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Thread: Atag IS24 Plus Evohome Plus Opentherm

  1. #21
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    Quote Originally Posted by bruce_miranda View Post
    Because accidentally leaning against a radiator that has 82C of water flowing through it is going to hurt.
    A radiator at 70oc is still going to be hot. As far as I remember the maximum the boiler got up to was about 75oc. But still I can still see Viessmans logic from disabling the controls on the front, if you had a end user reducing the temperature down to say 55oc then the system would never heat your hot water quick enough!

  2. #22
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    Quote Originally Posted by mtmcgavock View Post
    A radiator at 70oc is still going to be hot. As far as I remember the maximum the boiler got up to was about 75oc. But still I can still see Viessmans logic from disabling the controls on the front, if you had a end user reducing the temperature down to say 55oc then the system would never heat your hot water quick enough!
    And yet they allow 99% of the users who don't use OT the ability to alter the max flow temperature via the front controls?

  3. #23
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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.

  4. #24
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    Quote Originally Posted by Kevin View Post
    ... (65KW) ...
    Holy balls! I've got a 4000 sqft, 1950s house that leaks worse than a sieve and I'm about to replace a 30kW boiler with a 25kW one!

  5. #25
    Automated Home Legend paulockenden's Avatar
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    Actually, house size doesn't really matter when sizing a boiler. It's number of bathrooms (and specifically baths/showers) that need to be in use at one time.

    Maybe the person with the 65kW shower runs a guest house or similar?

  6. #26
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    Of course the house size matters. I have 24 radiators and I need a good amount of energy going in to the system in order to get a good amount of heat out of the radiators, otherwise it would take hours and hours to heat my house. I can guarantee that a person in a newly built one bedroom starter home will not need a boiler as big as mine.

    By the same argument, I can turn my boiler off and still quite happily use all three showers because I have stored hot water! So boiler size has no bearing on the number of showers that need to be in use at one time. What it does have a bearing on is how quickly I can re-heat my cylinder full of cold water. Or my house full of cold air. And I really don't want to be doing the latter in my (large, old, leaky) house with a piddly little 12kW boiler.

    So yes, house size (and stored hot water tank size) does matter.

  7. #27
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    Quote Originally Posted by paulockenden View Post
    65kW? That's enough to heat a palace!
    Presumably you're thinking of a small one-bedroom starter palace here?
    Last edited by dty; 10th April 2017 at 12:00 PM.

  8. #28
    Automated Home Legend paulockenden's Avatar
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    I was talking about mains fed showers (so combi), rather than with stored hot water.

    Are all of your 24 radiators scheduled to heat from cold at exactly the same time?

    P.

  9. #29
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    You should be asking my Evohome that, not me!

    The 10 that cover the bits of the ground floor where we never close the doors are all in the same (multi-room) zone, so I expect Evohome turns them all on at the same time, yes.

    But that's not the point. The point is that heating stored hot water is the same as heating stored warm air. One is held in a lagged metal tank, the other is held in a (hopefully reasonably well insulated) brick box. Apropos the combi-boiler hot-water-on-demand situation, these two things are essentially the same thing, but with different volumes of different specific heat capacities being heated. And the thing you probably care most about in both cases is how quickly you can heat them. And the thing that gets them heated faster is more energy going in.

    Yes, you can schedule different zones for different times if that happens to fit with your lifestyle. I have my hot water scheduled for 4am to avoid the Evohome optimisation period, and my study set to be warm by 9am instead of 7am for the bedrooms. But that's just about managing the peak load on the system. (Which is also how I get away with 25kW in this house.)

    The heating system that was here when I moved in seemed to just about manage with a 30kW boiler. Hence by being a little bit smarter about scheduling (moving DHW earlier), having zoning (Evohome) and moving to a brand new condensing boiler, I should be OK with a smaller boiler. But even if I scheduled things in the worst possible way, I can't see myself needing much more than 30-35kW.

    All of which is to say... of course house size has a bearing on the size of the boiler! The other major link between the two being that a larger house has (generally) more windows and certainly more surface area through which to leak heat.

    Of course, the poster could, as you say, be running a guest house! Although he does talk about a domestic installation. Nevertheless, he could also live in a 10,000 sqft house!

  10. #30
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    FWiW, my installer carried out a detailed analysis of the size of my home, rads etc, and he concluded that I could replace a 24kW Ideal boiler with a 18kW condensing system boiler - provided that we were happy to wait for the HW to re-heat. My property is 5 bed with a 250L unvented cylinder and 19 radiators. Eventually, a decision was made to go for a 24kW boiler. I did meet with two potential installers that quoted for boilers in excess of 30kWs.

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