I saw something really weird the other day... I was in a basement, and saw a 90%+ furnace. Two pvc pipes coming out of it, intake and exhaust. Here's where it got strange... The pipes went up from the furnace, turned and went under the floor joists, and CONNECTED together and ran out of the house as ONE pipe. This did not make any sense to me at all. All of the 90%+ systems I have ever seen have had two pipes all the way out.
Can anyone explain this to me?