My 520 is heating only 3500 sq.ft....
3 different buildings...
#1- 1972 Mobile home @ 12 x60,with additions on 1 side makes it almost 24 X 60,LOUSY vinyl windows that leak,trailer is 2x3 walls,little to no insulation...
#2 - 30 x 40 Shop,mostly block walls,1 side addition is 4" pole barn walls..
#3 - 1987 14 x 70 Mobile home ,2 x 4 walls,old windows..
Approximately 1,000 ft. of 1" pex underground to connect the boiler located in a 53' truck box to the 3 buildings...
When temps drop near zero, 7 teeth of feed is needed but the 520 is able !!