[jeff5may]

Ok, let me clarify a bit. In a sense, both ormston and I are correct. The design looks horrible on paper. A traditional split system has all this latent heat flowing straight to where you want it. You can make a cool looking graph and calculate everything distinctly. The one pipe looks messy and super leaky and ineffective when you plot points and make diagrams.

The one pipe setup is a heat pump, just not a traditional or "proper" design. It isn't going to start from scratch and heat up a cold room quickly. The analysis on paper leads engineering types to believe that it won't do it's job very well. After all, there is no such thing as a free lunch.

However, the rig does its job well for the price and size. It is meant to be a supplemental heat source (or sink) anyway. When you put it downstairs or in a cool, dank basement, and it is cool or cold outside, the make-up air tends to fall towards the unit. This cooler air is heated and then rises to the ceiling and finds its own way up.

The utility of the unit comes from its ability to exhaust comfortable warm air inside the envelope in large quantities. Unlike a standard heat pump, this rig will keep pouring out the same temperature air when it gets frigid outside. Rather than using it's leverage to move raw Watts from one place to another, it extracts what you don't want (heat or anti-heat) from the indoor air and exhausts the waste outside.

This is where the unit can fool you if the whole premise is not considered. As others have stated, when it gets cooler outdoors than the exhaust air, the unit loses effectiveness due to the incoming make-up air. When a standard air source split unit starts losing efficiency, the condenser temperature drops and so does current draw. The one pipe setup will continue to draw the same power and the same temperature air will be spit out indoors.

In most situations, by the time the outdoor temperature dropped to this level, the one pipe system would not be able to keep the cold from creeping in, even if it was a split unit. Being under a ton of refrigeration capacity, it has its limits. As the Delta T between indoor and outdoor air temperature rises, the primary heat source will be called upon more and more anyway. If saving money is s major concern, a low ambient control could be rigged to cut power to the unit below a certain balance point.

FWIW, the Sub-Zero hyper heating air source mini split units act the same way: they force a higher indoor discharge temperature by speeding up the compressor. This lays waste to the COP of the unit. Useful heating is still achieved, but at great expense.