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Thread: Atag IS24 Plus Evohome Plus Opentherm

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  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 ;-)

  5. #5
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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.

  6. #6
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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.

  7. #7
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    I have a Viessmann 111-W storage combi so don't experience HW issues, but lack of max CH flow temp control has been a disaster so far so have to disagree strongly with Viessmann setup locking out the control dial in combination with 3rd party (e.g. Honeywell R8810) Opentherm.

    Overshoots are constant as it appears even though I'm in an old house, the radiators were sized for more moderate and importantly safe to touch briefly without getting scolded temperature (50-65 depending on season).

    The HR92's demand max heat (which is then translated to max flow temp) far too often - sometimes even to maintain a room temperature. They don't react fast enough either when the radiators are at 80 odd degrees and especially in this weather when the heat loss is a lot lower than middle of winter. That's not really surprising either as even when heat demand stops and the HR92 closes, the radiators are left with 80 odd degree water in them which keeps pumping out significant heat into the room.

    Looks like I'm going back to BDR91 Relay control because of this as it provided for a combined setup with much much better control with very minimal overshoots as I was able to set the CH flow temperature appropriately to the heat loss at that time of year.

  8. #8
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    Your home sounds very similar to mine (sealed S plan with unvented tank and heat only boiler). I've been going through the same procrastinations and seriously considering Atag.

    What made you go for the system boiler? Or did you have a system boiler previously? I'm currently heat only, but could convert to system when I upgrade.

  9. #9
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    Quote Originally Posted by fergie View Post
    Your home sounds very similar to mine (sealed S plan with unvented tank and heat only boiler). I've been going through the same procrastinations and seriously considering Atag.

    What made you go for the system boiler? Or did you have a system boiler previously? I'm currently heat only, but could convert to system when I upgrade.
    I have a 19 radiator/5 bed home with an unvented 250L OSO cylinder. The IS24 has replaced an Ideal Classic system boiler.

    I am still trying to get my mind around Opentherm and condensing boilers. At the moment, about 50% of the house is calling off and on for heat. The boiler currently has a flow temperature of 60C and a return temperature of 54C. There are considerably less zone temperature variations than was the case; no boiler cycling and the boiler itself is whisper quiet.

    Edit: I have just checked. Three zones and 10 radiators all at set temperature. The boiler has flow/return of 50C/49C. Pretty clever stuff.
    Last edited by HenGus; 13th April 2017 at 05:47 PM.

  10. #10
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    Quote Originally Posted by HenGus View Post
    I have a 19 radiator/5 bed home with an unvented 250L OSO cylinder. The IS24 has replaced an Ideal Classic system boiler.

    I am still trying to get my mind around Opentherm and condensing boilers. At the moment, about 50% of the house is calling off and on for heat. The boiler currently has a flow temperature of 60C and a return temperature of 54C. There are considerably less zone temperature variations than was the case; no boiler cycling and the boiler itself is whisper quiet.

    Edit: I have just checked. Three zones and 10 radiators all at set temperature. The boiler has flow/return of 50C/49C. Pretty clever stuff.
    Hi HenGus, really appreciative of all the info you've been posting. I suspect I'm going to end up with EXACTLY what you've got. A couple of questions about your cylinder: did you choose it, or was it already there? Did you have any problems installing the EvoHome temperature probe? And if you chose it, was there anything specific about that brand (I'd never even heard of it), and how did you decide on the 250L size?

    Thanks again!

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