Great guess on the fuel use number. I used 388CCF the month of January. So, 38.8 million BTUs in and 27.2 million out using 70% efficiency. At 65ºF base temp there were 966 degree days. January was brutally cold here, with the highest fuel bill we've ever seen.
27.2 million BTU / (966 degree days x 24 hours) = 1,173 BTU per degree hour
65ºF less 15ºF (99% for Philadelphia) yields 50º - so if I understand correctly, the heat load is 58,650 BTU/hr
I lowered the high limit from 200ºF not long after we moved in. Probably should have gone lower after the windows were replaced. Seems like condensation issues shouldn't be a problem. This information has been extremely helpful.
The raw combustion efficiency might still be over 70%, so it's conservative to worst case it at the full nameplate efficiency. With the full nameplate 78.5% the load becomes 58,650 BTU/hr x 78.5/70= 65,772 BTU/hr. That's probably higher than reality due to the gas used for domestic hot water, but could be somewhat lower than reality if your house leaks a lot of air. (It's probably not crazy-leaky if the window replacement job was done reasonably well, and the attic floor was sealed prior to insulation.)
Using ASHRAE's 1.4x multiplier recommendation the biggest cast iron boiler you'd ever want to install there is
---1.4 x 58,650 BTU/hr= ~82 KBTU/hr if it's close to 70% efficiency
---1.4 x 65,772= ~92 KBTU/hr of output at the 78.5% efficiency is still valid (probably not)
With a mod-con boiler 1.2x is almost always more than adequate which would put the bracket between:
--- 1.2 x 58,650 BTU/hr=~ 70 KBTU/hr
---1.2 x 65,772= ~79 KBTU/hr
So, if you were to continue with building envelope upgrades an 80-85K-in condensing boiler would get you there, but for now a 100-110K-in condensing boiler would for sure be enough, and if the boiler has a 10:1 turn down ratio would still modulate well even after a few more efficiency improvements on the house. You have sufficient radiation to deliver the full output of a 100K mod con, and even broken up into two zones a 10:1 turn down boiler wouldn't short cycle on zone calls.
Assuming a design load of 60K. with 175' of baseboard that works out to 343 BTU/hr per foot, which is going to take an average water temp (AWT) of about 145F with a return water temp in the 130s F, which will be a bit above the condensing zone, but at
your average wintertime temp of ~35F your heat load would be about 35-36,000 BTU/hr or about 200 BTU/hr per foot, which the radiation would emit at an AWT of 120F, where it would be edging into the mid-90s for efficiency.
A 100K with a 10:1 turn down can modulate as low as about 9500 BTU/hr out, so with the outdoor reset curve dialed-in perfectly it would run pretty much continuously whenever the temperatures dropped into the mid-50s F or cooler, and may even beat it's AFUE tested efficiency by a percent or two. You don't have to settle for a 5:1 turn down or lower, which used to be the standard.
A few boilers that would fill the bill would be the
Navien NHB-110,
Lochinvar KHB-110 (or
NKB-110) ,
HTP UFT-100W, or
Burnham K2WT-100B. There are others. The fire-tube heat exchanger types like the KHB , NKB or UFT have more design forgiveness. Stainless heat exchangers have a much better track record than other materials (avoid aluminum). The HTP-UFT is substantially cheaper than most of the others and simpler to set up than most, and would be the best choice if attempting a DIY (with some pro help at the point of commissioning it.) But choosing can boil down to figuring out which vendors are better supported in your area. Since you're not really in a hurry (and may not even do the project at all) you can take your time to ponder it all. With all of these lipstick-on-mirror basic design calculations already in the can you're in a pretty good position to specify what you need to see in any proposals from installers.