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smoke test
Saturday, August 8 2009
When I was building the greenhouse foundation back in the fall, I'd had the idea that I could use all the stone and earth on the outside of the foundation as functional thermal mass for the interior if only I had ports to allow me to get greenhouse air from the interior to the subsurface exterior. So I built four two four inch ports into the walls, one on the south wall and one on the north wall, each with long (ten foot) perforated PVC pipes leading off into the rocky fill around the greenhouse. I routed the port in the south wall to a pipe running in the top part of the deep trench carrying the greenhouse drainage eastward to daylight at the escarpment. The north pipe then ran westward along the outside of the greenhouse's north wall, terminating a couple feet beyond the building's northwest corner. Now that all those pipes are buried beneath fill and the greenhouse can be sealed up fairly tightly, I wanted to test to see if air really could be made to flow from the north port through the spaces in the fill to the south port. So I built a crude little fan-powered smoke machine. It consisted of a powerful 120 volt computer muffin fan, an L-shaped four inch galvanized sheet metal stove pipe fitting, and a bean can with many tiny holes drilled in the bottom. I filled the bean can with a little oregano and a lot of old dry White Pine needles (a great source of pleasant-smelling smoke), placed its bottom against the pipe fitting, the other end of which pointed to the intake of the fan, which I'd positioned against the port through the wall.
When I got the fan going and lit the pine needles, it was like I was spraying a fire hose of smoke into that hole, and very little wasn't going in. Within a few seconds I'd generated enough smoke that, had it been in our living room, it would have been rendered uninhabitable. But this was within the much smaller volume of voids in the fill around a small building. So where was all that smoke going?
After a couple minutes, smoke started pouring out of the the port in the south wall, indicating that air really could be circulated from one port to the other. But if I was celebrating, I was doing it too soon. I'd blocked the drainage system pipes and so couldn't see that most of the smoke was entering the drainage system and pouring in a huge cloud from the distant end of the drainage system, some sixty feet away at the edge of the escarpment. During the construction of the greenhouse, I'd been careful to isolate the lower part of the system from the higher levels in the fill, but evidently there were enough leaks for the smoke to find its own way. There was also a small amount of smoke pouring out of two of the fill's retaining walls, both the one around the door well and the one stretching westward from the greenhouse's southwest corner. Happily, though, I didn't see any smoke pouring from the ground.
Despite the objective measures of failure, an outside-the-foundation air circulation system could be implemented if I used spray foam to seal gaps in the retaining wall and then put some sort of flap at the far end of the drainage pipe to keep it closed except when there is drainage water pressing against it.
For linking purposes this article's URL is:
http://asecular.com/blog.php?090808 feedback
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