My Mother's House Part IV
(Page 2 of 3)
January/February 1982
By the Mother Earth News staff
However, earth sheltering doesn't eliminate the need to exchange a home's interior air at least once every two hours. (Such a change is required to maintain adequate oxygen in a house, and to prevent the accumulation of carbon dioxide and toxins emitted by gas appliances, woodstoves, and the building materials themselves.) And you may be surprised to know that, in a very well-insulated dwelling, the task of heating or cooling the "new" air to room temperature can account for a substantial portion of the building's thermal requirements.
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In order to limit the difference between the temperature within a house and that of the exchange air that must be introduced, a few pioneers have been experimenting with arrangements of buried "breathing" tubes . . . by means of which the natural heat sink of the ground tends to warm or cool the indrawn air almost to its own temperature. For the most part, such systems have been used to provide cooling, so they've been nicknamed "cool tubes". In truth, however—since, incold weather, they bring in outside air that's already warmed above ambient temperature and doesn't require as much energy to be brought up to indoor comfort levels—they can contribute to efficient home heating, too.
Because the use of passive ground-source heating or cooling systems is so new, little technical information is currently available. So we based our design decisions on a few rudimentary facts gathered mostly from folks who'd already tried similar setups. First, we knew that the thermal conductance of metal pipe isn't substantially better than that of the plastic variety, so we opted for less expensive, durable irrigation-type PVC. Originally, we were in the market for the 12" size (though any diameter between 8" and 18" would likely have been acceptable), but we eventually picked 15" . . . simply because it was readily available.
To take advantage of the most stable ground temperatures possible, we buried our pipes as deep as was practical . . . a full ten feet. At the same time, we ran perforated plastic drain lines around the building's foundation and laid their outlets beside the cool tubes. The slope of the hillside on which our house is being built made the task of draining the pipes an easy one. Water that condenses in the tubes (the situation arises in summer, when the relative humidity of the warm air is increased to saturation by the cooling effect of the earth) runs downhill to drains located at the air entrances.
The available data suggest that cool tubes are effective for only about 60 feet in one run: After that distance, the temperature difference between ground and air—in a tube where air is flowing at an adequate rate—becomes too small for thorough heat transfer. Consequently, we used two 60' lengths of the 15" tubes . . . with one entering at the front of each end of our structure. A ducting channel in the slab then runs the length of the mass wall, to allow us to distribute the cool (or warm) air evenly. (This trench also serves as the runway for the 2" PVC kitchen sink drain.)
