First of all thanks for the input. The house is a 1955 cape, I insulated the hell out of the 'cubby holes' R32 on the floor and R19 on all the knee walls. Unfortunately I don't know how the sealed walls are and the rafters above. I was thinking of different methods to shove insulation up there without ripping down the sheet rock or the roof (any ideas are appreciated). The windows are all new andersons and as for oil consumption, I can usually go a month and half with approx 240gallons (I filled up on jan.15 and it's only on half). The old boiler is an RSA-125 which I feel is a little oversized DOE 144 and IBR 125 vs. 88 & 77 for the new guys. So
I'm feeling the contractors did at least some of their homework.
Unless they have you a room-by-room heat loss calculation based on a realistic outside design temp I'm betting they didn't do much homework at all- just eyeballed it, took a WAG, then upsized it slightly "just to be sure" they never get the 5AM call from the now-cold client. Even downsizing 20% from a Manual-J type heat loss calc RARELY falls behind the true heat load, and most contractors UPSIZE at least 20% from there.
The K-factor (= heating degree days per gallon of fuel use) on an oil bill tells a lot, and establishes a firm upper bound on the boiler sizing.
Blower door testing and air sealing can take the heat load down a LOT, if you think the place is a bit leaky. A well insulated wind tunnel is still expensive to heat. Your place is
better insulated than this, but could easily be as air leaky.
Without exact dates and fuel use quantities to work from the error bars are huge. WITH exact dates you can look up base 65F heating degree days from a nearby weather station from
degreedays.net , and use the boiler's efficiency to come up with a BTUs per degree-day number. Divide that by 24 to come up with BTUs per degree hour, then multiply it by the difference between your
outside design temp (about +15F in Northport) and +65F to come up with the BTU/hour output. To have any accuracy you have to look it up.
As an example, say you used 240 gallons over a period that added up to 900 heating degree days, and your boiler tested out at 85% for a DOE output number. That's 240/900= 0.27 gallons per heating degree-day. There are 138,000BTU/gallon, and at 85% you're getting 0.85 x 138,000= 117,300/btu per gallon, so at 0.27 gallons/HDD, that's 31,671 BTU/HDD. Per degree-hour that's 431,671/24= 1320 BTU degree-hour. At at design temp of +15, at design temp the delta is 65-15= 50F degrees, so your heat load at design temp is then AT MOST, 1320 x 50= 66,000 BTU/hr.
But if you're heating domestic hot water with that boiler, knock off 10% from that number. If the existing boiler is more than 2x oversized (=yes), knock of another 10-15%. If the place has never been tightened up with blower door verification, knock off another 15% (and do the air-sealing work- it's not huge money, and a better investment than a high-efficiency boiler.) I would be shocked if your house had a true heat load over 50K @ 15F, and I wouldn't be too surprised if it were under 30K @ 15F. But run the real numbers, see what you come up with.