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Energy Assessment: Measuring real life U-values

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  1.  
    • CommentAuthortony
    • CommentTimeDec 8th 2012
     
    In a recent thread Paul suggested this

    http://www.professionalequipment.com/content/how-to-determine-r-value.asp

    Can we make it work for Celsius and output U-values?

    Probably simple maths not sure.

    I would be extremely interested in any results for "blobbed and dobbed" walls too
  2.  
    It's easy to convert into Celsius - just divide the deltaT and surface values by 1.8. The curves are still in R (imperial) though. To convert these to RSI, divide by 5.678 (an easy to remember constant). Then, finally, take the reciprocal to get the U value.

    Sorted :)

    Paul in Montreal.

    p.s. Tony, you should try it for your basement floor to see what the effective U value is of your heat store.
    • CommentAuthorEd Davies
    • CommentTimeDec 8th 2012
     
    I assume they're just using an estimate of the resistance of the internal air to wall interface to estimate the heat flow through the wall to get an estimate of the wall's resistance. The trouble is that, compared to any sensible level of insulation, that interface does not present a large resistance so the result is very sensitive to any errors in the measurement.

    E.g., the two bottom lines: R=15 corresponds to U = 0.379 W/m²·K and R=20 to U=0.284 W/m²·K. Look how close those are when the interior to exterior temperature is in the sort of ranges we're likely to meet in the UK, say up to about 40 °F. A 0.1 °C error in opposite directions in each of the air and interior wall temperature measurements could move you from one of those lines to the other.

    It's worth noting though that their estimate of the air to internal wall interface resistance does seem to vary with the temperature difference. I get R-US values of 0.6825, 0.6875 and 0.75 for different points on the graph. I assume this is because they're taking increased convection with bigger differences into account.
    • CommentAuthorEd Davies
    • CommentTimeDec 8th 2012
     
    Posted By: Paul in Montrealp.s. Tony, you should try it for your basement floor to see what the effective U value is of your heat store.

    Hmm, they do say it's for “walls, windows, ceilings and floor” but it's also worth noting that the interface resistance varies quite significantly due to orientation (because of convection) so I can't see how it'll be very accurate at all.

    I think even the first significant figure on the result would be dubious.
    • CommentAuthortony
    • CommentTimeDec 8th 2012
     
    My living room wall outside is 2.7C inside 20.9C and the room 21C not sure the charts go low enough.
    • CommentAuthorEd Davies
    • CommentTimeDec 8th 2012 edited
     
    Assuming your thermometer has a resolution of 0.1 K, is that really inside at 20.86 and room at 21.04 (0.18 K difference) or inside at 20.94 and room at 20.96 (0.02 K difference)? Difference will multiply the estimated U-value by about 9. With those sorts of temperature differences this technique doesn't even reliably get you within an order of magnitude of the right answer.

    Maybe a few hundred sets of readings over a couple of hours as the temperatures change by a degree or two might begin to give a sensible answer.
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 9th 2012
     
    Ed, the error gets worse depending on orientation and season as you have to add these figures to take convection into account.

    Surface position, Direction of heat transfer,RSI (K·m²/W)
    Horizontal, Upward,0.11
    Horizontal,Downward,0.16
    Vertical,Horizontal,0.12
    Outdoor surface 6.7 m/s,Any Direction,0.030
    Outdoor surface3.4 m/s,Any direction,0.044

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