You don't need a heat pump to recover large amounts of heat from the vent - just look at an HRV.
The problem is the reliability of the system. The air going up the vent is incredibly oily. The oil in the air will gum up your heat pump or your HRV core or whatever you use. At a minimum, you destroy your efficiency. More likely, you destroy your equipment.
The problem is the reliability of the system. The air going up the vent is incredibly oily. The oil in the air will gum up your heat pump or your HRV core or whatever you use. At a minimum, you destroy your efficiency. More likely, you destroy your equipment.