Last week a stray (tom) cat got closed under our house and didn't make noise for 4 days. By that time we had a lot of cat pee! Seems that it drank water from the drip pan under the air handler and proceeded to mark every square foot under there. The smell started coming in the house, so we began pulling up plastic under there, thinking we'd just lay more and fix the problem. But we found a mucky mess under the plastic - major moisture. Lots of mold. We had a leaky shower drain that had gone unnoticed. Mixed with the cat's activities, it was really odiferous. so we have several issues. But my big question at this point is why outside air (the pee and mold smell) is coming into the house. If I pull out the filter and stick my head in a return duct when the air isn't blowing, it's like I'm sitting in the crawlspace. I don't see any ducts disconnected down there. I do see several places where the wrapping on the thick insulation has popped, and you can see the pink insulation, but I assume there is a plastic duct somewhere inside that and smell shouldn't get in. Am I correct that the air handler shouldn't be sucking outside air under there?

I have a downdraft furnace and my question is this. On top of the furnace where the return duct work for the house collects, there is a 6" round duct that goes through the wall to the outside, with a damper of some type which is spring loaded. What is this for??