Atag IS24 Plus Evohome Plus Opentherm

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  • HenGus
    Automated Home Legend
    • May 2014
    • 1001

    Atag IS24 Plus Evohome Plus Opentherm

    Given the recent spate of posts about boiler/opentherm issues, you might be interested in the following (then again you might not).

    For the past year, I have looked at (my wife says ‘procrastinated over') options for a replacement boiler which has involved me taking many soundings from tech support teams, installers, experts and forums. Each time that I thought that I had reached a decision, reports of a potential compatibility issue would arise. This resulted in me dropping Ideal, Intergas and Veissmann boilers from my short list. The decision-making process was not helped by a lack of knowledgeable installers: most just wanted to fit a new boiler on to my existing system. Thanks to some excellent advice from Top Brake (I hope that your ankle is getting better) and Atag Tech Support, I was able to find an installer that was happy to work with my existing Evohome system.

    As of yesterday, I have a replacement Atag IS boiler connected to my 3 year old Evohome system via Opentherm. Use of the latter was conditional on me being able to switch back to a BDR if any Opentherm issues should arise.

    Configuration Changes: A conventional ’S’ plan configuration has been modified with the replacement of the heating zone valve with an inline open/motorised closed valve connected in series to the existing HW BDR. The existing HW BDR has been disabled and unbound from the controller. When the HW BDR demands heat, the HW zone valve opens and the CH inline valves closes giving HW priority. A boiler relay has been established using an Opentherm bridge - a 5 minute job.

    Boiler Changes: The boiler has been configured to P1 - max boiler temperature 70C.

    First Impressions. Opentherm just works - no ‘if or buts’ as far as I can tell but it is still early days. When HW is demanded, the boiler seems to set a target temperature of about 10C above the existing cylinder temperature - no mad rush to 92C or max boiler temperature. Boiler temperature increases as the cylinder water temperature rises to the maximum P1 setting of 70C. Given our life style, I do not foresee any issues setting HW heating periods outwith the times when we want heating. That said, in Winter, I suspect that we will need to change the boiler target temperature to P0 - 80C to get a quicker cylinder re-heating time.

    This morning when I got up at 7am to make a cup of tea, HW was heating (about 2C below the Evohome set temperature) and the boiler temperature was 65C. About 40 minutes later, the selected zones were all at set temperature and the boiler was running on pump only at 45C flow/41C return.

    Summary: Noting all the other posts about boiler/Opentherm issues, I am pleased that, with an Atag boiler, Opentherm appears to work (as it is designed to do?). I appreciate that I am tempting fate by writing this!!

    Sorry that there are no fancy graphs etc. I would be very interested in monitoring my system (other than by popping into the garage to look at the boiler) but I confess that without something that is just 'plug and play', I am out of my depth (its an age thing). I will update this post when the system has been running for a few more days.
  • paulockenden
    Automated Home Legend
    • Apr 2015
    • 1719

    #2
    Just shows how important it is to do thorough research.

    Looks like you've found the ideal (as opposed to 'Ideal'!) boiler!

    Comment

    • HenGus
      Automated Home Legend
      • May 2014
      • 1001

      #3
      Sadly, my man is back trying to rectify an underfloor leak. He is blaming the quick fix fittings favoured by plumbers in the 90s - now no longer used. I suspect that power flushing the system didn't help. Having replaced one fitting, he now has a leak from another. It could be a long night as preparations are being made to take out part of the hallway ceiling. That wasn't part of my cunning plan: clearly, my risk assessment wasn't good enough.

      Comment

      • paulockenden
        Automated Home Legend
        • Apr 2015
        • 1719

        #4
        Radweld ;-)

        Comment

        • mtmcgavock
          Automated Home Legend
          • Mar 2017
          • 507

          #5
          Surely with wiring as HW priority without the controller knowing this causes issues with TPI and the optimisation? As if a heating zone is calling for heat and your HW is up to temp then the zone may only take 15 minutes to warm. Where as if you HW is cold and takes 30 mins to warm then an additional 15 minutes to warm the zone bringing a total of 45 minutes, surely this is confusing matters for the EvoHome?

