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.

    • CommentTimeJan 15th 2022
    Posted By: WillInAberdeenMHRV must comply with an obscure BRE guide from 1994, which suggests that (leakage+deliberate) ventilation should add up to 0.5-0.7ach, and that leakage will be 0.7ach for a typical house, or 0.2-0.35ach for a specially detailed house. So MHRV should be sized for 0.3ach or less, maybe 0, which is hardly prescriptive!
    Are the leakage numbers there values for testing at 50 Pa or values for actual infiltration under 'ordinary' conditions? (However 'ordinary' conditions are defined)

    A value of 0.2-0.35 ACH sounds like a plausible test value at 50 Pa for a PH (i.e. a specially detailed house). According to our PH certification PHPP we have a test result of 0.42 and a consequent infiltration of 0.033 ACH, i.e. a bit less than 8% of the test value. The effective ventilation rate will be 0.033 plus the MVHR rate 0.38 = 0.413 ACH so I can't see how the rules make sense taking either interpretation of the leakage number. :confused:
    • CommentAuthorbhommels
    • CommentTimeJan 15th 2022
    I am not very aware of regulations but have come across a factor of 10 to translate from ACH @ 50 Pa to average real life ACH before. Not sure where it comes from, but applying this factor 10 would make the figures in the quote by @WillnA add up. 0.7 ACH for a typical house would mean 7 ACH @ 50Pa, which is scandalous but below the 10 ACH required by BR, and could be achieved typically for newbuilds.
    Similarly, MVHR would be sized for 3 ACH@50Pa, which is in line with what some quote as the upper limit for MVHR. For a standard build, <3 ACH would require well above average attention to AT detailing.

    PH standards demand <0.6 ACH @ 50Pa, which is not easy to achieve. I think djh should be very pleased with an achieved 0.42 ACH@50Pa.
    It's not great that the Scottish building standards require compliance with a 25-year old document which is not available as an electronic text. Athough some scans of original paper copies are available online, including in the BRE shop. It was written before passivhouses were a thing!

    In short it suggests that 'natural infiltration rates [in the existing housing stock in 1994] as low as 0.2 ach are the exception rather than the rule; 0.7 ach is probably typical.... Generally a mean natural infiltration rate of 0.2 ach will have an air leakage rate at 50 Pascals of about 4 ach.'

    4ach @50 Pa is roughly 10m³/h/m². For comparison the airtightness target in the 2020 building standards for setting TER is 7m³/m² @50Pa (=~3ach @50Pa) - so new buildings are expected to be detailed to be more airtight than was seen as exceptional for the average housing stock in 1994. Clearly you can aim for better than 7m³/m² @50Pa and that will help you beat the TER but require more artificial ventilation.

    The other referred document in the building standards is CIBSE B2 which advises against sizing ventilation based on ACH figures at all, or by implication m³/h/m² or l/s/m² figures. So a bit of a tangle.
    • CommentAuthoran02ew
    • CommentTimeJan 29th 2022
    A more thorough read of 2021 part F i found this.

    which i read as-
    A dwelling is perseved less airtight and can rely on natural ventilation at a designed air permiability of >5 m3/m2hr and an as built >3 m3/m2hr.

    Dwellings that have better airtightness than these figures will require continuous ventilation.
    • CommentTimeJan 29th 2022
    an02ew might have meant: "A dwelling is DEFINED as less airtight and can rely on natural ventilation at a designed air permeability of >5 m3/m2hr OR an as built >3 m3/m2hr."

    What's also interesting is that the guidance doesn't seem to admit of the possibility of a passive ventilation system, although probably it could be proposed and supported by an expert advisor.
    • CommentAuthoran02ew
    • CommentTimeApr 23rd 2022
    Further poking around and I discovered this document from VA I can’t find the source material online and I was wondering if anyone had any ideas
    It states a cut off figure of 5m3/m2 h at which the natural infiltration would overcome the need for continuous ventilation. Under this figure and a calculation applying a set infiltration rate to the volume of the house is deducted from the balanced values of the commissioned MVHR.

    Any ideas why this infiltration rate is fixed? And could we not use the airtightness results to create a unique infiltration rate and apply that to the balancing values?
    • CommentAuthoran02ew
    • CommentTimeApr 23rd 2022
    VA document
    • CommentAuthortony
    • CommentTimeApr 23rd 2022
    Always worthwhile in my book
    From the example calculation, looks like the 5m³/m²h figure is at test conditions, and is the same part L figure that an02ew quoted a few posts above, 29th Jan.

    They actually subtract 0.04l/(s m³) which is a totally obscure unit but is equal to 0.15ACH.

    (Edit : does that actually comply with the new building regs? Supply rates look too low?)

    Scottish Standards are being updated now, and will look like the English ones :cry:
    • CommentTimeApr 23rd 2022 edited
    The document appears to be http://www.vent-axia.com/files/catdownloads/part%20fand%20l%20brochure%20web.pdf and that refers to the 2010/13 version of the regs. So it's now pretty much out of date. I have no idea whether that matters. Sorry haven't looked any further.
    • CommentAuthoran02ew
    • CommentTimeApr 27th 2022
    Posted By: djhThe document appears to behttp://www.vent-axia.com/files/catdownloads/part%20fand%20l%20brochure%20web.pdf" rel="nofollow" >http://www.vent-axia.com/files/catdownloads/part%20fand%20l%20brochure%20web.pdfand that refers to the 2010/13 version of the regs. So it's now pretty much out of date. I have no idea whether that matters. Sorry haven't looked any further.

    There doesnt seem to be any more informative in the revised partF just advise to seek expert advise, which is a bit of a waste of time considering its a 2121 revised doc dealing with ventilation. Could it not decide an optimum AC/h rate from which it should be easy enough to work back from the AT results and calculate the air exchange rate for the MVHR instead of the arbitrary figure in the above documant.
Add your comments

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

© Green Building Press