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.

    • CommentAuthordelprado
    • CommentTimeSep 18th 2017
    Can anyone recommend a good one? I can't even seem to find one where I could put in the actual u value of my wall (which I know) rather than just saying "insulated cavity"

    • CommentAuthortony
    • CommentTimeSep 18th 2017
    Hot 2000
    • CommentTimeSep 18th 2017
    • CommentTimeSep 18th 2017
    Posted By: SteamyTeaSomething simple like this?

    • CommentTimeSep 18th 2017
    What is all that then, I just click on the link and it work at this end.
    Works for me. I do want to find out what resources Steamy Tea's Santa has got, though.
    • CommentTimeSep 18th 2017

    Are the answers right
    Yes, seem to be, but I suspect the OP probably wants a whole-building version, which works without having to total everything up - and probably one which also estimates, or calculates, ventilation heat losses.
    • CommentAuthorgravelld
    • CommentTimeSep 19th 2017
    I think djh is on his Javascript high horse again :devil::devil:
    • CommentTimeSep 19th 2017
    I could knock up a ventilation one easy enough.
    • CommentAuthordelprado
    • CommentTimeSep 26th 2017
    SteamyTea - or anyone else - can you explain the simple calculator?

    So say for example I am trying to work out the heat loss through a wall which is 12sqm, has a u value of 0.6, and the temperature difference I am trying to design for is 21 degrees, as I want to maintain a room temp of 21 degrees when the temperature outside if zero (no point going lower as Bournemouth rarely gets colder). If I put those numbers in the power comes out at 141.12. What does that figure actually mean?

    Does it mean I then repeat the exercise with all other forms of heat loss in the building and then add up all of my power figures to give me a total heat loss?

    • CommentTimeSep 26th 2017
    The m^2 and the K cancel out, so you are just left with the W, or watts.

    0.6 [W.m^-2.K^-1] x 12 [m^2) x 21 [K] = 151.2 W

    W is the power you need to supply to keep the temperature stable.
    If you want to keep that temperature stable for 1 day, which is 24 hours, so that you can work out the energy needed, then you multiply the power by the time.

    151.2 [W] x 2h [h] = 3628.8 Wh

    More commonly called 3.62 kWh
    • CommentAuthorDarylP
    • CommentTimeSep 26th 2017
    U-value units are Watts per Kelvin per m2, so as ST said before,
    if there is a temp difference across a wall of 21degrees (21 inside, 0 outside),
    the U-value = 0.6W/m2.k,
    you will need 151.2Watts to keep the interior of the 12m2 wall at 21degrees.
    Do the same calc for all heat-loss thermal envelope elements, and that will give you your fabric heat loss.
    However that is ONLY fabric heat loss....:wink:
    • CommentAuthordelprado
    • CommentTimeSep 26th 2017
    Thanks Daryl.

    So basically I do as I outlined in my post, add all the numbers up, and I find out how fabric heat loss. What else do I need to add? Air? Is there anything else?
    Thermal bridging - where the fabric U value is not really what it purports to be.
    • CommentTimeSep 26th 2017 edited
    Posted By: Nick ParsonsThermal bridging
    Yes it is tricky.
    But can be worked out like a u-value, if you know where the bridges are.

    Corners can cause a few problems, but if you take the greater values, usually the external measurements, then you should be loosing less.

    Air losses are worked out slightly different as the losses are measured in J (joules).
    The joule is the unit of energy and is very small, so usually the kJ or the MJ is used.
    3.6 MJ is 1 kWh

    The things you need to know are the density of air, the specific heat capacity of air, how often that air is changed, the volume of the house and the temperature difference.

    The density of air is around 1.25 kg.m^-3
    The specific heat capacity is around 1 kJ.kg^-1.K^-1
    Air changes what they are, as is the temperature difference.


    1 [kJ.kg^-1.K^-1] x 1.25 [kg.m^-3] x 160 [m^3] x 0.5 [air changes/h] x 10 [K] = 1000 kJ [1 MJ]

    1 MJ = 0.27778 kWh

    Multiply this by 24 hours in a day:

    0.27778 x 24 = 6.667 kWh.day^-1

    (it is late and I may have made a mistake, but others will tell me)
    • CommentAuthorDarylP
    • CommentTimeSep 27th 2017
    @delprado, basically, yes. :bigsmile:
    Fabric losses - U-valus for walls, floors, roofs, windows, external doors
    air-filtration losses
    Psi-values for Htb per linear metre (y-value in SAP)
    DHW storage losses

    I suppose it depends on your required level of accuracy, and what you want to do with the data? :wink:
    • CommentAuthordelprado
    • CommentTimeSep 27th 2017
    It doesnt need to be perfect, its just for seeing whether underfloor heating can provide enough output and if it can, can I reduce the internal area at all.

    My only though re that calculator is presumably even with the same u value, wont floors (for example) lose less heat than a roof?

    Air loss - the house will be broadly airtight to a non-passic haus standard. Ie, careful plastering and sealing all holes. Some use of membrane
    • CommentTimeSep 27th 2017
    Posted By: delpradoMy only though re that calculator is presumably even with the same u value, wont floors (for example) lose less heat than a roof?
    Also depends on the surface area, the temperature differences, wind direction and speed, solar gain, ground water movement, ground shading from trees and buildings.

    So you just have to model each hour, day, week, month or year depending on how good a model you want.
    As an example, when the wind is northerly,we tend to have clear skies, but lower windspeeds and less rain. All these affect temperature loss.
    • CommentAuthordelprado
    • CommentTimeSep 27th 2017
    Thank you so much. That is incredibly helpful.

    Do you reckon for a "decent" calculation, I could just whack the u values and areas in that calculator, add em up, and if I get 400w result, put in, say a 600w rad and be ok? Its almost like I need an approximate standard uplift to account for the other variables (excluding air tightness as I am assuming heat loss there will be minimal)
    • CommentTimeSep 27th 2017
    I think you should be able to do that OK.

    Generally you need to oversize a little on the heating as you have to be able to raise a temperature, not just maintain one.
    But as you say, Bournemouth is pretty mild. Lived in a flat in Surrey Road when I was a student and the 2 kW heater kept the place pretty warm.
    And it is useful to build in a bit of provision for extra heating, just in case - easy if you are building or adding something well=insulated.

    eg connections for an extra radiator(s).

Add your comments

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

© Green Building Press