"When they were built most houses with cast iron radiators weren't very well insulated and often didn't have storm windows. Since then many have had energy saving improvements so the total radiator heating capacity is now much greater than the minimum needed according to the building's actual heat loss. It is, however, very comforting to really "cook" all those radiators on bitter cold days.
Replacement steam boilers must have enough power to fill every radiator with steam but replacement hot water boilers for the improved home can be selected according to the actual heat loss instead of radiator heating capacity if the residents can accept the lower water and radiator temperature which will result.
My energy audit confirmed there is only about 4 inches of about 1970s era, insulation in the gable. The energy auditors says they only need to cut a 14" hole to go up there to blow in 15" of cellulose and since they spotted knob and tube, stated they would build a channel over it since it could be a fire hazard, not to. They found lots of places I did not know about where I need to use Great Stuff, around an expansion tank, and in places where holes had been cut for pipe. I am going to do interior plexi storms on 4 small windows with a Southern exposure.
Given what my improvements will be, would it be a gamble to insist on a Weil-MacLain Ultra 150 rather than a 230?
I am not good at math so doing a heat loss calc myself is daunting. Clearly Desco has dumbed it down in their survey, they must input the user data and do it themselves.