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Thread: Which OpenTherm boilers work well with Evohome?

  1. #51
    Automated Home Ninja
    Join Date
    Jul 2014


    A HCC80R allows you to zone a single UFH manifold. Whether you need it of not completely depends on how your UFH is configured. I have UFH in one 100sqm open plan zone. So there was little point of a HCC80R, I just use a single thermostat and a BDR91. Also when zoning a UFH you will need to have intimate knowledge of the pipe runs and the valve placements on the manifold.

  2. #52
    Automated Home Legend
    Join Date
    May 2014


    Quote Originally Posted by StephenC View Post

    Viessmann have no idea themselves about Opentherm, that is quite clear, and at best are designed with vendor lock in, so anyone considering Opentherm should stay away from Viessmann boilers (I will be in the future, and it looks like ATAG is very good at out of the box Opentherm interoperability from what I am reading on here)
    I would agree that getting information about Opentherm control from the various technical teams can be a bit like pulling hens' teeth. I have tended to ask the question in a different way given that some of the bespoke manufacturer's control are Opentherm in all but name. My question has been along the lines of 'this is my set up, how would you expect your controller/boiler to work'?

    Some of the manufacturers (Intergas for example) told me that they have run boiler bench tests with Evohome, using an Opentherm connection, with Honeywell representatives present.

    As far as my Atag is concerned, Opentherm is controlling 100% to the T-Set (calculated Flow) temperature - not the max flow temperature. I have absolutely no idea how T-Set is calculated but it is set via an installer profile. The boiler has no manual control of the actual flow temperature. As I write, the boiler is on 70C Flow, 61C Return and 70C T-Set. One zone (3 large radiators) is on heat as the 5C window open temperature reduction kicked in without me noticing. I think my wife was feeding the birds when I was in the shower.

    My only comment about Opentherm is that it seems, at least to me, to be slow to get the boiler into the full condensing mode but, having never owned a modulating condensing boiler before, I can only assume that under Opentherm control closing the gas valve reduces the effective boiler size from 23.2kW down to 5kW as a priority. Most certainly, at the moment, the boiler is just purring with little obvious external plume.

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