Since I am coming into this thread late,...
Heat pumps compressors "in residential applications" have an electric band around them to keep the freon heated and moving.
There has always been a concern that if it is too cold and all the freon is sitting in the compressor it will pop its internal non resetable overload switch, and it does happen.
Up here if we "have" to install a heat pump we always install electric back ups in the air handlers.
And most all manufacturers of residential a/c units say do not turn on below 50 degrees.
You can get around that by installing the electric band around the compressor.