code and see what values are being compared see if (most likely) one is being rounded and the other not.
    Having had Evohome installed for over 2 1/2 years now, I have never seen a HW temperature over-run above my set target temperature UNLESS I have had pump over-run set and then it was only by a couple of degrees. On the odd occasion, I have seen a temp a degree or so below the target temperature but I attribute this to someone running a hot tap when the CS92 has called a stop but before the Evohome temperature has updated. I cannot say that I am bothered provided the temperature is within the range that I have set

    I appreciate that those with a technical background are keen to look deeply into the system; however, my simple view is that Evohome does what it says on the tin. That said, I rarely use the App having had a senior moment when applying a QA only to find that when I returned home the controller had frozen.

  3. #33
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    @DBMandrake Might this be a CS92 transceiver issue?

  4. #34
    Automated Home Legend paulockenden's Avatar
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    I was just going to say that I've never seen any DHW overshoots of this nature. But looking at my Domoticz graphs it seems I have.

    chart (5).jpg

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  5. #35
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    Quote Originally Posted by g6ejd View Post
    I also think you have uncovered a firmware issue because my DHW is set to 50, but it never reads more than 49 (by the CS92), i.e. the boiler is switched off at 49 so it never stops at 50C the set-point. It would be really easy to check the Evohome source code and see what values are being compared see if (most likely) one is being rounded and the other not.
    That's interesting - you have the new 1.03 firmware don't you ? I'm still on 1.02.

    Mine always waits until it reaches an indicated 50 degrees before it turns off the hot water relay. The valve does shut before the reading changes to 50 but that is just a delay of about 5-10 seconds before the display updates. I have never seen it get to 49 and switch off and then the reading stay at 49 as you describe. I wonder if there is a subtle change in the logic in 1.03 ?

    Looking at the output of the V1 and V2 web API's and comparing the displayed figure on the Evotouch is interesting - it appears that both the Evotouch display and V2 API's round hot water temperature down, not to the nearest degree as you would expect.

    In one instance I saw the V1 API reporting 48.87 degrees while the V2 API reported 48.0 and the Evotouch displayed 48. The figure always appears to be rounded down.

    I wonder if the 1.02 firmware does something like use the rounded down figure to decide whether the temperature has hit the set point yet, so that 49.9 still would not satisfy the hot water demand, and yet at the same time the CS92 rounds 49.9 up to 50 and decides now is the right time to send a temperature update ?

    In that case the CS92 believes it has sent a temperature update at the set point that will trigger the hot water demand as being satisfied, but the Evotouch is not satisfied so it keeps heating until the next (considerably delayed) temperature update by which time the temperature has overshot.

    Whether the problem is hit would depend on the rate of temperature rise and how that coincided with the sampling periods of the sensor, so it might hit under only very specific semi-random circumstances, and probably only with hot water that heats very quickly. (EG a small cylinder like mine) I've noticed that when the reading hits 47 or 48 during re-heat it always successfully changes to 50 shortly thereafter and shuts off, its only when it goes to 49 that there is a problem due to the reading then staying on 49 for anywhere from another 10 to 20 minutes, which is an eternity in hot water heating time.

    If we assume that the behaviour of the CS92 can't be modified (firmware updates aren't possible in the field) then it would appear the only workaround would be for the Evotouch to be a bit more lenient about the set point target - if it sees a temperature within half to one degree of the set point maybe the 1.03 firmware says "ok close enough, lets switch off hot water" to avoid the risk that the temperature sensor won't send another update for a long time. The fact that yours always stops at 49 (which is probably something like 49.8, rounded down for display) would possibly suggest that they have done exactly that.

    Quote Originally Posted by HenGus View Post
    Having had Evohome installed for over 2 1/2 years now, I have never seen a HW temperature over-run above my set target temperature UNLESS I have had pump over-run set and then it was only by a couple of degrees. On the odd occasion, I have seen a temp a degree or so below the target temperature but I attribute this to someone running a hot tap when the CS92 has called a stop but before the Evohome temperature has updated. I cannot say that I am bothered provided the temperature is within the range that I have set
    Glad its working well for you but that's twice now that it's done this in the first week that I've had the hot water kit, and the issue is not related to pump overrun - it's definitely a lack of temperature updates being sent to the evotouch during the critical time when the target temperature is being reached that is the issue.

