I have a strong odor coming from one of my heating vents. This occurred for the first time (in the 5 years I've lived here) last year (in mid-February, as opposed to late January - coincidental?). At that time, I had a heating duct replaced. The old duct was wet inside, and the HVAC tech said the odor was coming from the water in the duct. The techs saw no sign of animals under the house or in the duct; no leaking pipes or areas of standing water were observed. With the new duct, the odor went away. Just this week, the problem recurred in the same vent (the duct serves only this vent). Rather than replace the duct again, I want to discover and address the source of the water in the duct. I live in Santa Cruz, CA, where the air is cool and moist. I tend to keep my house cool (59 at night, low 60's during day, mid to high 60's in evening). My heating system includes AC, which I haven't used in 3 years, and then only for a few days during a hot spell. I assume the water in the vent is coming from condensation, but am not sure how that happens. What can I do to solve the moisture problem before I replace the duct again? The contractor who installed the new duct is coming out to look at it this week, but seemed surprised to learn that I'm having the same issue and didn't want to speculate as to why when we spoke on the phone. The odor is sickening. I have the vent closed and covered over, but still smell it - just not as strong as when it's open. While it rained yesterday, the smell came back a few days ago, when there hadn't been rain for about 10 days. I appreciate any insights you may offer.

'Sickening' to me is dead mice or birds. At first it's a sweetish smell but gets so bad you want to vomit. But it can also be urine and feces, not so sickening but still bad.
Sewage gas smells like rotten eggs, sulfur.
Standing water should smell musty.
Despite what they said, I go with dead animals, on some annual cycle.
I've watched guys doing demolition and not even notice dead mice falling out all over the place.