          Ideally Honeywell need to add a feature into the software where HW priority can take place software based so that the controller knows this is happening. On Honeywells part this would be so easy to do as well.

          Comment

          • HenGus
            Automated Home Legend
            • May 2014
            • 1001

            #6
            Originally posted by mtmcgavock View Post
            Surely with wiring as HW priority without the controller knowing this causes issues with TPI and the optimisation? As if a heating zone is calling for heat and your HW is up to temp then the zone may only take 15 minutes to warm. Where as if you HW is cold and takes 30 mins to warm then an additional 15 minutes to warm the zone bringing a total of 45 minutes, surely this is confusing matters for the EvoHome?

            Ideally Honeywell need to add a feature into the software where HW priority can take place software based so that the controller knows this is happening. On Honeywells part this would be so easy to do as well.
            I passed that exam question on to Honeywell yesterday. In truth, it is not an issue first thing in the morning as HW heating can be set well in advance of the 1 hour maximum optimisation period. If it proves to be an issue, then HWP can be ceased simply by disconnecting the two valves as the inline valve is powered off open.

            As far as the controller is concerned, it does seem to know when there is a HW demand. It should set the required default temperature to 92C which, with my set up, leaves the boiler to ramp up the a flow max temperature of 70C. Yesterday evening, I stood watching the boiler (not the most exciting of things to do) when it re-heated the cylinder from 44 to 60C - with the heating on at the time modulating at 51/41C. It took 18 minutes to reach cylinder set temperature with a maximum boiler flow temperature of 70C. When DHW heating demand ceased, the boiler very quickly went back into its low temp modulating operation.

            My installer also said that there is an argument which suggests that it is better just to leave HW set to ON to minimise the time when hot water is being re-heated; i.e., only being re-heated from, say, 55 to 60C rather than 40 to 60C. Clearly, that will result in more but shorter re-heat periods. Every day is a school day with Evohome.

            Comment

            • bruce_miranda
              Automated Home Legend
              • Jul 2014
              • 2307

              #7
              Your ATAG seems to work exactly like my Vaillant, except that I needed an extra module to convert OT to eBUS. But otherwise well done. A lot of us on the forum have been waiting to see what you came up with in the end. :-)

              Are you able to confirm what I saw as Evohome OT behaviour. i.e. if any zone is 1.5C below the setpoint the boiler is pushed to it's set max by OT before it modulates back. But thank goodness the ATAG doesn't have the same issue like the Viesmann.

              Comment

              • HenGus
                Automated Home Legend
                • May 2014
                • 1001

                #8
                Originally posted by bruce_miranda View Post
                Your ATAG seems to work exactly like my Vaillant, except that I needed an extra module to convert OT to eBUS. But otherwise well done. A lot of us on the forum have been waiting to see what you came up with in the end. :-)

                Are you able to confirm what I saw as Evohome OT behaviour. i.e. if any zone is 1.5C below the setpoint the boiler is pushed to it's set max by OT before it modulates back. But thank goodness the ATAG doesn't have the same issue like the Viesmann.
                I had thought (and I have e-mailed Honeywell about it) that the max temperature was somehow linked to the HW ON time period (rather than a specific demand for HW) set in the controller but didn't make any sense - but it stacked up with what I thought that I was seeing. What you have reported does make more sense. When I checked the boiler flow this morning - about 15 mins after HW reached its set temp - the boiler was at 70C as the zones were about 3 degrees below target. I am not getting any temp overshoots. I will run a heating cycle this evening with HW set to OFF to check. Thanks for raising the question.

                Comment

                • bruce_miranda
                  Automated Home Legend
                  • Jul 2014
                  • 2307

                  #9
                  I know for a fact that HW demand will kick the boiler into max. The question is what it does when there is CH demand. At what point does it go to max. I find that the boiler will go to max depending on how far from set point any zone is.

