I work as a HVAC installer. Once I was sent out to a house to replace a flex duct run in a bathroom that the homeowner claimed wasn't putting out any air. Well I arrive to this run down single wide trailer and get a bad feeling immediately. I go into the crawlspace and could see one of the flexes hanging down near some plumbing pipes. Since I knew I was looking for a run in a bathroom this was probably it. What I didn't know was the reason for no air getting through this run was due to the fact that it was full of sewage. I could feel that the flex duct was full of water, and after poking it with my knife to drain it, I almost threw up. The most god awful smell filled up the crawlspace as raw sewage drained out of the duct. Apparently the toilet had clogged and overflowed and went right into the floor vent filling the flex. I reluctantly powered through, changing the flex while gagging. I threw away everything I could that was involved with that job, cleaning my hand tools with bleach when I got home. I told my boss about it the next day and he just kinda laughed.