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Thread: Evohome & Opentherm - flow temps UFH and radiator zones

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    Default Evohome & Opentherm - flow temps UFH and radiator zones

    Just wondering how this works.
    Say I have Evohome controlling radiators, UFH zones and hot water, how does it decide the flow temperature to demand from the boiler?

    I'd imagine that UFH would need 35-50c (say)
    Radiators would be 35-70c
    DHW would need to be 70c to heat the cylinder to 65c
    I can imagine the rads getting up to temp while the UFH is still heating so would want UFH to be full strength while rads could be controlled using TPI.

    Is all this stuff calculated automatically?

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    Automated Home Legend top brake's Avatar
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    Quote Originally Posted by jdp80 View Post
    Just wondering how this works.
    Say I have Evohome controlling radiators, UFH zones and hot water, how does it decide the flow temperature to demand from the boiler?

    I'd imagine that UFH would need 35-50c (say)
    Radiators would be 35-70c
    DHW would need to be 70c to heat the cylinder to 65c
    I can imagine the rads getting up to temp while the UFH is still heating so would want UFH to be full strength while rads could be controlled using TPI.

    Is all this stuff calculated automatically?
    TPI doesn't control flow temperature, for that you need OpenTherm
    I work for Resideo, posts are personal and my own views.

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    Quote Originally Posted by top brake View Post
    TPI doesn't control flow temperature, for that you need OpenTherm
    So would Evohome decide on different flow temperatures depending on the type of zone calling for heat?

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    Automated Home Legend top brake's Avatar
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    Quote Originally Posted by jdp80 View Post
    So would Evohome decide on different flow temperatures depending on the type of zone calling for heat?
    Yes indeed it does. It modulates flow temperature according to the heat demand, ramping up to suit hot water for example
    I work for Resideo, posts are personal and my own views.

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    I know that the opentherm does change heat demand between hw and ch.

    But i'm not sure if it can distinguish between radiators and ufh. Maybe it can if you pair the devices as UFH. However in my case the ufh is just another motorised zone valve that is controlled by evohome as a zone valve as part of the CH. I'll see if the opentherm provides a lower flow temperature when only the ufh is calling for heat.

    It does however vary the flow temperature based on how many CH demands are there.

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    Automated Home Legend paulockenden's Avatar
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    Isn't the temp of UFH usually controlled by a separate manifold mixer valve? To reduce the temp of the heating loop down to a sensible level?

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    Yes, dependent on the floor type etc. Ours is set to about 50 degC

    I wonder what will happen if there's a period when radiator zones are in a low demand situation, but the UFH is high demand - could the boiler modulation cause flow temperature to drop below what is required to run the UFH circuit?

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    Quote Originally Posted by paulockenden View Post
    Isn't the temp of UFH usually controlled by a separate manifold mixer valve? To reduce the temp of the heating loop down to a sensible level?
    Yes that is true. But it's such a waste for the boiler to heat the water to 50+ only for the manifold to cool it down to 30+ before sending it down the loops.

    Hence I was just keen to find out if evohome was intelligent enough to know and set a reduced target flow for UFH demand only?

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    Quote Originally Posted by bruce_miranda View Post
    Yes that is true. But it's such a waste for the boiler to heat the water to 50+ only for the manifold to cool it down to 30+ before sending it down the loops.

    Hence I was just keen to find out if evohome was intelligent enough to know and set a reduced target flow for UFH demand only?
    a regular fixed flow temperature UFH manifold doesn't cool it down, it simply bleeds in hot water as required
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    It doesn't 'cool it down', more 'ekes it out' - no heat is wasted (other than losses in the system). It's no different from a rad valve being turned down.

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