"Yong Huang" wrote in message
...
In this 18-year-old house, the A/C vent at the entrance to the master
bathroom sometimes gives out strong bad smell. We can't find a
pattern, whether it's related to weather, or we just used the toilet
or shower, etc. The smell is sometimes like urine, occasionally like
sewage. It happens maybe one out of ten days. I can't see anything by
shining a flashlight into the hose, and can't visually see anything
unusual when I go to the attic. No other A/C vent in the whole house
has any smell at all. How do I go about troubleshooting? Thanks for
advice.


You may have a critter or two living in the duct. Remove the entire
flexible section and bring it outside. Just get new flex to install or
better yet get solid steel duct to eliminate the problem from happening
again. Insulate the steel with duct wrap.