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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    I am at the stage of choosing an MVHR system and the supplier is pushing a rotary wheel system. I'm not entirely sure whey and I can't find much info on the pros and cons vs a counterflow system but I am concerned about humidity control. Does anyone have any thoughts on this?
    • CommentTimeSep 28th 2020
    Can you state the particular brand/model?

    (I need a horizontal unit as putting it under a sloping roof in the loft).
    • CommentTimeSep 28th 2020
    How will you access the top to change the filter? Or is there some access underneath that is not described?

    There are several brands that sell horizontal counterflow units - Vent-Axia & Brink to name just two.

    In theory, a rotating wheel system is what's called an 'enthalpy exchanger' because they capture and recirculate some of the water vapour. Whether that's good or bad depends on circumstances. I don't know much more than that, sadly.

    PS Is your loft warm (in the heated space)?
    • CommentAuthorjms452
    • CommentTimeSep 28th 2020
    if you are in the uk the drying inside in the winter is a benefit, our counter flow unit keep us 40-65%rh all winter.

    If we were somewhere really cold a rotating wheel would make sense but in the UK I'd go for less moving parts (and probably higher efficiency) of counterflow.
    I suspected that adding the humidity back in the UK is not really necessary! I can understand the benefit in a colder/drier climate but I don't often find myself wishing for more humidity in these climes.

    I'm planning on installing the other side of the purlin wall in the loft. The whole area will be insulated so it won't be in cold space. Its not ideal but I'm struggling with where else I could put it.

    The Vent Axia horizontal version looks worse than their standard vertical version on most parameters. So I'm still on the lookout for a decent horizontal system.
    • CommentAuthortony
    • CommentTimeSep 30th 2020
    I did a rotary wheel 25 yrs ago still working fine, commercial system though. Twin fans in a box should be ok for yours
    I definitely have cold (damp) feet about the rotary wheel model and I can see difficulties installing/changing filters in the eaves space. I'm going to try and change tactic and place the MVHR above the toilet in the downstairs WC. Hoping I can use a standard vertical Vent Axia in this space.
    Perhaps this is a moot point now but the summary that Justin Bere prepared on the risks of COVID 19 is only becoming more relevant as we discover more about the virus transmission (i.e. mainly air not surfaces).

    He makes the point that:

    "A rotary heat exchanger presents the risk of contamination of the fresh air supply by droplets of condensation formed out of the extracted air. The risk is normally considered very small, but where a deadly virus is concerned, perhaps even this small risk is too much. We should therefore use the cross-flow system if at all possible."

    Hopefully this won't be a long term consideration and this particular virus will pass soon, but it adds more weight to the argument for separate chamber cross-flow systems.
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