If I leave the furnace fan always running for an extended period of time (to circulate and filter the air), when the central AC turns on there is a not-so-fresh smell that is emitted through the vents. It almost smells wet or stale.
This smell eventually disappears if the fan is not run in between AC cycles.
There is a MERV-16 filter, which is regularly replaced, located within the system, between the air intake and the furnace.
What is this smell? What is causing it? Besides not running the fan, how do you prevent it?