We worked yesterday at -4 with absolutely no problems, just a bit slower as 5 inch snow on the ground. Today the first cleans were at -6 which was ok, and when as the day warmed up got like normal. I think we could work all day at -5, but it is borderline. Somewhere around there and lower the van inside starts to freeze up and i start to worry about the R/O filter.
We use a hot water system heated with LPG.