Posted By sailawayrb on 06 Jul 2018 02:05 PM
First off, if you have a good ACCA Manual J heat loss analysis, you can make a good decision on the size of the boiler. How comfortable are you that your 63,000 Btu/hour building design heat loss is accurate?
+1 on getting an AGGRESSIVE Manual-J (per the Manual's instructions) before picking the boiler.
For a 2530' ICF house with R60+ in the attic 63KBTU/hr would be the load with some windows open, or at an outside design temperature of -80F or something. (Even the colder locations in Alberta only have outside design temps in the -25F range: https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf
With decent performance windows most houses like that would come in under 30 KBTU/hr @ 0F outdoors, 70F indoors, and still well under 40K @ -25F.
For reference, my 1923 vintage 2x4 antique 2400' house + 1600' of insulated basement w/ clear storm windows over antique single pane double-hungs comes in under 40KBTU/hr @ 0F, at about half the R values of an ICF house and crummier than code windows.