Boiler BTU output, calculating home heat loss and replacement sizing – GreenBuildingAdvisor

I’m hoping you all at GBA can help me understand my 3500 sq. ft. house’s HVAC system (coastal Massachusetts). My house currently has 1983 Weil Mclain CGM-7 boiler with cast iron recessed baseboard radiators, which I supplement with mini-splits (three 1:1 mitsubishis, 12K and 18K) and a wood stove. As I understand it, the boiler has an input of 210K BTU and DoE heating capacity of 165K BTU (79.5% efficiency). At some point this unit will need to be replaced, so I have been looking into sizing a replacement.

I’ve been tracking my gas/electricity/firewood consumption for two years, and calculated heating loads using heating degree days for my location (after removing non-hvac loads for electricity and domestic hot water). For example, this past month we used 245 therms of natural gas, 1316KwH, and 1/4 cord of red oak (75% efficiency stove, 25Mbtu/cord). Subtracting out domestic use (600kwh and 25 therms), I calculate that we used 29MBTU for the month. With 1119 HDD, I calculated 1070 BTU/hr/ºF. At a 99% design temp of 9ºF outdoors and 68ºF indoors, I calculate that I would need 63K BTU/hr. I have done the same calculations for other months and get the same answer, within ~20%. This seems low, but my house is oriented SW with lots of glass so solar heat gain is substantial (we use cellular shades at night). Is it really possible that the boiler is oversized by a factor of 2.5x? If so, would I be ok going with a Weil Mclain Eco Tech 110?

The other question is efficiency. Based solely on boiler specs, I calculate that the payback period would be 30+ years going from a 78% to 95% efficiency unit at my current therm consumption (savings ~26% per year, or $600) and an install cost of ~$20K (ballpark). However, I know my old unit does not modulate output. Does this mean that when a single zone calls up heat (say 20′ of baseboard at 500btu/ft), my boiler will run at 210K BTU input to satisfy the heat needed, even if it’s only 10K BTU required? If so, wouldn’t the Ecotech 110 (10:1 turndown) use far less gas in the real world than my present setup, irrespective of stated efficiencies? Does anyone have any real world examples of replacement boilers in their homes, and how much their fuel consumption changed?

I’ve learned so much from this community over the years as a reader; you all are great.