HeatPump_Foul_Odor

3 year old Lennox heat pump and first time I've noticed a stinky odor coming from the vents, usually during defrost modes. Next service visit, I'll mention this to the technician but am wondering what are reasonable answers I should expect to hear to treat or remedy this (if there even is a remedy).

Thanks!
Greg

source https://hvac-talk.com/vbb/showthread.php?2223889-HeatPump_Foul_Odor&goto=newpost