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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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    • CommentAuthorborpin
    • CommentTimeFeb 9th 2012
    A while ago, there was a side discussion about MVHR air flow and rate of extract and supply being balanced or not. Having got a quote from Airflow for their system that was where I expected it to be on price, I am interested as to how good a design it is.

    It is using the Airflex Pro pipe (discussed elsewhere) and the DV 90 SE unit http://www.airflow.com/AIR1-SHOP/pdf/Heat_Recovery/Duplexvent_90SE.pdf.

    House Volume 485 m3, Ac rate 0.44 (213 m3/h), balanced in/out.

    Do these figures sound right for an airtight house?
    How does this HRV unit compare to others (efficiency etc)

    Any input much appreciated.
    • CommentAuthorjms452
    • CommentTimeFeb 10th 2012
    Hi borpin,

    Not sure how much the unit costs but with efficiency 80% and fan power 0.7W/L/s there are definitely better out there that don't cost a bomb.

    if you have tortuous paths that would need lots of 90° bends then their ducting may be the best bet but if you can do it with simple straight runs I'd go for larger bore rigid.

    How does there unit cope with sub-zero exterior temperatures? I know that some Aluminum heat exchanger HRV units just bypass the heat exchanger which seems nuts to me.

    Also note that there test certificate states that there test data is taken with large bore rigid tubing.

    I have no complaints with this unit and not that expensive for a whole house unit.

    • CommentAuthorborpin
    • CommentTimeFeb 10th 2012
    Hi John, The type of ducting does suit me, but I may buy the unit separate from the rest of the system which drove my question. Thanks for your input.
    • CommentAuthordocmartin
    • CommentTimeFeb 10th 2012
    Hi jms452, I note on the Nuaire summary that "filter replacement is every 5 years". Is the filter washable? What is the regular maintanence and suggested interval?
    • CommentAuthorjms452
    • CommentTimeFeb 10th 2012

    its disposable fine paper concertinaed up probably approaching 1m2 of it - I imagine that you could replace it with a block of washable of open cell foam after the warantee had expired for less than cost of two replacement filters (I think £50),
    • CommentAuthordocmartin
    • CommentTimeFeb 10th 2012 edited

    the vaillant filters are a similar concertina catridge of fleece, 390 x 180mm. I estimate the surface area to be .01728m2. The fresh air side is far cleaner than the exhaust as it has a pre-filter. The soiling of the exhaust filter is significant at the 3 monthly maintanence when I vacuum and replace. There is no way I could envisage the filters being efficient for 5 years without maintanence,
    Edit: Oops! decimal point, should be .1728m2.
    • CommentAuthorborpin
    • CommentTimeFeb 11th 2012
    Been doing some digging and for this size of air change (213 m3/h), it seems most units are around the 80-85% efficient mark. Unless anyone knows any different.....
    • CommentAuthorGreenPaddy
    • CommentTimeFeb 22nd 2012

    hope I'm not stating the obvious, but have you looked at the sap appendix q website - lists all the "approved" units (for inclusion in SAP calcs), with the operational characteristics, efficiencies etc. Really useful to have it all in one place.
    • CommentAuthordhutch
    • CommentTimeFeb 23rd 2012 edited
    Posted By: GreenPaddy...have you looked at the sap appendix q website - lists all the "approved" units ...
    Sounds good, do you have a link?
    • CommentAuthordhutch
    • CommentTimeFeb 23rd 2012
    Is this it? http://www.sap-appendixq.org.uk/search.jsp?parentID=1&categoryID=3
    • CommentAuthortony
    • CommentTimeFeb 23rd 2012
    The design rate and the actual in use rate can vary dur to resistance in the ducts

    In would say 0.44 is too low if that is the maximum.

    You should be able to purge the house and have a party mode over 1.0 ach mine is 1.5 (actual) I think

    I run at more than 0.44 during active periods during the day and drop down below that only at night

    I would think a higher capacity unit would be safer especially if the house is one with low infiltration rates as all houses should be

    Please check our where this 213m3/h comes from is it for the unit or the whole system there is a big difference, you dont want to find out after you have got it in.
    • CommentAuthorborpin
    • CommentTimeFeb 23rd 2012
    Posted By: tonyPlease check our where this 213m3/h comes from is it for the unit or the whole system there is a big difference, you dont want to find out after you have got it in.
    It comes from the House volume x .44 (see OP).

    Your thoughts are where my concern comes from. having seen on here where a 'specialist' seems to have got it so wrong I wanted to do a sanity check.

    @greepaddy Thanks for that I'll take a look.
    • CommentAuthorGaryB
    • CommentTimeFeb 23rd 2012
    Don't forget to look at the specific fan power, not just the efficiency. After all, you pay for the electricity.

    The best units are around 0.5 to 0.6 W/l/s. Avoid anything higher than 0.8

    The SFC figures make a big difference to the SAP calculations and hence EPC ratings.
    • CommentAuthorborpin
    • CommentTimeFeb 24th 2012
    Posted By: GaryBDon't forget to look at the specific fan power, not just the efficiency. After all, you pay for the electricity.
    Ah excellent info, thanks.
    • CommentAuthorchriskemp
    • CommentTimeFeb 25th 2012
    0.3 litres per second, per metre square, is the min trickle rate you would want to achieve.

    Then boost rate is the summed rates of the number wet rooms - so 13l/s for a kitchen, plus 8l/s for each wetroom.

    (assume no infiltration to be taken off - if you're building a very airtight dwelling)

    Filters should be changed annually (or cleaned) to ensure flow rates are maintained = "adequate ventilation".

    Otherwise you need a unit with constant duty function - which increases duty to overcome system resistance (blocked filters).
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