I'm also in PA...I'll be using thermopex from CB...my dealer said at least 12" and much more than that isn't needed...it's rated to run ON the ground and he's done that for winter installs then buries in the spring....snow doesn't melt off it above ground so I can see why going very deep isn't needed. I'd say it depends on your insulation of the pex your using....weak insulation go deeper, good stuff not as much need.
I have a central boiler and called hem on this question. My dealer told me to dig 12 inches. Central boielrs answer was------ We are in minnosota and the weather here is a lot colder than in Pa. The guy quoted his as being covered with 2 inches of dirt. I have 6 inches over mine and drive heavy equipment over it all the time with no problem(due to the dealer telling me he drives all his mahcinery over his. ) I was told from central boiler that if the fire goes out you have many hours to get things fixed before having to worry about pipes freezing. I just love this stuff ! I don't get any degree drop from furnace to hot water heater at all.