I actually have been thinking about this very problem, a diy, inexpensive air to air heat exchanger. I would like to do this for my crawl space. I am not a big believer in radon risks. I thought about doing a traditional heat exchanger with lots and lots of sheets of aluminum. One could do this in a rectangular duct.
But I thought that this would be simpler and very cheap: Take a three inch round aluminum vent pipe and put it in a four inch round vent pipe. Wrap it in insulation and make it pretty long, say 20-25 feet. The cold, outside air goes in the center pipe. The warm, moist inside air exits through the large vent pipe outside of the center (the "annulus" part). There would be condensation developing on the inside pipe, dripping down, so it would have to be inclined down somewhat with a collection pan.
One couldn't move the air too quickly, but it is my contention that with a sufficiently long pipe (again about 20 feet) the temperature of the air in the center pipe would be coming out warmed to almost the temperature of the inside air temperature.
Other variations: two three inch pipes in a six inch outer pipe. This would match the areas inside the smaller pipes with the outside remnant area. Or seven three inch pipes in a honeycomb pattern inside a nine inch outer pipe.
One would want the inner pipe to stick out somewhat so that one isn't sucking in what one is blowing out.
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