    Quote Originally Posted by HenGus View Post
    @DBMandrake Might this be a CS92 transceiver issue?
    What kind of issue are you suggesting ?

    Faulty from new ?

    A wireless signal issue ?

    Firmware bug ?
    Last edited by DBMandrake; 30th October 2016 at 03:30 PM.

  6. #36
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    Quote Originally Posted by paulockenden View Post
    I was just going to say that I've never seen any DHW overshoots of this nature. But looking at my Domoticz graphs it seems I have.

    chart (5).jpg

    P.
    Can you zoom in on a period where it appears there is an overshoot and see whether your reading stuck at 1 degree below the target for a long time, and also see what the temperature peaked up to after the reading started updating again to see if you see what I posted in my graph ?

    Are you graphing from an HCI80 or via the Web API, if the former do you get at least one decimal place in your hot water temperature out of interest ? I use the Web API but it looks like it should be easy to update the hot water reading in the Domoticz script to use the V1 API for fine resolution like I did with the heating zones. It's just a matter of getting a few quiet minutes to sit down and work on the code...
    Last edited by DBMandrake; 30th October 2016 at 03:38 PM.

  7. #37
    Automated Home Legend paulockenden's Avatar
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    I'm using an hgi80.

    Can't zoom in on past events because Domoticz seems to only save averaged data in the past.

    IMG_0257.jpg

  8. #38
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    Quote Originally Posted by paulockenden View Post
    Can't zoom in on past events because Domoticz seems to only save averaged data in the past.
    Ah that's a pity. Munin does this by default too but in my Munin config I have set it to keep all data at full 5 minute resolution for up to a year - it turns out that it only takes about 10MB per zone to do this so I'm happy to do that.

    Not sure whether there is a way to configure Domoticz to do the same or not, I haven't looked.

    I run both Munin with the evohome-munin plugin and Domoticz in parallel on the same Raspberry Pi - to be honest at the moment I mainly run Domoticz so I can easily look at raw un-rounded un-biased temperature readings on a nice web based dashboard.

    I've updated my domoticz evo-update.sh patch to also provide hot water temperature to two decimal place resolution so we'll see what the prior reading is next time there is an overshoot.

    My guess is it will be between 49.5 and 49.99, as I suspect the basis of the problem is a difference in rounding behaviour between CS92 and Evotouch.

    Although they both have access to the raw high resolution temperature reading, I suspect that nearest neighbour rounding is done in the CS92 before the test that says "has the set point just been crossed, if so send an immediate temperature update", but the code in the Evotouch rounds down before deciding whether the set point has been reached or not.

    Thus if the temperature is sampled at 49.75 the CS92 rounds up to 50 and decides yes, now is the right time to send an update, the Evotouch receives 49.75, rounds it down to 49 and decides that more heating still has to be done to reach 50, so the CS92 now doesn't seen another update for a long time...

    If the temperature is never between 49.5 and 49.99 during a sampling period the problem doesn't occur. That's my working theory anyway!

  9. #39
    Automated Home Legend paulockenden's Avatar
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    I'll try to keep an eye on the Domoticz graphs/logs over the next few days, to see if I can capture for a DHW overshoot.

    At first sight though, it does look like a bug. Those with particularly high flow rates and temps from their boiler (basically those with high kw boilers) are likely to see quite a large overshoot, I'd have thought.

  10. #40
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    My boiler is somewhere between 18kW and 23kW - depending on how it was adjusted when it was provisioned. (which I don't know)

    The cylinder is only 95 litres and I have the flow temperature boost for hot water demand set to about 78 degrees. I could set it lower as it does heat a lot faster than I expected but with my current thermostat wiring (internal and external flow temp stats wired in series) that would also reduce the maximum configurable flow temperature for heating, so I can't really set it lower than about 75 in case I need 75 for heating in the winter.

    I could get another (or different) external flow temperature controller and be able to set hot water flow temperature lower than maximum heating flow temperature if necessary with the internal stat retained as a safety over temperature cutout but I was hoping to avoid that.

    Reheat from 45 to 50 is only about 6 minutes, a further 10-15 minutes will take it to well over 60, so 10 - 20 minutes delay between readings if the set point is "missed" is quite a big overshoot.

    Maybe people with larger or slower heating cylinders wouldn't notice much of an issue.

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