I think he's just wants to keep the house above freezing. These heaters have the thermo inside so if it's located fairly high(say 3-4' below the floor joists), I would think it would mostly regulate this upper layer. They do direct the heated air down so it would mix it up some. I would think Max will track the temperature and see what he needs down there. Best is to get the pipes dry so you don't have to worry while you're gone but he knows his house in the cold from experience.

A present house I'm working on is uninhabited. Lloyds of London was one of the few that would insure it.