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Thread: Atag IS24 Plus Evohome Plus Opentherm

  1. #1
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    Default 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.

  2. #2
    Automated Home Legend paulockenden's Avatar
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    Just shows how important it is to do thorough research.

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

  3. #3
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    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.

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    Automated Home Legend paulockenden's Avatar
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    Radweld ;-)

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    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.

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    Quote 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.

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    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.

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    Quote 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.

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    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.

  10. #10
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    Quote 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.

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