                  Comment

                  • HenGus
                    Automated Home Legend
                    • May 2014
                    • 1001

                    #10
                    Originally posted by bruce_miranda View Post
                    I know for a fact that HW demand will kick the boiler into max. The question is what it does when there is CH demand. At what point does it go to max. I find that the boiler will go to max depending on how far from set point any zone is.
                    I am not disagreeing with you. It is the transition from HW heating to CH that I have struggled to understand. As I said above, the only reason that I could think of for the boiler staying at max temperature with just CH ON was the fact that I had a HW heat timed period set. Thirty minutes or so later, the boiler was modulating and HW heating was not available (i.e.; it was OFF). I put 2 and 2 together and came to the conclusion that the two factors were related, but I couldn't understand how so I e-mailed Evohome Support for some advice. Your experience that the boiler goes to max if a zone is 1.5C or more outside of its set temperature makes a lot more sense: that is, it gets the zone up to temperature and then keeps it there.

                    That said, if the Honeywell guys are reading this and, have some time to spare, an 'idiot's guide' to normal Evohome operation under Opentherm control would be helpful and, in my case, much appreciated.

                    Comment

                    • Kevin
                      Moderator
                      • Jan 2004
                      • 558

                      #11
                      I am interested in the Honeywell reply re. hot water priority systems. My (non opentherm) boiler enforces this so I can't easily defeat it as you can. I really would like EvoHome to correctly account for this, realising that sometimes when a central heating heat demand is requested it won't be available if the hot water is heating.

                      Actually in the meantime I have kludged this a little in that I have a mid position valve between heating and hot water and if demand is requested for heating whilst hot water demand is present it will go to mid position and supply both. However this creates a few other issues.

                      It's no longer strictly hot water priority.

                      The boiler now miscalculates how long it takes to heat the hot water cylinder. With the manufacturers boiler controller you set the heating 'on time' and it intelligently brought the hot water up to temperature just in advance. With evohome I have to estimate times and the optimised start just couldn't be accommodated easily. Not that the HW loses too much if early heated.

                      The flow temperature whilst heating both is the preset algorithm for heating my hot water. Interestingly my boiler seems not to use a max setting for this but about 15 degrees above tank temp so the rads don't get scalding which was my worry.

                      One benefit is that my boiler was overspecced (65KW) and cycled a lot whilst on HW. This eliminates some of the cycling and doesn't really impact the time taken to heat the HW cylinder.

                      So rightly or wrongly that's what I've done. I had intended to move to opentherm when this boiler goes so I'm watching this thread with great interest. Seems like I dodged some early adopter issues.

                      K

                      Comment

                      • paulockenden
                        Automated Home Legend
                        • Apr 2015
                        • 1719

                        #12
                        65kW? That's enough to heat a palace!

                        Comment

                        • mtmcgavock
                          Automated Home Legend
                          • Mar 2017
                          • 507

                          #13
                          I installed a Viessman VitoDens 100-W OV in a property with opentherm a couple of weeks ago and experienced the same modulation. With the Hot Water off and a heating demand when the zones were calling for a great temperature rise the boiler would go into full mode to supply these zones, and then gradually modulate down with as the set temperature was reached. With the Hot water on the boiler would modulate to max mode to get the temperature rise on the hot water.

                          Comment

                          • bruce_miranda
                            Automated Home Legend
                            • Jul 2014
                            • 2307

                            #14
                            Holy cows and I thought my 38Kw boiler was over specced. Can you even buy domestic boilers with a 65Kw output....goodness.

                            Comment

                            • mtmcgavock
                              Automated Home Legend
                              • Mar 2017
                              • 507

                              #15
                              Yes Vaillant do a ecoTEC 65kw wall hung boiler, classed as light commercial but can be installed as a domestic appliance.

                              Comment

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