We have a Trane Dual-Fuel unit. When the heat pump runs, it gives off a sweet, musty smell. It only smells when the heat pump runs - once the gas heat kicks in, it's gone. I have tested this by cranking up the heat to trigger the aux (gas) heat, and the smell disappears.
The smell is constant when the heat pump is on. From what I've read, Dirty Sock only causes smell during defrost mode. When does defrost mode occur, and when would that cause the smell? Does it make sense that the gas heat doesn't smell?
Also, if DSS is caused by micro-organisms living on the coil because heat pumps don't get hot enough to kill it, shouldn't running the gas heat for a certain period of time kill it?