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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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    • CommentAuthorRobinB
    • CommentTimeSep 4th 2009
     
    I've just had my SAP calcs re-run 4 ways trying to use it as a tool to help decide which features to add to our soon-to-be built house.

    The initial calcs were done based on Windows u 1.1, walls u 0.13, roof 0.2, floor 0.17. 4 square metres flat plate solar collectors. First variant increased roof U value from 0.2 to 0.14, the second added 1 KW PV, the third left off the solar hot water panels.

    I was surprised to see no change at all in energy requirement for space heating in any of the variant (12052.18kWh/year). Assuming it was an error I called SAP-man and he said that was correct and that it's not a design tool and some default value - based on he know-not-what was used.

    Does this make sense? I couldn't believe at first that there was no difference! Am I at the eco-point where the changes I can make will add very little benefit for quite a big outlay? And that perhaps the added materials would not recoup the cost of their manufacure (in carbon or money terms) in their lifetime.

    For example would it make much difference if the wall were just 0.19? ( I could save the hassle/expense of insulated plasterboard).

    Is there another way to go about this that isn't too expensive?
    • CommentAuthorCassie
    • CommentTimeSep 4th 2009
     
    I understand your frustrations

    My SAP calculations got worse when we added MVHR and if I dont insulate the house on the outside - insulate it from the thermal mass of the surrounding earth, then the Heatloss assumed by SAP will fail me as well.

    If the airtightness is as good as we think it will be then it will overcome this, but by this time the house is built and buried so the MVHR and external insulation remain.

    Of course if it ends up a high B or an A I will be gloating !
    • CommentAuthordelboy
    • CommentTimeSep 4th 2009
     
    I'm sorry to inform you but your SAP assessor has got it wrong. Your heating demand will change if your external wall u-values change (or any other aspect of the envelope, including bridging and infiltration).

    For those of us sad enough to know where to look, you can see it in the SAP worksheet (see http://projects.bre.co.uk/sap2005/pdf/SAP2005_9-82.pdf).

    Go to PDF page 124 (p 123 of the document), and look at to box 35. This is a sum of all heat loss through the envelope fabric (walls, doors, windows, roof & floor) - derived from the sum of boxes 26-31.

    You then add box 35 to box 36 to get box 37 - this is the Heat Loss Coefficient which is the amount of heat which leaves your building in Watts per degree difference outside (W/K).

    You multiply box 37 (the HLC) by the amount of degree days (and also multiply by 0.024 - god knows where that number came from), and this gives you box 81, which is the total annual space heating requirement.

    Therefore if you alter the u-value in box 29 (for example), this will change the number in box 35 which will change the number in box 37 which will change box 81. If you see what I mean.

    Is it possible that there is no change in heating demand when you are altering other things such as the renewables?

    As an aside, I'd say that 12,052kwh was a pretty hefty heat demand. Are you building a massive house?

    PS Your SAP assessor is right that SAP is not a design tool.
    • CommentAuthorsipman
    • CommentTimeSep 4th 2009
     
    Robin

    I would place more emphasis on workmanship i.e how well the insulation is utilised, rather than chasing notional "U" values. Good airtightness and a moderate level of insulation (well utilised) will result in a home that requires minimal heating
    • CommentAuthorRobinB
    • CommentTimeSep 5th 2009
     
    Sipman and Delboy, Many thanks for your comments and useful links. I previously worked out that I need about 6kw imput for space heating and 2.5 for water. Does that seem bad? House is detached, roughly 1/3 earth sheltered. 193m square over 3 floors.
    • CommentAuthorsipman
    • CommentTimeSep 7th 2009
     
    Robin

    This small development may be worth a visit, not far from where you are building

    underfloor heating to the GF only, from an airsource heat pump

    http://www.claysllp.co.uk/SIPS-Housing-Project-Stones-Methodist-Church---Phase-1-development/11444

    http://www.claysllp.co.uk/Second-Phase---Stones-Methodist-Church,-Ripponden-development/11748
    • CommentAuthorkotpat
    • CommentTimeDec 2nd 2009
     
    Hi Folks

    Hot water requirement

    it takes 1.16 watts to heat 1 litre of water

    say you have 4 people in a house CIBSE says 45 litre per person that is

    45 x 4 = 180 litre hot water cylinder

    main cold water is normally at 4 deg C/ and hot water is 60 deg C -hence 56 deg rise

    180 litre x 56 deg c rise x 1.16 watts = 9.744 kw/ hr
    hence it will atke 9.7kw to heat 180 litre in one house

    so please clarify were the 2.5 litre comes from

    a shower at 7litre/ min for 7 mins = 49 litre x 4 people = 200 litre

    200 litre x 56 deg c rise x 1.16 watts = 12.99kw/hr


    9.7 kw required / 2.5 kw as stated by your notes = 3.38 hours to reheat to reheat a 180 litre tank

    12.99 kw required / 2.5 kw as stated by your notes = 5.19 hours to reheat a 200 litre tank

    hence 6 kw for heat loss and 9.7/ 12.99 for dhw 15.7kw/ 18.7kw = 23 kw boiler

    cheers
  1.  
    as a user of the irish equivalent software (DEAP)... (written partly by the same personna as SAP)... i have noticed some things about these energy softwares.

    1. do not use default values.. they are terrible. if using HRV refer to SAP appndix Q and choose a good efficiency and lo fan power
    2. changing thermal mass should be reflected in the 'thermal mass' catagory... sounds simple but its a common error to simply choose 'medium' all the time.
    3. SAP (and DEAP) is only meant as to produce comparative ratings, using a lot of conventions and assumptions. It is not really any good for making accurate reflections on end user quantities.
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