Ever since I had a new HVAC (gas pack) unit installed 6 years ago, my house smells like plastic (not burning plastic) when using the air conditioner or heater. The HVAC installer was unable to find the source after multiple diagnostic efforts. They tried replacing the blower motor, and the lid which has a layer of insulation (and glue). The lid replacement seemed to help, but then it got worse. The smell is worse when using the A/C while the sun is beating down on the unit, and when using the heater when it is really cold. It seems to get worse as time goes by. What could be the cause? Could it be the air ducts?