A/C-Heating unit

[Lily Dimopoulos]

The old unit was replaced with a new one on March 2005. It was noticed on Summer, after turning the A/C on, that a bad odor (like sewage) was spread in the entire house and right now when the heater is on, an urine odor is noticed. At this point, the A/C tech came over and took a look on the ducting. He said it can't be maintenance because it is made of fiber glass...I heard different opinions regarding these bad odors: it could be from the main pipe, it could be from wetness under the house, etc etc. At this point I don't know what to do. My home is about 20 years old. Please advice. Thanks