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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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    • CommentAuthorgravelld
    • CommentTimeFeb 7th 2018
    Anyone with any experience of DCV?

    My understanding of how they work is that sensors on ventilation grilles dictate when the grilles are opened or not. A fan somehow knows when to run or not - I think it simply runs if the grilles are open because the same power is applied to it, so if the house is too positively pressured (the grilles are sealed) the fan doesn't turn.

    Is that right?

    Surely this needs an otherwise very airtight fabric?

    Other than that, seems like a nice solution for retrofit, assuming the sensors cover all the pathological cases.
    • CommentAuthortony
    • CommentTimeFeb 7th 2018
    I reckon that the fans run very slow most of the time, best would be to open up and speed up when an assumed CO2 monitor says to
    • CommentAuthorgravelld
    • CommentTimeFeb 8th 2018
    That's how I understand it works - the monitors are in each room, and opens the vent when the sensor says so. Because the resistance is lower, the fan then runs faster to maintain the same pressure. That's my understanding...
    • CommentAuthorgravelld
    • CommentTimeFeb 8th 2018
    I was reading around a bit today and I might have got the wrong end of the stick, at least slightly.

    DCV is also termed a strategy; i.e. only ventilating as required, dependent on the internal pollution. It isn't wedded to specific layouts of how the ventilation works such as my descriptions above. Generally it appears decentralised without ducting, which makes it attractive to retrofit, and with less ventilated heat loss than PIV or MEV.

    Anyone have any of these systems?
    • CommentAuthorgravelld
    • CommentTimeFeb 11th 2018
    Been learning a bit more.

    Apparently the term for the fan is a "constant pressure fan".

    I found a lot of what I'm talking about is sold as demand controlled MEV which appears ductless (great for retrofit) but less heat loss than vanilla MEV or PIV because the vents can close (I guess this also assumes half decent air tightness).

    Passivent make one called "iMEV". Although, from their website it appears the vents are a little keen:

    For example, the Passivent A121 extract increases from 4l/s to 21l/s when relative humidity passes 30%.
    • CommentAuthorgravelld
    • CommentTimeFeb 11th 2018 edited
    Looks like that last quote leaves out some detail. RH30% is when the vents begin to open at 4l/s. 21 l/s is actually the top rate, when RH is over 60%.

    In the UK we're not going to have RH under 30% so we're going to have an extract rate well in excess of 4l/s.

    Let's say we got RH about 50%. That would be about 16l/s per vent. Multiplied by a vent per room and this begins to look like a lot to me... Am I wrong to simply multiply the extract rate by the number of vents?
    • CommentAuthorgravelld
    • CommentTimeFeb 11th 2018
    Actually, I think I've misunderstood - these DCV MEV systems still require duct work. :shamed:
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