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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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Ventilation: MVHR drainage: icicles

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  1.  
    • CommentAuthorwookey
    • CommentTimeDec 12th 2022
     
    Has anyone else got icicles growing from their MVHR vents in the current cold spell? It's been below freezing for about 4 days here and the size of the icicles shows how much condensation is dripping out of my vents (more than I had realised).

    http://wookware.org/pics/online/greenbldg/iciclezoom.jpeg

    I remember carefully sloping the vents through the wall to drain outwards if there was any water in them, but I'm wondering if that was the right answer now, because there is an internal drain in the machine, so having the condensation drain internally might be better than having it dripping down the wall/render outside. I can't remember if I just made that up or read it somewhere. I think it was recommended.

    I had already adjusted the vent grills to be stepped out a few mm from the render surface so that any condensation dripped rather than ran down the wall making a stain, so I already knew that at least some of the time condensation occurs.
  2.  
    This pic was last winter, but it's looking similar now. This is a through-the-wall unit with no internal condensate drain.

    Our family probably generate 10 kg of water vapour a day (two gallons if it were all condensed) so it seems like only a small proportion of that actually does condense in the MHRV. If it did, the MHRV would recover substantially more heat, but the incoming air doesn't have enough heat capacity to achieve that. The heat recovery % numbers they quote on the datasheet only refer to the sensible heat, they ignore the latent heat, despite that being about the same amount of heat loss.
      mhrv icicle.jpg
    • CommentAuthorborpin
    • CommentTimeDec 13th 2022
     
    Fortunately my Komfovent unit doesn't produce any condensation otherwise the unit would have been off for the last few days!

    This will be the same issue as Gas Boilers condensate drains.

    Letting the vents drain internally might not be a good idea as the water draining back might end up in the fans.

    That in itself is an interesting thought as my vents are vertically above the unit. Mmm need to check as I'm having issues with condensation and my unit and had issues with the fan in the old unit failing.
  4.  
    Just to add, the cold air is very dry so we need less volume of ventilation than usual. We turned off the MHRV for periods yesterday and overnight, saving on electricity and heat losses, and the humidity in the house is still about 10%RH lower than it normally would be.
    • CommentAuthorborpin
    • CommentTimeDec 13th 2022 edited
     
    Posted By: WillInAberdeenJust to add, the cold air is very dry so we need less volume of ventilation than usual.
    MY issue is CO² and the limiting factor on the ventilation rate I need. I'd also be wary of condensation inside the unit from the cold outside air with it stopped.
    • CommentAuthorcjard
    • CommentTimeDec 17th 2022
     
    My MVHR condensate drains into my waste water system, no problems..

    So long as the ice hasn't blocked yoir drain it'll be ok..
    • CommentAuthorbhommels
    • CommentTimeDec 17th 2022
     
    On advice (from GBF members) I routed the MVHR drain internally. I can remove a bit of pipe and put a container under the drain so I get a 'free' source of demineralised water, more or less. Handy, as the water hardness here is off the scale (pardon the pun).
    • CommentAuthorjms452
    • CommentTimeDec 23rd 2022
     
    If it was easy I'd route it fully internally.

    Our 22mm MVHR condensate has a 4m run then drips onto an external roof and it does ice up in cold weather.

    After the first winter I made a 12V 24W heater that heats the external pipe for half an hour twice a day that fixed it
    • CommentAuthorborpin
    • CommentTimeDec 27th 2022
     
    All condensate drains should be routed to an internal drain to prevent freezing up.

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