Tech was out today for upstairs level having a weird smell. Performed 2 inspections. Upstairs level system was installed by a home warranty company. New 2025 carrier. While checking system tech seen the following. Return plenum was not sealed . This is pulling in hot attic air into return Plenum. There is no filter rack in Plenum, not even a filter. Supply Plenum is not sealed this needs to be mastic, sealed to keep it air tight. Reason for dirty sock smell in system is due to Plenum’s are not sealed correctly. Also, there is bacteria growing on the evaporator coil. A UV light needs to be installed to kill the bacteria. Tech recommends duct cleaning to also eliminate smell and have better air quality. Please note that adding a new return plenum is not going to get rid of the smell. You will still need to get the ducts cleaned and add a Uv light to kill the bacteria that is growing on the evaporator coil. There is no safety switches on the evaporator coil. Drain line needs to be clear