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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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    • CommentAuthorbardo
    • CommentTimeOct 22nd 2019
     
    Hi, I'm a first time house builder and looking to install moisture extractor fans from above a DIY kitchen cooker hood and in the bathroom (with bath and shower).

    The cooker and bathroom share the same party wall.

    Our walls are wide : 550mm (straw bales 450mm wide with another 100mm of lime / cladding/battens on the outside wall).

    Can anyone recommend a simple effective way of doing this Do we really need MVHR or just a simple extractor fan? Any brand/models?

    Thanks
    • CommentAuthortony
    • CommentTimeOct 22nd 2019
     
    Well, extract fans suck hot air out of the house wasting the energy in it. At the same time an exactly equivalent amount of cold outdoor air is sucked into the house through vents, gaps and cracks😢
    • CommentAuthorGreenPaddy
    • CommentTimeOct 23rd 2019
     
    Like Tony, I believe recovering heat (even better latent heat energy) is much smarter than chucking it all away, plus MVHR is constantly washing/freshening the rooms. Traditional extract fans just get switched off, as they're too noisy, leaving a very damp atmosphere.

    Try googling..."single room heat recovery"

    That will bring up various unit that are a bit like an extract fan, but have heat recovery built in. It should also bring up small MVHR units, which would involve some ducting (makes like Boulder and Quiet-vent 150) - a few hundred quid, rather than £1500 for the large units.

    See what you think, and maybe come back for more thoughts?
    • CommentAuthorjfb
    • CommentTimeOct 23rd 2019
     
    Whether or not to go for MVHR depends on how airtight you are planning to get the build. Anything under 3 ACH @50Pa (3 air changes per hour at 50 pascals) requires mechanical ventilation. Anything greater than 3 and MVHR is less useful (lower numbers are more airtight).

    Presumably in a straw bale house the render on the inside is the airtight layer. So long as you pay close attention to details of junctions between different materials, windows/walls, walls/roof, it is very possible to get a good air test result and get well below 3.
    • CommentAuthorCWatters
    • CommentTimeOct 23rd 2019 edited
     
    In my opinion it's all about control...

    If your house isn't air tight you end up with some unknown and uncontrollable amount of ventilation and a matching heat loss.

    If you build an air tight house you can/must control the ventilation by designing it in. You could specify adjustable trickle vents but with these you get limited control because ventilation depends on the wind. You also loose heat because air goes in some vents and out of others.

    MVHR is much better. Not only because air goes in and out via a heat exchanger recovering the heat but you can control which rooms get incoming fresh air and which rooms have extract vents (at the design stage).

    We opted for MVHR and was surprised to find we didn't need a separate extractor fan (or boost option on the MVHR) in the bathroom despite four of us using the shower. Your mileage may vary.

    We did fit an extractor fan over the hob but it's not used a lot.
    • CommentAuthorbardo
    • CommentTimeOct 23rd 2019
     
    Thanks all. We have gone for adjustable trickle vents in the windows despite the mentioned reasons. The calculated heat loss is around 3kw. What this will be with a vent hole in the floor for the woodfuel combi oven I don't know. I'll look into the single room MVHR as recommended and get back.



    Posted By: CWattersIn my opinion it's all about control...

    If your house isn't air tight you end up with some unknown and uncontrollable amount of ventilation and a matching heat loss.

    If you build an air tight house you can/must control the ventilation by designing it in. You could specify adjustable trickle vents but with these you get limited control because ventilation depends on the wind. You also loose heat because air goes in some vents and out of others.

    MVHR is much better. Not only because air goes in and out via a heat exchanger recovering the heat but you can control which rooms get incoming fresh air and which rooms have extract vents (at the design stage).

    We opted for MVHR and was surprised to find we didn't need a separate extractor fan (or boost option on the MVHR) in the bathroom despite four of us using the shower. Your mileage may vary.

    We did fit an extractor fan over the hob but it's not used a lot.
  1.  
    Posted By: bardoWe have gone for adjustable trickle vents in the windows despite the mentioned reasons. The calculated heat loss is around 3kW.

    3kW per what? hour? day? month? or a continuous 3kW loss?
    •  
      CommentAuthordjh
    • CommentTimeOct 23rd 2019 edited
     
    Posted By: Peter_in_Hungary
    Posted By: bardoWe have gone for adjustable trickle vents in the windows despite the mentioned reasons. The calculated heat loss is around 3kW.

    3kW per what? hour? day? month? or a continuous 3kW loss?

    Normally such figures, and the corresponding heat load figure, refer to the maximum rate of heat loss/supply needed to keep the house at a constant temperature at all times. The heat load will be greater if heating the house from cold. Normally, the maximum heat loss will be experienced when the outside temperature is at the minimum design temperature and the wind is blowing at maximum design speed.
    • CommentAuthorMike1
    • CommentTimeOct 23rd 2019
     
    Posted By: bardoThanks all. We have gone for adjustable trickle vents in the windows despite the mentioned reasons.

    Apart from the mentioned reasons, personally I'd also avoid trickle vents (at least in an urban situation) because of the noise they let through.
    • CommentAuthorowlman
    • CommentTimeOct 24th 2019
     
    Apologies bardo for hi-jacking your thread, but it's a similar theme.
    Does anyone know of a ceiling mounted single room MVHR unit? or, a wall mount for directly under the ceiling where the inlet/outlet has a top entry and can be angled to outside in the loft void?
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