<div dir="ltr">so John, are you saying that when measuring heat flow from one mass to another, it does not matter what the density of the cooler mass is, and that the only thing of significance is the difference in temperature?<div>
<br></div><div>-Grasshopper</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On Wed, Aug 21, 2013 at 9:42 AM, John Straube <span dir="ltr"><<a href="mailto:jfstraube@gmail.com" target="_blank">jfstraube@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">The heat flow under steady state from indoors to 55F soil is the same (within a few percent) of indoors to 55F air. More heat flows to outdoor air at 45F because the temperature difference is larger, but for the same R-value, the heat loss to 45F soil is the same as it is to 45F air.<br>
<br>
The only difference for steady state flow is that the surface film coefficients change. Yup, there is no surface film used for concrete to soil heat exchange (we assume the film has an R-value=0) but the value for the film on a windy night for the outside of a wall is only R0.2 or so, (a rounding error in R20 assemblies).<br>
<br>
The only difference of density and heat capacity is caused by dynamic swings. Because the soil is so massive, moderately conductive, and connected to a stable deep earth temperature, it definitely does not vary much over time, whereas the outdoor air temperature does. This has no impact on assessing the heat loss of over the heating season for a floor slab (e.g., the slowly changing soil temperature makes steady state heat loss calculations very accurate, whereas mass / heat capacity in walls matters more because of the fast changing air temperature).<br>
<br>
Not sure what rules of thumb are being used.<br>
<br>
<br>
On 2013-08-21, at 9:26 AM, Alan Abrams <<a href="mailto:alan@abramsdesignbuild.com">alan@abramsdesignbuild.com</a>> wrote:<br>
<br>
> John, your points are well taken, but just to clarifiy: what was meant by that is different heat flow rates to materials of different densities and specific heat capacities. the important question is then, does heat flow at a greater rate to air at an average winter temperature of maybe 45d F, or to the earth at maybe 55d F, per square foot.<br>
><br>
> and is it still air or windy?<br>
><br>
> it goes back to the original point, that there are some limits to rules of thumb.<br>
><br>
<br>
John Straube<br>
<a href="http://www.JohnStraube.com" target="_blank">www.JohnStraube.com</a><br>
<br>
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</blockquote></div><br><br clear="all"><div><br></div>-- <br><font><font><span style="color:rgb(102,0,0)">Alan Abrams</span></font><b style="color:rgb(102,0,0)"><br></b><i><span style="color:rgb(102,102,102)">certified professional building designer, AIBD<br>
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