Evohome - firing boiler for no reason?

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  • paul777
    Automated Home Lurker
    • Nov 2015
    • 9

    Evohome - firing boiler for no reason?

    Hi,

    I have a question about the behaviour of Evohome I hope someone can answer. I having logging setup to record
    - Boiler firing
    - Pump Active
    - Themostat triggered (by Evohome controller)
    - Actual temp measured by HR92
    - Configured target temp
    - Actual target temp as chosen by the Evohome controller optimization
    - Boiler out and return temperatures

    What I see is the evohome regularly requesting the boiler to fire for just over one minute even when all the rooms are at or above temperature.

    This pattern can be seen in this image of 50mins worth of logging. Most rooms are over target, with just study and dinning room at temperature. In the 2nd chart the Evohome thermostat is represented by the grey bars (labelled "front" in the legend).

    What is the EvoHome controller doing, firing for this short space of time has little to no effect on the temperatures of the radiators or rooms, it just seems to be wasting gas warming up the pipework and the boiler?

  • paulockenden
    Automated Home Legend
    • Apr 2015
    • 1719

    #2
    That's TPI. It's why, over those 50 minutes, the rooms hold their temperature. Without this proportional input from the boiler they'd be cooling down.

    Looking at those nice steady temperature logs I'd say it's making an excellent job of it! With old style heating controls those room temps would be oscillating within a few degrees hysteresis band.

    Comment

    • paulockenden
      Automated Home Legend
      • Apr 2015
      • 1719

      #3
      Incidentally, Paul, what are you using for your monitoring / logging / graphing? Looks like a nice little setup with lots of inputs!

      P.

      Comment

      • rotor
        Automated Home Guru
        • Aug 2015
        • 124

        #4
        That's an amazing setup you have for capturing all that data!

        As Paul Ockenden said, the whole reason those rooms are holding so steady is because of those short bursts on the boiler that just manage to keep the radiators ever so slightly warm, and stop the rooms from cooling down. It's a beautiful thing.

        Comment

        • paul777
          Automated Home Lurker
          • Nov 2015
          • 9

          #5
          Thanks for explaining - I guess that all makes sense - it just seemed that the effect of the rads was so small.

          The monitoring is done using a couple of Raspberry Pi boxes. One monitors gas usage - my gas meter has a built in reed switch which ticks every 0.31KWh. One monitors the boiler and thermostats - optical sensors on the leds on the thermostat receivers and on the boiler leds for it being active and being lit. The temp sensors on the output and return pipes. The RPi script sends a network broadcast with the status of all the sensors every second or so. A c# program on my server picks these up and when something changes writes an entry into my sql server database. The service on my server also polls the Evohome API every 5 seconds and records actual, target and configured target temps for each zone. The reports are then done using SQL Reporting Services. (Evohome API I have made public here https://github.com/paul-777/Evohome.Net )

          Comment

          • paulockenden
            Automated Home Legend
            • Apr 2015
            • 1719

            #6
            Originally posted by paul777 View Post
            polls the Evohome API every 5 seconds
            Eeek - you're not supposed to poll at anything shorter than 10 minute intervals!!!!

            P.

            Comment

            • paulockenden
              Automated Home Legend
              • Apr 2015
              • 1719

              #7
              Originally posted by paul777 View Post
              Thanks for explaining - I guess that all makes sense - it just seemed that the effect of the rads was so small.
              The effect on the rads is EXACTLY the right amount! Your graphs demonstrate brilliantly just how good Evohome is at holding a room at a given temp.

              P.

              Comment

              • paul777
                Automated Home Lurker
                • Nov 2015
                • 9

                #8
                Sorry that should have read 5 minutes! (5seconds would make for a very large database) But who defined the 10 minute rule? It seems Honeywell are resistant to actually publishing the API and associated rules about its use. I should also mention that it seems rather lame that I have to sign up to and use an external web service to access something which sits on my network - Honeywell really should work on providing a direct local interface, I can't believe it would be a difficult change to the controller firmware and would eliminate the security risks associated with having it talk externally.

                Comment

                • paulockenden
                  Automated Home Legend
                  • Apr 2015
                  • 1719

                  #9
                  The 10 minute thing was posted by one of the Honeywell guys here.

                  You don't HAVE to use the web service. Honeywell does offer a local interface. It's called HGI80 and allows you to poll the data locally. Go as fast as you want to, then

                  P.

                  Comment

                  • paul777
                    Automated Home Lurker
                    • Nov 2015
                    • 9

                    #10
                    Interesting but a google of HGI80 yields very little, a few references to it but no product documentation or anywhere offering it for sale - do you have any links that would help?

                    Comment

                    • paulockenden
                      Automated Home Legend
                      • Apr 2015
                      • 1719

                      #11
                      Have a read through this thread: http://www.domoticaforum.eu/viewtopic.php?f=7&t=5806

                      There's now support for it baked into (open source) Domoticz, so it should be possible to grab the basics from the code there. of course, under .net on Windows you'll need to access the USB port differently.

                      P.

                      Comment

                      • erik
                        Automated Home Guru
                        • Feb 2015
                        • 244

                        #12
                        TPI in itself is indeed is a good way to keep a certain temperature. You should be able to set the duration of the TPI cycles by changing the value in the advanced settings. Default is 6x per hour. You can also change the minimum run time to 2 minutes, if for example your boiler is a slow starter.

                        1 thing to note: the 'actual' temp measured by the HR92 is not really the actual temp measured by the HR92. It's a value that is influenced by Evohome and Evohome rounds to values of 0.5 but also tends to 'correct' its values towards the setpoint. So if HR92 measures 18.2 and you setpoint is 19, it might display 18.5. If HR92 measures 18.6 and you setpoint is 17, it might display 18.0. If you want a true reading of the temperature, you need your own external temperature logging.

                        And paul777, have you been able to verify that the HR92 for dinner and study is opened enough in this situation (radiator getting slightly warm?)?
                        Last edited by erik; 21 November 2015, 10:35 AM.

                        Comment

                        • paul777
                          Automated Home Lurker
                          • Nov 2015
                          • 9

                          #13
                          Yes, valves were open, it is just that given the size of the water circuit they are attached to firing for little over a minute doesn't allow for much curculation. I have since found and changed the minimum run time setting to 5minutes and this seems to behave better although the weather has changed sharply here 16C to 5C outside temps so hard to draw conclusions. I'm also not sure how long it will take the Evohome controller to adapt - I know it take an initial 10 days when you first start the system up to learn properly. I'll probably leave it a few days then swicth back for a few days and see the results.

                          Comment

                          • paulockenden
                            Automated Home Legend
                            • Apr 2015
                            • 1719

                            #14
                            Originally posted by paul777 View Post
                            The RPi script sends a network broadcast with the status of all the sensors every second or so. A c# program on my server picks these up and when something changes writes an entry into my sql server database.
                            Paul, are you using commercial shields for the IO, or just plugging into the available ports? And are you doing all of this over Wi-Fi / Zigbee / something else?

                            Sorry for all the questions, I'm just keen to replace some of what I'm doing here, and if I can do so without reinventing the wheel so much the better!

                            Thanks,

                            P.

                            Comment

                            • paul777
                              Automated Home Lurker
                              • Nov 2015
                              • 9

                              #15
                              Sorry - forgot about this for a while. I'm using some optical sensors (TSL257-LF) onto the GPIO pins (with 1K in series with IO and 4K7 pull down) and some DS18B20 temp sensors hooked up via a DS2482S-100+ to the I2C pins and using owfs. I have attached my script (a bit hacky but it does the job).
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