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Heating and cooling: Air to water heat recovery in summer

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    I'm trying to find a MHRV system that during summer time the latent heat in the vented exhaust air can be exchanged to DHW rather than be exchanged to the incoming (fresh) air which in the warmer months is not required. Any help would be appreciated.
    •  
      CommentAuthordjh
    • CommentTimeSep 7th 2009
     
    This sounds like a solution looking for a problem. The difficulty is that DHW is at a greater temperature than the exhaust air so there's no direct way to use it. You would need to include something like a heat pump. (MHRV with a heat pump is called a "compact unit" in passivhaus literature). A better idea would be to use solar thermal to provide your DHW in summer, IMHO.
  3.  
    The only way I can see this working would be to use some form of heat exchanger to pre-heat the incoming water into a main heating system (be it solar, biomass, fossil etc.). Incoming water is at around 12C, air is probably at least 20C otherwise you'd be wanting to pre-heat the incoming air, so in theory there is some scope to pre-heat the water.

    However:

    The heat carrying capability of air is much lower than water (as previously discussed for systems using MVHR to provide space heating), so I don't think there is actually very much heat there to recover, at least from a DHW perspective. (I haven't done the sums though.)

    You're likely to have a much higher level of condensation in the exchanger (virtually guaranteed) than normal and you'll have to make sure this is drained well and doesn't cause corrosion or mould. At least you won't have to worry about freezing.

    In the UK, there probably aren't many days when you're not pre-heating the incoming air and the internal temperature is much higher than external ambient temperature (perhaps with high thermal mass at night?) In this case, why not just heat exchange from the outside air rather than complicate it with the MVHR?

    Due to the low number of days where such a system would be used it would have to be very low cost to justify any investment.

    In the end I think there is so little heat available in relative terms and the costs/practicalities of extracting it are sufficiently high that it isn't worth considering. Far better to find somewhere to put a solar thermal panel.
  4.  
    Thanks for you comments,
    Plan to use solar thermal

    ""why not just heat exchange from the outside air rather than complicate it with the MVHR?"

    Plan to implement AGS (PAHS) with earth tubes (40m) that bring the fresh air in under an insulalated waterproof "cape" there should be some ground to air heat exchange and vice-versa (exhaust) is there much point in going any further by utilizing a HRV??

    The water feed to the DHW is under this "cape" I am hoping to bring this supply up from 10c-12c to at least 17c (depending on contact time)before entering a HWC
    • CommentAuthorunguided1
    • CommentTimeSep 26th 2009 edited
     
    <blockquote><cite>Posted By: earthshipally</cite>I'm trying to find a MHRV system that during summer time the latent heat in the vented exhaust air can be exchanged to DHW rather than be exchanged to the incoming (fresh) air which in the warmer months is not required. Any help would be appreciated.</blockquote>


    You could try an ESP ecocent, I have had great succsess installing these units.
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=2693&page=1#Item_28

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