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    • CommentAuthordelprado
    • CommentTimeApr 17th 2017
     
    If PIV works from a ventilation perspective, then why hasn't anyone invented an extractor that runs to an input unit in the loft, which takes a fresh input from outside, and then mixed with hot and positively pressurizes the house?
    • CommentAuthorlineweight
    • CommentTimeApr 17th 2017 edited
     
    When you say "mixed" do you mean the air is mixed or heat-exchanged?

    If mixed - what's the point of extracting air to blow it back in again?

    If heat-exchanged, then why positively pressurise?
    • CommentAuthordelprado
    • CommentTimeApr 18th 2017
     
    I mean something working exactly as a conventional unit does (I dont know how they worK), but basically just one unit for the whole house without ducting - and it works because it establishes positive pressure?
  1.  
    Conventional MVHR extracts air from the house then passes the heat to the incoming air which is then imputed to the house.
    Typical MVHR uses ducts to better collect air from decided locations and to deliver incoming air to where it is wanted.
    MVHR without ducts (e.g. FreshR units) do not use ducts but rely on diffusion/migration of gasses and moisture to balance out within the house.
    Any (?) MVHR unit with individually adjustable input and extraction fans could be set up to provide positive pressure. Positive pressure is sometimes used when the house has a wood burning stove to help prevent the ingress of smoke when loading the stove. Sometimes houses with such stoves set a positive pressure because the implications of negative pressure are dangerous.
    Giving a positive pressure would mean that the air would be looking to escape through any gaps/cracks and alike and so would not be available for heat collection.
    PIV works by pressurizing the air which is then lost through gaps either accidental or designed and with it goes the heat
    MVHR works by collecting the air removing the heat and giving it to the input air.
    Any system that pressurizes the air and so caused losses would mean that the efficiency would fall, the more lost (not returned to the MVHR unit) would mean that the efficiency would get closer and closer to the losses involved with PIV
    Just having a pressurizing fan in the MVHR would mean that it would be luck as to how much air went through the units heat exchanger and how much would be lost through other available gaps/hols in the house. Air will always take the path of least resistance so there would be very limited control of such a system.
    • CommentAuthorlineweight
    • CommentTimeApr 18th 2017
     
    <blockquote><cite>Posted By: delprado</cite>I mean something working exactly as a conventional unit does (I dont know how they worK), but basically just one unit for the whole house without ducting - and it works because it establishes positive pressure?</blockquote>

    You'd be using the positive pressure to move air through the house but you'd be losing energy, as post above mine says. There are other ways of moving the air without ducting though.

    You might like to have a look at this thread:

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14972&page=4#Item_30
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