anyway. And it's probably close enough over the range we play with, given that there are large other variables we don't play with (like infiltration, or the effect of convection -- wind -- on boundary layer heat loss on the outside walls, and other esoterica...).
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
Delta T is difference between 55F and outdoor temperature. 55F is design temperature of the envelope of the house. Heat is lost from envelope of the house to outdoor. But huge factor in heat loss are infiltrations. They might be larger than heat loss trough the envelope itself.