We live in Western North Carolina in a well-built double-wide manufactured in 1994 with an Intertherm heat pump. In February '09 we replaced the system with a Trane XL15i which has performed well. In the Fall of '09 we began noticing an unpleasant odor when the heat came on - not the burned dust smell the first time the heat comes on in the Fall but a persistent odor like a dirty washcloth.
Our dealer inspected the unit and found that the coils (condenser?) were covered with mold. They agreed to replace the coil unit with new 'coated' coils which would not allow the mold to regrow. This replacement was performed in December 2010 and the problem was resolved; the odor went away.
In March 2011, after a period of warm weather with the system in cooling mode for a while, we had to switch back to heat and the odor returned. The dealer has checked the unit and can find nothing wrong, insisting instead that some chemical in our house is depositing on the coils causing the problem. They advise us to have a $450 environmental evaluation done to identify 'our' problem.
The problem seems to me to be clearly related to the mold inside the unit but I'd like some advice on how to proceed. Thanks.