Checked a radon system for a customer who had gallons of water in his crawlspace. The Builder-installed system had an upside-down fan in the crawlspace. The bottom of the pipe had a roof boot around it, but was sitting on a concrete slab. The vent pipe runs next to the fireplace flue pipe through the attic and roof and terminates next to the chimney flue under the chimney cap. So, the fan pulls water into the crawlspace, pushes radon into the living area, and, if the Owner.built a fire, he would have ash, soot, carbon monoxide, and maybe sparks in there!