My downstairs unit that I need to replace is a 2.5 ton unit that heats/cools about 1100 sq.ft and a room above the garage that is about 300 sq.ft. This bonus room has the longest supply lines from the air handler which is located central to the downstairs. It appears to have smaller ducts and a return that is sized the same as the supply (at least the return register is the same size as the supply - for a long time I thought they were both supply registers since they look alike).

Anyway, everyone who has looked at this arrangement has said that the only way to do this is to have a third heat pump for the 300 sq. ft bonus room. It is currently tied in to the downstairs and I'm wondering if that small return in this room is causing inefficiencies. Is this the way to go, or can I have a design that handles the downstairs and this one upstairs room that gets very low air flow currently??