Does this shut down when very cold (-10F), or just get less efficient?

Great question. Per the Submittal Form, pg. 1, Outdoor Temperature Operation Range for Cooling is 15-115 F (-10-46 C). For Heating it is 15-75 F (-10-24 C). For further technical assistance, we recommend that you call in and speak to one of our experienced customer service representatives as they would be able to better assist you. Thank you and have a wonderful day.