Accuracy of Nest TPI control?

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  • MJNewton
    Automated Home Jr Member
    • Jan 2021
    • 21

    Accuracy of Nest TPI control?

    Hi everyone,

    My first post here, although I've been reading with fascination many of the threads in the archive.

    For many years I've been using a Honeywell CM927 thermostat that has managed rock-solid control of room/house temperatures. You can see the stability in the graph below - noting in particular the grey line representing the temperature in the room with the stat an the red line showing the temperature of the air being extracted from the house by an MVHR and hence representing something of a whole-house average. The heating isn't on overnight so it's the heating periods (high sections of the graphs) that's the area of interest. Even squinting at the graph reveals a very obvious change in stability on Sat 30th Jan and that point being when I swapped out the Honeywell for a Nest...

    (Here's a link to the full size image - the forum-attached image is really small: http://www.newtonnet.co.uk/permanent/preandpostnest.png)

    Both the Nest-read temperatures and MVHR-read temperatures show a fair bit of oscillation which leads me to believe the TPI control of the Nest is nowhere near as effective as the Honeywell. Or am I being too impatient in reaching this conclusion and should give the Nest longer to learn the responsiveness of the house and heating system?
    Attached Files
  • MJNewton
    Automated Home Jr Member
    • Jan 2021
    • 21

    #2
    Just to follow up on this for the sake of the archives (although the lack of responses suggests it's only me that's had an issue!) I've found that the Nest's control of the temperature has improved somewhat now that's it's been installed for a week or so. Perhaps it did just have to be given a bit of time to learn the response rate and in the meantime was using some fairly liberal settings.

    Comment

    • mikey8156
      Automated Home Jr Member
      • Dec 2011
      • 37

      #3
      I am not sure if the TPI control is self learning.

      Comment

      • MJNewton
        Automated Home Jr Member
        • Jan 2021
        • 21

        #4
        Originally posted by mikey8156 View Post
        I am not sure if the TPI control is self learning.
        Are you sure? The plots definitely show clear improvements in stability over time so I'm wondering what else it could be?

        Last edited by MJNewton; 13 February 2021, 01:54 PM.

        Comment

        • kevinsmart
          Automated Home Ninja
          • Sep 2018
          • 257

          #5
          I don’t think Nest employs TPI in the same sense as the CM927, switching on/off in 10 minute intervals.

          But it’s True Radiant algorithm does learn how long your room takes to heat up or cool down and from memory, when I had one, eventually uses a very narrow maintenance band, something like 0.6C.

          I also switched from a CM927 to Nest when they came out and was initially disappointed by its temperature control.
          Last edited by kevinsmart; 13 February 2021, 01:40 PM.

          Comment

          • MJNewton
            Automated Home Jr Member
            • Jan 2021
            • 21

            #6
            Originally posted by kevinsmart View Post
            I don’t think Nest employs TPI in the same sense as the CM927, switching on/off in 10 minute intervals.
            Can you expand on that? How else could TPI work if it's not pulse-width control? Nest describes itself as a Class IV thermostat and hence 'An electronic room thermostat that controls both thermostat cycle rate and in-cycle on/off ratio of the heater proportional to room temperature.' I don't know if the Nest employs a 10 minute cycle (unlike the CM927 you can't see/adjust it) but it must surely work in the same way regardless of cycle length.

            Comment

            • kevinsmart
              Automated Home Ninja
              • Sep 2018
              • 257

              #7
              There’s an explanation here:

              True Radiant helps keep homes with in-floor radiant or radiator heating systems more comfortable by helping to reduce temperature swings typical of radiant systems. With True Radiant, your Nest therm


              Whereas with Honeywell TPI where there is a regular on/off cycle when within 1.5C, Nest learns when it should turn off early to meet the set-point and later when to turn back on.

              In practice in my house at the time it would be off for over an hour which didn’t suit some of the colder rooms in the house. Whereas the Honeywell thermostat kept pulsing to balance the heat loss.

              Comment

              • MJNewton
                Automated Home Jr Member
                • Jan 2021
                • 21

                #8
                Originally posted by kevinsmart View Post
                when I had one
                I forgot to ask: what do you have now? Evohome? I actually thought this was an Evohome forum when I first arrived as it seems to attract the lion's share of discussion.

                Comment

                • MJNewton
                  Automated Home Jr Member
                  • Jan 2021
                  • 21

                  #9
                  Originally posted by kevinsmart View Post
                  Whereas with Honeywell TPI where there is a regular on/off cycle when within 1.5C, Nest learns when it should turn off early to meet the set-point and later when to turn back on.
                  I think you might be mixing up two aspects within True Radiant? Google uses the term to cover not just the TPI element but also what others call Optimum Start.

                  Originally posted by kevinsmart View Post
                  Whereas the Honeywell thermostat kept pulsing to balance the heat loss.
                  That's exactly what my Nest does - sometimes pulsing for only a minute which is annoying, not least given 20s of that is just the zone valve opening and boiler preparing to fire.
                  Last edited by MJNewton; 13 February 2021, 02:12 PM.

                  Comment

                  • kevinsmart
                    Automated Home Ninja
                    • Sep 2018
                    • 257

                    #10
                    Originally posted by MJNewton View Post
                    I forgot to ask: what do you have now? Evohome? I actually thought this was an Evohome forum when I first arrived as it seems to attract the lion's share of discussion.
                    Yes, Evohome

                    Comment

                    • kevinsmart
                      Automated Home Ninja
                      • Sep 2018
                      • 257

                      #11
                      Originally posted by MJNewton View Post
                      I think you might be mixing up two aspects within True Radiant? Google uses the term to cover not just the TPI element but also what others call Optimum Start.



                      That's exactly what my Nest does - sometimes pulsing for only a minute which is annoying, not least given 20s of that is just the zone valve opening and boiler preparing to fire.
                      Maybe they changed the firmware, my Nest V2 never pulsed, just appeared to stay off far too long. OpenTherm will improve matters if your boiler is compatible.

                      Comment

                      • MJNewton
                        Automated Home Jr Member
                        • Jan 2021
                        • 21

                        #12
                        Originally posted by kevinsmart View Post
                        Maybe they changed the firmware, my Nest V2 never pulsed, just appeared to stay off far too long. OpenTherm will improve matters if your boiler is compatible.
                        Sorry kevinsmart; I missed this reply. Funny you should mention it staying off for too long - I've sometimes thought similar with mine (v3).... There's no pleasing me is there! ;-)

                        Re Opentherm, yes, it'll definitely be a must-have feature of our next boiler but for now our Ideal Icos is soldiering on (touch wood) and so it's a simple on/off control for the time being hence why I'm keen on the TPI control to get the most out of it.

                        Comment

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