Replacement boiler - which technology to use?
My 100+ year old wood frame/stucco house has a 28-year old gas boiler with a power vent connected to a 100+ year old brick chimney. When I replace the boiler I want to disconnect from the chimney because I don't want the expense of lining the chimney, and the chimney leaks both through flashing and corbelled brick joints into the attic. I would prefer to demolish the chimney when I redo the roof.
Some specifics about the heating system: The boiler is in the basement, and connects to 12 old tube-style cast iron radiators using a pump for circulation. There is about 3' between the top of the foundation and the underside of 1st floor joists. I have options for locating the flue at code-required distances from windows, equipment, etc. The existing boiler has 150Kbtu input, but nothing listed for output. I ran a heat loss calculator for the house which analyzed walls, windows, floors, ceiling, etc and came up with a load of 75Kbtu. (1500sf of heated space, 2 stories, heated basement, unheated attic, some insulation, storm windows, no fireplace, Chicago area)
My options for a replacement boiler seem to be a single stage power/direct vent exhausted by a b-vent through a basement side wall, or a condensing boiler exhausted through PVC. I was leaning toward the higher efficiency condensing boiler, but from what I've read my radiator system runs too hot to condense the latent heat out of the exhaust, and therefore much of the efficiency is lost. However, each of the 3 mechanical contractors I've had bid a new system were extremely hesitant to do a side-wall vent, and recommended lining the chimney if I wanted to go with the lower-cost single stage boiler. Since that option is not preferred they recommend a condensing unit with modulation responsive to outdoor temperature.
I see diagrams illustrating closed combustion and side wall venting for single stage equipment. Why are these contractors steering away from them? Other than the additional profit to be made selling a condensing unit? Or is the condensing unit really the best way to go with this house?