Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)


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.

Buy individually or both books together. Delivery is free!

powered by Surfing Waves

Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.

    It's great to draw pictures, really helps thinking! It is true that the lines will curve exponentially, but maybe unnecessary to get into that detail on the sketch charts.

    Could you add a line to both the charts showing the value of [pollutant concentration * ventilation rate] at each point in time? That's equal to the amount of pollution being removed, in kg/h, or whatever unit you're using.

    If you double the concentration and halve the ventilation, you still get the same kg/h of pollution removed.

    The area under that line equals the total mass of pollution that is been removed, say over a cycle. So long as that area is equal to the mass of pollution that is generated during a cycle, then you're at equilibrium.

    You can optimise the ventilation rate (hence heat loss) by ventilating more at the times when pollution is most concentrated, so the [concentration * ventilation] product becomes greater. If you take this optimisation to its limit, and do maximum ventilation only when the pollutants are at maximum concentration, you end up with a purge ventilation cycle.

    (Edit for posterity: by 'concentration' I mean the excess concentration indoors, compared to the background concentration in incoming ventilation air, which obvs already contains humidity, CO2 etc.)
    • CommentAuthorlineweight
    • CommentTimeApr 21st 2022
    Adding that line might go beyond my maths skills ...

    I can see that optimising for ventilation night end up with a purge type cycle

    But the original question wasn't about optimising for ventilation only - it was about ventilating with the minimum amount of energy loss.

    My thinking was that the more you can rely on diffusion (which doesn't really necessitate energy loss) rather than physical movement of the air (which does) then the less energy you lose.

    That's the thing that I don't have the information for, to try and quantify. I am imagining that in the ideal "trickle" scenario (which I guess would be on a windless day) hardly any air is moving in or out, but pollutants are being removed by diffusion. Of course it might be that what can actually be removed purely by diffusion is trivial in the scheme of things. This is what I have no grasp of.
    That scenario could be achieved (hypothetically!) by covering the window opening with a sheet of something that allows humidity and pollutants to diffuse out, but stops cold air blowing in. (A permeable membrane).

    Plasterboard or woodfibre board or breather membrane would be ok.

    Now imagine how much faster the humidity would be cleared if you removed that sheet and just had the window opening - or try the smell experiment on the previous page!

    Old houses are traditionally very vapour-open with lath/plaster ceilings and t&g pine floorboards, but even with those huge areas of permeable membranes, you still need airflow to properly ventilate them, usually deliberately as well as through air leakage.

    BTW, even diffusion and MHRV have large energy losses in the form of the latent heat of the water vapour, this can be as much as the sensible heat of the warm air extract. One way to recover both the sensible and latent heats is to use an Exhaust Air Heat Pump EAHP for ventilation. Even on a warm day, the EAHP can cool the outgoing air enough to condense the outgoing water vapour and so recover the latent heat, often for use in DHW.
Add your comments

    Username Password
  • Format comments as
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   

© Green Building Press