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Fundamentals: How does air leakage work?

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  1.  
    • CommentAuthortony
    • CommentTimeNov 8th 2011
     
    How does air leakage work?

    I have heard it quoted several times that MHRV is not economic in air leaky houses.

    Air leakage is a measure of how draughty a house will be.

    However when it is calm (not windy) even the most appallingly leaky buildings wont leak much.

    When it is windy they will leak big time.

    This is a very good reason for having air tight buildings not leaky ones.

    There is more to MHRV than economics. On a windless day all buildings need it, it is nice in all buildings but draughty buildings are losing so much heat and wasting so much energy that they should be made air tight. It is not that MHRV is uneconomic, inefficient, not needed or useful BUT that the building needs sorting out!

    FUNDAMENTAL
    • CommentAuthorJonti
    • CommentTimeNov 8th 2011
     
    Toni,

    even on a day without wind if it is warmer inside the building than outside then won't warm air leak to the outside through any holes?

    Jonti
    •  
      CommentAuthorjoe90
    • CommentTimeNov 8th 2011
     
    Not only will warm air leak out (through holes at high level) it will pull cold air in through holes at a lower level (even when it is not windy).
  4.  
    Posted By: tonyI have heard it quoted several times that MHRV is not economic in air leaky houses.
    I think this is just based on the calculation that heat recovered by the heat exchanger is small compared to the overall heat lost through infiltration. So the capital cost of the equipment & the running cost of the fans is seen as being wasted.

    However, this ignores the comfort benefits & ignores the fact that without MVHR you will be forced to cut even bigger holes in the fabric to provide trickle ventilation.

    David
    • CommentAuthortony
    • CommentTimeNov 8th 2011
     
    I agree David

    Joe, yes air will still leak but on a windless day almost none at all compared to on a windy day so I am ignoring it for the purpose of this thread
    • CommentAuthorDarylP
    • CommentTimeNov 8th 2011
     
    Tony,

    I think I get your point, it is not worth installing MVHR into a dwelling that is not 'air tight'.....?
    Well YES, spend the money on the building fabric first (BFF is our mantra) and after that look at MVHR if you want to.
    Whey spend energy of trying to recover heat from the exhausted air, if most of it is being blown out of the fabric in an uncontrolled manner...?

    Cheers....:smile:
    • CommentAuthorEd Davies
    • CommentTimeNov 10th 2011
     
    Posted By: davidfreeborough
    Posted By: tonyI have heard it quoted several times that MHRV is not economic in air leaky houses.
    I think this is just based on the calculation that heat recovered by the heat exchanger is small compared to the overall heat lost through infiltration. So the capital cost of the equipment & the running cost of the fans is seen as being wasted.


    Isn't it more fundamental than that? HR is only economic in the sense that it reduces the heat loss from MV. If the house is leaky enough that you don't need MV in the first place then adding MV is a complete waste of resources and heat. Adding HR increases the resources used but reduces the heat lost but doesn't improve the situation over just using the natural ventilation.

    It seems to me the possibilities are:

    1) Leaky house: cheap but with unnecessary losses due to excess ventilation in many circumstances (windy or strong stack effect).

    2) Airtight house: condensation, mould and, in extreme cases, suffocation.

    3) Airtight house with MV: right level of ventilation so avoiding losses from excess ventilation.

    4) Airtight house with MHRV: right level of ventilation with further reduction in losses.
  8.  
    I wrote down the figures somewhere but can't lay them to hand but from memory in an average 150m2 airtight house with 1ACH there's 50m3 of air being changed every hour through leakage which is enough fresh air for 2 people. So with a more leaky house, the air quality is probably not too bad, so the benefit in air quality by using HRV is reduced. The leakier the house is, the more air is being exchanged that's not going through the heat exchanger.
  9.  
    Posted By: Ed DaviesIt seems to me the possibilities are:

    1) Leaky house: cheap but with unnecessary losses due to excess ventilation in many circumstances (windy or strong stack effect).

    2) Airtight house: condensation, mould and, in extreme cases, suffocation.

    3) Airtight house with MV: right level of ventilation so avoiding losses from excess ventilation.

    4) Airtight house with MHRV: right level of ventilation with further reduction in losses.
    Obviously everyone should be aiming to be in category 4) above, but this is not always achieved in refurbishment situations.

    My point was that if you are in category 1) then the Building Control officer will still insist that you cut slots in your nice new triple glazed windows unless you fit some other form of ventilation. Continuous Extract & Positive Input Ventilation both require trickle vents or other air inlets/outlets. So the only way to avoid the cold draughts caused by cutting even more holes in the fabric, while complying with BC requirements, is to fit MVHR.

    David
    • CommentAuthortony
    • CommentTimeNov 11th 2011
     
    you don't get the cold draughts from trickle vents if they are closed!
    • CommentAuthorborpin
    • CommentTimeNov 11th 2011
     
    From a slightly different angle, would fitting MHRV to an old (leaky?) house reduce condesation? The assumption must almost be that the house is not that leaky else you would not get condensation; but I'm not sure my logic is right there.
    • CommentAuthorDarylP
    • CommentTimeNov 11th 2011
     
    Yes... possibly?

    If you can exhaust the warm moisture-laden air, and replace it with drier air from o/s the thermal envelope, it may be beneficial in your case.

    Condensation is a result of a combination of factors, some you can control, some you can't....

    Good luck :smile:
    • CommentAuthorborpin
    • CommentTimeNov 11th 2011 edited
     
    Ah that throws up another question. If it is one of those cold misty damp mornings (Scotland), do you really want that air forced into your house (airtight or leaky). Do MHRV systems have a means of detecting the outside humidity? You could well create condensation, could you not?
    • CommentAuthortony
    • CommentTimeNov 11th 2011
     
    If your house is warm that would exceeding unlikely or even impossible, the inlet filter could get wet I suppose though mine never has and we have some very misty days
  15.  
    Posted By: tonyyou don't get the cold draughts from trickle vents if they are closed!
    That's the other think I hate about trickle vents. We live on a new estate of 57 houses & every other one I've been in so far has all its trickle vents closed! It makes a joke of all the regulations.

    David
    • CommentAuthortony
    • CommentTimeNov 11th 2011
     
    agreed -- and nationally probably the same

    It pains me to the nation wasting so much money like this -- nother thread on its way
    •  
      CommentAuthorJSHarris
    • CommentTimeNov 11th 2011 edited
     
    The house we had in Scotland (a Scandinavian kit house that was thermally pretty good) had trickle vents that, when opened, made a hell of a row in any sort of wind (west coasts, cliff top site......). Ours stayed permanently closed, too.

    The subject of condensation is one worth it's own topic, as some thing I've done to improve the insulation and air tightness on my present house have created significant condensation problems.
  18.  
    We have a pretty leaky Barn convertion that was done 35 years ago when air tightness and energy bills wernt as considered as they are now. We have insulated well, draft proofed where we can and installed good double glazing. Our house uses solid fuel heating that needs combustion air. As a result of the air `leaking` into the house for combustion we have a high air change rate. The consequence of this high air change rate is a very dry house. We have no bathroom or shower room extract and no kitchen extract. All this moisture goes into the house in winter but it is still not enough to get the house upto a reasonable RH level. typically the house is circa 26%RH and a little too dry. In the depths of winter we often have a pan boiling on thre stove to humidify the air.
    We never get condensation and MHRV would be a waste of time. Washing dries nicely in the house over night and the solid fuel system is cheaper to run than ground source, air sourece or mains gas.

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