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Thread: Atag IS24 Plus Evohome Plus Opentherm

  1. #11
    Moderator Kevin's Avatar
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    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

  2. #12
    Automated Home Legend paulockenden's Avatar
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    65kW? That's enough to heat a palace!

  3. #13
    Automated Home Ninja
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    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.

  4. #14
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    Holy cows and I thought my 38Kw boiler was over specced. Can you even buy domestic boilers with a 65Kw output....goodness.

  5. #15
    Automated Home Ninja
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    Yes Vaillant do a ecoTEC 65kw wall hung boiler, classed as light commercial but can be installed as a domestic appliance.

  6. #16
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    Quote Originally Posted by mtmcgavock View Post
    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.
    This sounds perfectly normal behaviour except that on the Viesmann I thought the max was not the user set max but the physical max of the boiler I.e. 80+C

  7. #17
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    Correct, once Opentherm was wired in the control temperature on the front of the boiler was made redundant and the Opentherm fully controlled the temperature.

  8. #18
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    And that is actually a problem with the Viesmann!

  9. #19
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    Quote Originally Posted by bruce_miranda View Post
    And that is actually a problem with the Viesmann!
    Why is this actually a problem? You are leaving the Opentherm to do it's job of deciding what flow temperature the system requires. If you dial back the temperature of the maximum flow temperature you are just defeating the object of having the Opentherm as you are telling the system the maximum you want. The greater the flow temperature the quick the temperature rise in the zone, the quicker rise on your hot water. Why wouldn't you want the maximum flow temperature?

  10. #20
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    Because accidentally leaning against a radiator that has 82C of water flowing through it is going to hurt.

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