I have an unusual situation: Three buildings. Two are 2,000 sq. ft. One, on two floors is 4,500 sq. feet. These are external building dimensions. Radiant heat everywhere. Propane. The original house is 2,000 sq ft. and is currently heating by a Bradford-White Combi Hotwater Heater. We are replacing it. We are about to put in a boiler in the larger building, which will supply heat for the radiant floor and for domestic use for all of the buildings. I have gotten it down to deciding on either a Weil-McClain or a Lochinvar Knight. I would like advice as to which is best and most trouble-free. Also, estimate of size needed? I estimate about 200,000 btu. Is this correct? The buildings are well insulated. Many thanks.