[jeff5may]

IT CAN BE DONE. I am considering doing the same thing with a prefab window unit. Run the two heat exchangers in SERIES (not parallel) by swapping the cap tube out for a straight pipe. Pipe the condenser end of the series connected assembly into the reversing valve, and install a txv and check valve (in parallel) on the evaporator end. The arrow on the check valve should point away from the evaporator. This way, the condenser will not burp liquid or fog into the compressor in heating mode. The inlet to the TXV and the outlet of the check valve should merge, then that pipe would be the liquid line running indoors. The gas line of the set connects to the other open pipe on the reversing valve outside.

ECOMODDED has started a thread recently, asking about the same basic idea, on a smaller scale. He is planning on doing this with a dehumidifier as an indoor unit. For outdoors, he plans on using an automobile condenser.

You are correct in assuming that running both heat exchangers together will be more efficient. The more surface area and airflow you have, the more heat you can harvest from outdoors. How much more efficient? No one knows yet.

Indoors, a much larger heat exchanger takes longer to heat up (or cool down) as the cost of higher efficiency. Some like it, but most don't. The "warm" air the unit is spewing out the first few minutes at super-high efficiency doesn't warm the soul like a buck stove. I understand the first two minutes (while the liquid is building up in the hx) and the last two minutes (while the fan cools the hx after the pot stops spinning) are when the unit saves me the most money. To mom and the kids, that air just isn't warm enough.