Sanity Check

First post here, want to start by thanking all who post here, I have learned a ton just by clicking around.

I purchased my first home about a year ago, a 2,300 sf, 2.5 bath high ranch built in 1963 on Long Island, NY. Boiler is the original oil burner, still chugging along. Original insulation in walls, blown insulation in attic, crappy original aluminum frame double-slider windows which will be changed for quality casements in <1yr. I am converting to gas and the line has already been ran from the street. 190 linear feet of baseboard heating element.

My plumber, who is a personal friend, is recommending the Burnham Alpine 150 boiler (96% AFUE) w/ indirect heated 40 gal HW storage tank. My utility company offers good rebates on this boiler so I am OK with the model. Size is the issue (insert joke).

I have done a few heat loss calcs using different free apps/spreadsheets, and have come up with numbers ranging from 40-60k BTUs based on different inputs (mostly lowering design day temp by 5 degrees, changing window type).

I also looked at historical usage (only have data for last winter since house is new to me), my calc looks like this:

Coldest month = January, 1,316 Heating Degree Days
High estimate of oil used during this month (based on fill history) is 180 gallons
Estimated boiler efficiency of 80% (probably lower)
180 gallons x 140k BTUs x 80% / 31 days / 24 hours = 27k BTUs/hr during this month
1,316 HDDs / 31 = 42; I believe this means the difference in temperature between actual temps and 65 degrees was 42 degrees (ie, the avg temp this month was 23 degrees)
My design day = 10 degrees (15 is listed value, fudging it a little)
Adjustment Factor: ( 70 degree inside temp - 10 degree outside temp ) / 42 = 1.41 (not sure if this is a linear relationship)
1.41 x 27k BTUs = 38k BTUs design day actual energy use

"Rule of thumb" calc: 2,300 sf x 30 BTUs / sf = 69k BTUs required

So, my thoughts are:
150k BTU boiler is not right, minimum modulation is at 30k BTU which would cause short-clycling in shoulder season
105k BTU is oversized but probably not terrible, modulates down to 21k BTU, probably OK most of the time
80k BTU is probably the right choice, and I can't really calculate a scenario where it would not provide enough heat, but I am scared to pull the trigger on it because it is so drastically different from what my plumber recommends


My current dilemma:

Realistically, would the getting 105k BTU model really cause any difference in efficiency, since I believe most of the time my system will be running above 21k BTU which is the minimum modulation? Should I just go this way to 'be safe'?

Is there any scenario, any rule of thumb, anything I am not thinking about that would make the 80k BTU boiler insufficient for my needs? Anything wrong in what I wrote above?


All thoughts are welcome, many thanks...