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General: Heat loss of walls

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  1.  
    • CommentAuthortony
    • CommentTimeNov 8th 2007
     
    Do walls loose the same ammount of heat whether or not it is windy?

    In other words are U values dependant on wind speed?

    Heat is lost from the surface of a wall by; conduction straight into the air (this is small?), radiation ( this will be independent of wind speed ), and convection.

    Convection losses can be increased by forcing air over surfaces, forced convection. How much of this goes of for walls?

    Do hot box tests take account of any of this?
  2.  
    Tony, yes of course heat loss is related to wind speed.

    Here in Canada in winter, wind chill is measure in watts per square metre - extreme windchills are around 2-3kw per m2 - but these figures are usually used to determine how many seconds of exposure skin can stand before frostbite sets in.

    For houses, though, wind speed is more likely to affect air leakage - the models in hot2000 ask for the "exposure" of the building to help determine the effect. I'm sure the air leakage impact is more than an order of magnitude greater than the "wind chill" effect.

    I'm sure hot box testing doesn't account for any of this - unless it's done in a wind tunnel.

    Paul in Montreal.
    • CommentAuthorjon
    • CommentTimeNov 8th 2007
     
    The figures I have for this show the heatloss for exposed compared to sheltered conditions to be relatively insignificant Paul:

    Do you think there may be an error in standard assumptions?

    It does strike me as odd that a fast flowing turbulent gas does not carry away significantly more energy than still conditions but I have no data that shows otherwise. On the other hand I suppose that for gases as opposed to fluids, local Brownian motion within the gas may be much more significant than the macro scale turbulence and air movement.
    •  
      CommentAuthorfostertom
    • CommentTimeNov 8th 2007
     
    Another factor of great significance at the surface is whether it's wet or not - heat lost through evaporation - and the wetness effect becomes more significant as air speed goes up. You bet hotbox testing doesn't account for this, either - or maybe some standardised adjustment is made to the test results. Like the equally chancy standard adjustment that's made to average out the considerable degradation of performance that creeps in as the blowing gas in a foam gets gradually replaced by air.
  5.  
    Is the hotbox test unrealistic if it doesn't take into consideration wind blowing over surfaces?

    A simple example is a hot cup of coffee which becomes drinkable far quicker when you blow on the surface.
  6.  
    Steve,

    the coffee analogy is flawed because the cooling of the coffee is accelerated by the evaporation of the coffee itself. Try cooling a cup of coffee by just blowing on the side of the cup (it will speed it up of course - but to a smaller extent).

    This is not the same as wind blowing over a wall (unless the wall is dripping wet). The reality is somewhere between the two extremes.

    The only real test is to build an actual house and monitor it over a period of time - the problem is that there are then many interlinked variables which makes it extremely difficult to separate them out.

    Paul in Montreal.
    • CommentAuthortony
    • CommentTimeNov 8th 2007
     
    I am thinking that the surface texture of the wall will have an effect too, surface are per unit area for example, porosity and not just to water, smoothness all have differing effects.

    So are we saying that U values are not a reflection of the real world? Indeed we seem to have already said that wind speed has an effect on heat loss.
    • CommentAuthorjon
    • CommentTimeNov 8th 2007
     
    No, they're a reflection of the real world as a standard by which to compare

    Whether or not the surface condition emits at a slightly higher or lower rate is insignificant to most walls where the rate of energy throughput is determined by the most resistant elements (the insulation generally)

    We're just discussing the margins here
  9.  
    U-values calc contain some inplicit design assumptions. In reality the fluctuate. The "standard conditions" are a wind speed of 7m/s and a temp of -1C externally. For most inland areas of the UK, -1C is relatively infrequent and the hourly mean wind speed that is exceeded on only 10% of occasions is between 8 and 9 m/s. (These are meteorological wind speeds urban development and wind breaks result in about 20-50% reduction in wind speed.)

    Mark
    • CommentAuthorjon
    • CommentTimeNov 8th 2007
     
    Thanks Mark

    So, does this mean that for a sheltered (or exposed) condition, we may be allowing too low a U value for the external interface?

    If so, what is the margin likely to be? From a boundary check, if we assume a norm of 0.35 for a low emissivity cavity then each interface skin at 0.175 form the upper boundary I would guess

    Rso would be 0.03 normally or 0.11 for sheltered (at least that's what I've got in my software)
    So the lower boundary for sheltered is 0.11 and upper 0.175

    Doesn't seem a big difference so the impact of macro conditions must be overruled by mico effects of brownian within the gas?

    Or have I missed something obvious?

    jon
    •  
      CommentAuthorfostertom
    • CommentTimeNov 8th 2007
     
    Posted By: jondoes this mean that for a sheltered (or exposed) condition, we may be allowing too low a U value
    For all conditions, we're allowing much too high a U-value, by a factor of 2.5, by assuming steady-state like the test conditions, whereas real-life dynamically fluctuating conditions result in a much lower effective U-value. This in addition to any variations caused by air speed and/or wetness.
    • CommentAuthorjon
    • CommentTimeNov 8th 2007
     
    You've lost me Tom: Isn't the U value defined by a steady state?
    •  
      CommentAuthorfostertom
    • CommentTimeNov 9th 2007
     
    True, and if wanted an 'effective U-value' that indicated actual heat loss under fluctuating temp conditions, it wd be a lot lower.

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