Have been dealing with layers of odor from HVAC system for the last 3-4 months.
When we started up A/C in May it had a dirty maple-brown sugar smell. It eventually died down in several days, but still had nasty dank odor when A/C would kick off. We run our fan continually to filter the air so we would really notice it when the cool shut off.
House is new construction from March of 2006. Has always had slight dusty dirty odor from vents and A/C was smelly when it kicked off, but not as bad as this year. Finally decided to fix problem and started with duct cleaning.
Had ducts and coil cleaned about mid July on a Friday. That weekend we had terrible overpowering odor. It smelled like a muggy-dirty-stoney odor that was overpowering, especially when A/C was on and immediately following shut off.
Duct cleaning company believed it was a defective A/C coil and came back out the following Tuesday and replaced it. Smell improved significantly. Still had an off smell from A/C however, especially when it would kick off.
I decided to hire an environmental company who believed that the problem could be from the furnace. I mentioned to them that the furnace was run in the winter during drywall construction and was probably filthy and that we had always had a slight burning dirty smell when the heat was on. They pulled the furnace and cleaned it completely. The technician remarked that it was as dirty as 20-year-old funaces he had cleaned.
Noticed a marked improvement for about a week, but still had an off dirty smell. Called the environmental company again who sent out a team to do a recovery level fogging of the duct system. They noticed a smell that settles in ducts when furnaces have been run with burning dust and debris on them. They also thought they smelled the sickly sweet odor of something dead coming from the return when they had the furnace opened in the basement.
Again, marked improvement, but could now smell the odor they believed to be a dead animal that got stronger as the week progressed. The more heat and humidity the worse it would be. The day that it rained, the smell from the the vents was especially strong. It was a very heavy musty, dirty smell. We had always noticed that the odors we've had were worse with higher humidity and this smell was no exception.
Environmental company believed we could have a dead animal in the return flex in the attic, thus the differences in smell with heat and humidity. Upon thorough investigation of the attic and the flex they could find no signs of mice, breaks in the flex, or water and mold in the attic. They inspected most of the flex with a camera also, and saw nothing. They inspected the rest of the return duct system with camera and could find nothing either, other than the company we hired to clean the ducts left more drywall dust than they should have.
At this point, we are at a total loss as to what our problem is and where to go with it. One technician doesn't beleive it is a dead animal, but can't pinpoint what the odor is or where it's coming from. He doesn't think the remaining drywall dust that was sprayed and incapsulated should be the source either. The only recommendation at this point is an expensive HEPA filtration system. We would like to find the cause and not spend lots of money on a band aid that may not work.