There was no noticeable "odor" in the basement. Actually, at first I thought the clothes dryer vent was leaking because that is what it felt like, moist air. yes, we have a horizontal run going from the heater, through crawl space, to the outside..right before it gets to the outside wall, there is a small fan of some type...i assume it helps push things out..will have to look at that again (saw it a while ago when moving in 6 mos ago, just now getting pool going). I have been doing some reading, and I read about a two-pipe system, which we may have. There is another duct run, and the one end of it sits right next to..kind of just above, the heater..could not figure out its purpose before. The other end just kind of stops in teh crawl space..is this one supposed to bring in "fresh" air for the heater to use? It has a baffle in it too..to control air flow. Thanks for all replies thus far. FYI, we have a CO2 detector on the main level (our house is a two-story w/ basement). The basement, right now anyway, is for storage, laundry, heater, hot water tank. Will probably finish a portion of it at some point. I tried to attach a pic, but this site will not let me upload it (too big), will have to figure out how to take a smaller size pic.