The manufacturers in general design different unit for different regions or climates. If you are searching for a model that has a distinct energy advantage in the winter season, you have your work cut out for you. Regardless of the sales brochures' claims, the air-source units all suffer when the mercury drops. The older cap tube metered systems had a more or less linear COP and BTU output that corresponded directly with outdoor temperature. With the newer EEV metered systems, this is not so much the case any more.
The newer systems are much smarter in their operation, and they switch operating modes when it gets cold outside. How they do what they do really depends on how the manufacturer programs the control board. As always, the more expensive units in the lineup are programmed to behave more aggressively and/or have increased functionality versus the more economical units. Some of these units have the same guts in them, and the control board program is the only real difference in the design of the model. Naturally, the more robust unit will have a remote control with more buttons on it, and a more stylish indoor unit to look at.
There is an increasing trend among manufacturers designing units that perform much better than the old-style units in very cold outdoor temperatures. These units pretty much all have variable speed compressors in them, and operate on two basic platforms. The super turbo models sacrifice overall energy savings to provide rated BTU output when it is frigid out. They do this by spinning the compressor as fast as it can to boost head pressure. The more advanced models have a dual-stage compressor in them, with some sort of intercooler between stages. They switch compression ratio when it gets cold out, allowing the compressor to change its operating parameters to fit real-world conditions. The electronic expansion valves in the rigs take care of the refrigerant metering side of the circuit in both types of model, extracting as much heat as possible from the outdoor air.
A good few of our fellow ecorenovators have expressed their satisfaction with Gree units. The cold climate, sub-zero operating, variable speed unit they offer is the Crown series. The Terra series is also a variable speed inverter, but it doesn't dig so deep into the negative temperature zone. Gree makes other lines, but they are geared more for air conditioning mode, and may actually stop heating completely when the temperature drops, due to automatic low ambient cutout.
Naturally, if you have the means to install a water or ground source unit, the efficiency is not going to suffer when it gets cold outdoors.
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