I own 2 36K split heat units 1 2 zone system with 2 18K indoor units and 1 3 zone unit with 1 18K and 2 9K units. The cooling in summer is fantastic however during the winter the units are start to become inefficient at below 20 degrees. Below 10 degrees they are extremely ineffective and costly to operate. Below zero and they stop working until temp gets back above 10 degrees.