Apologies for the long post, trying to be thorough here!
For reference, the current oil boiler is 170K btu, and in the work estimate I'm requesting that the installer provision an additional 6th heating zone for a room that has a through wall heat/ac unit (like a hotel) - which is 21x22 with cathedral ceilings. Also, I've added an indirect water heater (60 gallon) that runs off of the oil boiler and will be integrated with the new gas unit.
Plumber "A" installed the water heater, did a good job. He quoted me a high efficiency wall mounted Burnham K2FT boiler @210K
btu. He didn't ask for any information regarding linear sqft of baseboard or do a heat loss calculation/assessment.
Plumber "B" estimated I need approx. 135K btus (Dunkirk) and advised that I should NOT have a big unit (ie 200K btu) as it will create short cycling and that's not good for boilers, particularly high efficiency machines. Also mentioned Burnham boilers aren't great and prefers Dunkirk (I have no understanding of the brand hierarchy here, but I feel like each plumber has their own strong opinions). He also requested the linear feet of baseboard in the home and will provide a final estimate shortly.
My own research has led me to believe that a heat loss assessment is the best way to ascertain how big of a boiler is needed, though it's fairly complex and I'm not sure I can populate it accurately. That said, I know not everything you read on the internet is true, and I'm there's something to be said for an experienced plumber knowing what's required to do the job right.
Finally, I've also been forewarned that while the high efficiency units are >95% efficient, they do require much more maintenance, possibly so much so that any savings are lost in the shuffle, including any MassSave incentives that might come from purchasing the more efficient unit.
Appreciate the inputs!