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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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  1.  
    Never got round to installing my MVHR unit due to location issues. I had a brainwave last night I could actually install it in the same cupboard as my HW cylinder downstairs in the kitchen.

    I could then put the supply and exhaust 1m vertically above eachother on the external wall (albeit 450mm stone)

    2 questions

    1. As my ASHP outdoor unit is in close proximity will I be pumping freezing cold air into the house?
    2. With duct running issues, could I get away with installing room supply and intake low level or must they be ceiling mounted? I'd potentially like to duct from the bathroom low level perhaps?
    • CommentAuthortony
    • CommentTimeDec 28th 2018
     
    Outdoors no problems also you can point the exhaust sideways flowing with the prevailing wind direction

    Inside almost anything goes, I would much prefer to see bathroom extract at a high level
  2.  
    External supply and exhaust vents should ideally be at least 3m apart, they should also be on the same wall so equally affected by wind pressure. You will find it a lot easier if they are easily accessible as you will find the supply vent will need brushing off regularly to remove accumulated crud.
  3.  
    Posted By: Victorianeco2. With duct running issues, could I get away with installing room supply and intake low level or must they be ceiling mounted? I'd potentially like to duct from the bathroom low level perhaps?

    The FreshR MVHR unit much liked by Viking House of this parish is a wall mounted self contained unit without ducts, having the flow and return at the top and bottom of the unit. One unit is said to be enough for a smallish house. So if this works without ducts then why not an ordinary MVHR unit without ducts? (The theory is that nature abhors unbalanced situations so RH and CO2 will balance out throughout the house thus extensive ducting not needed). In your case V-eco I would put the ducts where convenient.
  4.  
    I forgot about the first floor joists and I'd have to core drill some distance away...

    The best I could probably get is 60cm apart vertically. One could face one way and the other in the opposite direction...

    Considering the house is very unlikely to be very airtight anyway is there even a point I wonder?
  5.  
    I also found this website and it appears it's system extracts from the bathroom and supplies back into the kitchen...
    https://www.paulheatrecovery.co.uk/news/first-passivhaus-certified-decentral-mvhr/

    Another thought is I could actually duct externally 3m apart if that is absolutely essential? Could mount anywhere then I think
    • CommentAuthorTimSmall
    • CommentTimeDec 30th 2018
     
    It might be useful to get a rough idea how airtight the building is. With energy efficient MVHRs, they break even at about 5ach. Tighter the better of course...

    Not too much problem with the intake and outlet close to each other unless both are facing a confined space (particularly with a wall opposite the intakes and closer than say 3 meters away).

    The extracted air will be a little more boyant (slightly warmer and more humid than the surrounding air), and also will have some momentum, so will tend to go away from the building and a little upwards. If you can, select an outlet cowl which doesn't get in the way of this motion by diffusing or directing the airflow downward too much - remember that the MVHR unit will have a condensate drain in it anyway, so a little bit of wind driven rain coming into the outlet duct occasionally isn't going to be a problem so long as the ducting is properly sealed.

    It's also worth paying attention to prevailing wind direction - best to avoid having the outlet so that it's normally upwind of the intake. If you can satisfy those, then I wouldn't be too bothered about them being too close if it's going to be very hard to manage. Most balanced flue boilers have their intake and outlet approx 0cm away from each other...
    •  
      CommentAuthordjh
    • CommentTimeDec 30th 2018 edited
     
    Posted By: TimSmallthe MVHR unit will have a condensate drain in it anyway, so a little bit of wind driven rain coming into the outlet duct occasionally isn't going to be a problem so long as the ducting is properly sealed.

    Wind driven rain shouldn't get into the MVHR unit, that's not what the condensate drain is for (the clue is in the name). Rain should drain out of the duct, which should always be installed with a fall for that reason.

    FWIW, @Victorianeco, yes you can run duct externally to separate the terminals if desirable.

    Most balanced flue boilers have their intake and outlet approx 0cm away from each other...

    Most balanced flue boilers don't supply air from either the intake or the outlet directly to the inhabitants either! Or else they get swiftly condemned :)
    • CommentAuthorTimSmall
    • CommentTimeDec 31st 2018 edited
     
    Posted By: djh
    Wind driven rain shouldn't get into the MVHR unit, that's not what the condensate drain is for (the clue is in the name). Rain should drain out of the duct, which should always be installed with a fall for that reason.


    I'm not suggesting actively trying to get rain into it, my point is that many cowl designs bend over backwards avoid rain ingress at all costs, when in reality a few drops occasionally entering an MVHR *outlet* duct will do no harm because of the condensate drain (not to mention the slight fall, and airflow direction).

    If there is a choice of cowl, and one looks like it will direct the extract air towards the intake, then consider another design (and if that means that it might occasionally get a few drops of rain into the duct, that doesn't matter).


    Most balanced flue boilers don't supply air from either the intake or the outlet directly to the inhabitants either! Or else they get swiftly condemned :)


    That is true, but I don't think its relevant?

    My point is that such a "0 cm" design causes little or no "recycling" of air between the outlet and intake.

    Most single rooms MVHRs have "0 cm" designs like this.

    When I've checked previously different MVHR manufacturers vary wildly when it comes to intake/outlet separation.

    I don't consider situations where the outlet and intakes are forced to be close as much of problem in most cases, and 60 cm is probably OK unless you're extracting into a semi-enclosed area. If you can easily achieve more, then why not do-so, but I wouldn't consider it a "show stopper".

    Don't take my word for it, why not do a test with a smoke machine to see how much extract air ends up being pulled back in before going to the trouble of running external ductwork.
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