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Thread: Atag IS24 Plus Evohome Plus Opentherm

  1. #31
    Automated Home Legend paulockenden's Avatar
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    Every heating engineer I've ever met has told me that boiler sizing has everything to do with water heating (recovery time for stored, or simultaneous taps/showers for combis) and very little to do with radiators. And I've met several!

    P.

  2. #32
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    Because people expect their stored hot water to reheat super quickly, that's all. If people were happy to have their hot water take all day to reheat, suddenly you'd find the size of your house would be the most important factor in boiler sizing. In other words, hot water heating is the dominant factor, but it is not the only factor - especially when you get into large, leaky, old houses.

    1) Would you agree that heating water in a boiler and transferring that heat to water in a tank or to air in a room is essentially the same problem?
    2) If so, would you agree that heating a larger cylinder of water in a given timeframe would require more energy than heating a smaller cylinder of water in the same timeframe?
    3) In which case, surely you can agree that heating a larger volume of air (using radiators) in a given timeframe requires more energy than heating a smaller volume of air in the same timeframe?
    4) Therefore, if you have the expectation that you can turn your heating on at 6:30 and have the house warm by 8:00 then you must surely expect to have to put more heat in to a larger house with a larger volume of air than in to a smaller house with a smaller volume of air.

    And you put more heat in (over the same time period) by:

    1) Prayer, or
    2) Burning dinosaurs faster

    And the unit measure of how hard you pray... errr... I mean how fast you're burning dinosaurs is...? Anyone? Bueller? Bueller?

  3. #33
    Moderator Kevin's Avatar
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    At the time my boiler was installed it was a commercial model but now 65KW is just within the domestic category where Gas Safe engineers can work on them. I had to vacate my previous property earlier than expected approaching winter and rushed to get a heating system installed. I made a lot of mistakes in my choices and was provided with a lot of incorrect information. This is a large listed property 6000 sq ft 25 rooms dating from 1720 with an abundance of very thin glazed Georgian sash windows and was as much a sieve for heat as you could probably imagine, much improved now but still not good. Originally there were two boilers one upstairs and one downstairs plus an Aga used for hot water for one of the three cylinders.

    There were reasons to use this particular incumbent installer but I soon learned this was beyond his capabilities. I think he calculated the KW required in each room and added them all up to reach a figure as well as 4 bathrooms (stored hot water) and that's why he recommended 65KW. He sized new radiators accordingly although there's a lot of older cast iron ones still in use. 65KW is maybe right to heat all rooms in winter in a reasonable time if required and the hot water with quick recovery. The problem of course is that's an extreme situation which we never need and once the house is warm the boiler just can't modulate low enough. I believe at minimum modulation it still kicks out nearly 30KW so we have a lot of cycling. I quickly learned a wide modulation range is a really useful feature.

    Zoning at room level with evoHome of course makes it even worse as often we only occupy / heat a few rooms. The hot water storage cylinders just can't absorb anything approaching that amount of heat. If the house was fully occupied then maybe the boiler is sized right but our usage is different. I should have made this room zoning and occupancy pattern clearer to the installer so it's not really fair to blame him. I was very naive re. heating and would make many different choices now. It's absolutely the worst aspect of my home automation. Servicing on a commercial unit is ridiculously expensive.

    Aside from respecifying the system when this boiler eventually dies one other idea is that we also own an attached house that we rent. It is not a huge job to marry the two heating systems to be served by this one 'dinosaur' boiler , opportune as their boiler is very much on its last legs. I would like to better understand options of retaining their boiler as a backup and/or indeed adding a secondary much smaller boiler to my own system to serve our hot water and lower heating demands outside of winter. Some integration perhaps with evoHome to fire the smaller boiler when only a small number of zones request heat or are near setpoint. Not dual stage but switching from the smaller boiler to a larger one in the winter months. But maybe this is just throwing good money after a bad decision.

    I didn't want to divert this thread as I am really interested in how your Atag works out as I would like to incorporate OpenTherm in any future boiler I considered and I also wanted Honeywell's view re. evoHome usage with hot water priority (a feature it seems I should never have been talked into).
    Last edited by Kevin; 11th April 2017 at 02:43 PM.

  4. #34
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    Also, how did you work out that ATAG supported Opentherm? I can see no obvious mentions of it except that their own control was suspected to be running OpenTherm.

  5. #35
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    Quote Originally Posted by bruce_miranda View Post
    Also, how did you work out that ATAG supported Opentherm? I can see no obvious mentions of it except that their own control was suspected to be running OpenTherm.
    A lot of research on Netherlands websites about Atag boilers ( using Google translate); passing references to Atag boilers using Opentherm in the press; confirmation on this forum from TopBrake that Atag boilers will connect to Evohome using Opentherm; an exchange of e-mails with and a number of calls to Atag Technical Support, and frequent questions to my very patient installer (an EcoTechnician).

    My Plan B was to re-instate the heating BDR if things didn't work out as planned.

  6. #36
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    I really wonder why it's not a fact they want to publicise more? Which average Joe would go through all that trouble.

  7. #37
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    that's why he recommended 65KW
    the boiler just can't modulate low enough. I believe at minimum modulation it still kicks out nearly 30KW
    I have just had a Worcester Bosch GB162 65kW condensing boiler installed. That replaced an old Potterton 75kW boiler. As you note, one of my motivations for dropping to 65kW is that it lets domestic registered gas fitters work on a 65kW boiler. (Although I still needed a commercial fitter to sign off the gas pipework as the volume of the meter and the internal pipework is greater than that which a domestic fitter can certify.) But unlike you I am very pleased with the performance. The boiler modulates down to 14kW. I can see from my Loop energy monitor (which records gas consumption every 15 minutes) that the boiler runs at full power first thing in the morning to heat the house for up to an hour (depending on the external temperature) and then modulates down. I would not want a smaller boiler as that would take longer to heat the house.

    I didn't want to divert this thread
    Sorry! Back to Atag and OpenTherm.

  8. #38
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    Quote Originally Posted by Edinburgh2000 View Post
    I can see from my Loop energy monitor (which records gas consumption every 15 minutes) that the boiler runs at full power first thing in the morning to heat the house for up to an hour (depending on the external temperature) and then modulates down.
    How can you tell that from your Loop data? How can you distinguish whether it's using, say, 30kW in a given hour because it's modulating down to 50% from it switching on and off at 50% duty cycle under TPI?

  9. #39
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    How can you tell that from your Loop data? How can you distinguish whether it's using, say, 30kW in a given hour because it's modulating down to 50% from it switching on and off at 50% duty cycle under TPI?
    Because in the first hour of a winter's morning Loop is showing 65kW in each of the four quarter-hours. And thereafter - being something of an anorak - I can watch the boiler and all the zone valve controls and see that all zones are still demanding heat (i.e. no TPI demand reduction) but the power output of the boiler drops because the CH output temperature has reached the set point (70oC in my case). The boiler only switches off when the outlet temperature goes six degrees above the set point. That rarely happens.

  10. #40
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    Quote Originally Posted by bruce_miranda View Post
    I really wonder why it's not a fact they want to publicise more? Which average Joe would go through all that trouble.
    I think that the answer lies in the fact that they like to sell a heating system (boiler and controls).

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