On Tue, 8 Jul 2008 15:19:36 -0700 (PDT), Yong Huang
wrote:

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.

Yong Huang

I had this. Only in one vent, when the AC was on, which I only use
about one month out of the year. When the heat was on months later,
I smelled nothing, but I did smell it again when the AC was on the
following year.. It smelled like a damp dirty basement, maybe a
weak sewage smell, but never urine. (My basement where the
furance/AC is is very dry and didn't have a smell.)

Maybe a mouse got in there and died, but I don't think I had mice that
year. If it was a mouse, in the open, like the basement floor, it
woudl dry up entirely in a month or three. I don't know how this
smell lasted to the following year, but I think that was the end of
it.

The ducts to this register are between the first and second floor, and
in the wall. I hoped it would go away, and it did after 13 months.