I use a Rinnai(?) tank less also. Morton Building, 30 X 60 with 16' ceiling, fully insulated, and steel interior. Three garage doors 12 X 14, 10 X 8, and 8 X 8, walk door and two windows. Six zones in the floor, 2" foam insulated footings and 1" 4 X 8 foam on the perimeter only under the slab. I...