Yeah, with mixed fuel use it's harder and ultra-deep setbacks it's tough to do a fuel-use heat load calc. A simple I=B=R approach works though.
The
99% outside design temp for nearby Gloucester is + 5F so we can use that as the outside temp, 70F for the inside temp for a delta-T of 65F.
Assuming it's a ~23'x 35' retangular cape/ranch and fiber-insulated 2x4 construction that's a perimeter of 116', times 10' of height (includes joist depth) comes in at 1160' of gross wall area.
At a 15% window/floor ratio that would be 120' of window, figuring two exterior doors at 20' each brings the total clear-wall area down to ~1020', with a U-factor of about 0.1. So the wall losses would be:
U0.1 x 1000' x 65F= 6500 BTU/hr.
With 2" solid doors and single-panes with storms you get 160' of U 0.5. So the window & door losses add up to:
U0.5 x 160' x 65F= 5200 BTU/hr
Assuming R19 in the attic,that's about U0.06, for 800' comes in at:
U0.06 x 800' x 65F= 3120 BTU/hr.
Assuming you have NO insulation in the basement, 1.5' of exposed foundation & band joist at an average U-factor of 1, with 116' of perimeter and a 55F basement (when it's +5F out), you have ~175 of U1, and a 50F delta-T, which is a loss of:
U1 x 175' x 50F= 8750 BTU/hr.
Add it all up and it's around 23,570 BTU/hr.
If it leaks air like a sieve at the attic & foundation add another 4-5K, call it 29,000BTU/hr, which is the MINIMUM fire output of the -110. If your foundation is insulated & air sealed your design heat load is almost certainly under 20K. Eyeballing the
Weatherspark.com cursors when zoomed out to cover the whole heating season, binned-hourly mean temp January temp in Wenham is about 27-28F, so even in the leaky-house scenario the average mid-winter heat load is about 19,000 BTU/hr, 15K if it's pretty tight. The min-mod output of the -60 is about 15K, which means you'd be much better off with the -60, since it could actually track your load, whereas with the -110 it's always cycling, even on design-day.
This is a really common error, for both DIYers and pros alike, but even a rule-of-thumb guy should have gotten it right for an 800' house- even a grossly-oversizing 40BTU/foot x 800' only comes in at 32KBTU/hr, well within the non-condensing output of the -60. Most of those hacks use 35 BTU/ft, which would hit the above I=B=R estimate for a VERY leaky house. If your windows & insulation levels & tightness are than that, your heat load will of course be much lower. Code min new construction in MA is usually under 15BTU/ft, with rare exceptions.
So I guess the good news on this only slightly better than a WAG heat load calc is that assuming a 24,000BTU/hr design load and say 650' of exposed radiant floor you'd only need ~35BTU/ft out of the floor. At a more likely (or at least possible, if you insulate the foundation & band joist) 15,000 BTU/hr puts you under a very reasonable 20-25 BTU/ft requirement from the radiant, with more comfortable floor ~80F peak temps.
I have relatives in both Sheridan & Greybull- the Bighorns are very pretty indeed! I originally hail from the Seattle area- got way too used to seeing glaciated volcano horizons for the backdrop on clear days, and I too miss it, living in the comparatively flat swamps of MA.