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# How many BTU's of HEAT rise up out of the ground per square foot in the winter ?or... how can i measure it myself? Answered

It is a little known fact, but 60 degree HEAT is under the ground, and it RISES UP out of the ground. I know, because i used it to "heat" my house last december. My house didn't get colder than about 60 degrees, even though the outside temperature got down to 35 degrees... and i kept my furnace OFF. I tried to search the internet for information but cant find anyone else who mentions it.

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It gets a little complicated - one version of the classic Heat Transfer Equation is:

Qt = h*A(Th- Tl)

Qt = heat transferred in BTU/ft2-hr.
h = Heat transfer coefficient
A = Surface area through which heat is being transferred
Th = Temperature on the high-temperature side of the heat-transfer surface
Tl = Temperature on the low-temperature side of the heat-transfer surface

The tricky bit here (besides putting up with the English-unit nightmare construct of BTU/ft2-hr), is that little term h, the heat-transfer coefficient, which depends heavily on the various components of the heat transfer system. Data for each combination of component and type of component surface, usually has to be determined empirically, which means it's usually not available for whichever specific components you happen to have.

I'd guess that, while the earth did a nice job of insulating the underside of your house last winter, it was actually you and your appliances that did the heating. The average human being puts out about 50 BTU/hr.; and your fridge, freezer, water heater, washer, dryer, TV, computer, etc., probably have waste-heat loss rates higher than that. In a well-insulated house, if wind and precipitation levels are more-or-less moderate, maintaining a 60 F interior temp with a 35 F outside temp is not too surprising.

orksecurity (author)2009-09-29

That last paragraph strikes me as Not Unlikely, especially if these devices are older and less efficient. Add light bulbs too. Almost every watt of electricity you use winds up as heat, directly or indirectly. As I found out this summer, when my computer was making my study uncomfortably warm even when the rest of the house was fine.

Kevin_in_Denver (author)2010-10-19

All three other answers are correct, but you are also correct. There is a significant amount of heat "rising out of the ground" as well.

I've found enough data to make a rough calculation:

http://www.greenbuildingtalk.com/Forums/tabid/53/aff/13/aft/45292/afv/topic/Default.aspx

orksecurity (author)2009-09-28

Not necessarily 60 degrees. What happens is that, once you're below the frost line, ground temperature settles at roughly average temperature for the year -- heat soak in slowly, heat leaks back out slowly. (A few feet of soil is a reasonable amount of insulation.) There are companies which specialize in this, installing heat pumps which cool the house during the summer by pumping heat into the ground, and which then heat the house during winter by pulling stored heat back out. A search on something like "geothermal heat pump" or "geothermal heating and cooling" should find those products, and I'd bet their manufacturers have the numbers you're looking for. Note that the answer in BTUs depends on the differential between ground temp and the temperature above it.