A 40 gallon tank can recover with a 30KBTU/hr input (like most standalone gas-fired tanks) or a 500KBTU/hr input (like a sidearm indirect in an apartment building). The only difference is the amount of time it takes. Recovery time performance has no bearing on the continuous flow can be achieved. Continous flow rates are determined solely by the rate at which BTU/hr in=BTU/hr out, which is a delta-T x lbs/hr number.
What are you using for an outside design temperature? Being near "the coast" means almost nothing. Outside design temps quite differ by more than 20F if you're near south coast of Long Island vs. the northeastern coast of the Gulf of Maine or the Bay of Fundy. But for yuks, lets work backward from K-factor to calculate the implied outside design temp:
Since K-factors are a base-65F derived number be sure to use the delta between the 99% outside design temp and 65F (the presumptive balance point) rather than say an 70F indoor temp when using fuel use to derive heat loads- it's not a big difference, but a difference nonetheless.
A K-factor of 9.5 with with an 85% efficiency oil burner implies...:
(0.85 x 138,000 x (1/9.5) )/24 = 514 BTU/degree-hour.
To hit a heat load of 70,000BTU/hr means a design temp of:
65F- (70,000/514)= -71F
That's pretty damned cold- are you on the northeast coast of Baffin Island or something? ;-)
Naw, it's warmer than that even there!
The
99% outside design temp for Fairbanks AK is fully 40F warmer than that! "Near the coast" in Maine you're looking at no colder than about 0F for a design temp, coastal New Brunswick no colder than -5F. If we take -5F as an outside design temp, the delta is (65F- -5F=) 70F, and your heat load is about (514 BTU/degree hour x 70F = ) ~36,000BTU/hr. Even if you keep the place at 80F, and a balance point of 75F you're still looking at under 50KBTU/hr. Most 2200' homes in New England with any sort of air-tightness will come in under 50K @0F, many come in under 40K, even 2x4 houses w/storms over single-panes, not tight newer double panes and R19 2x6 framing like yours, with probably R30+ in the attic.
To have heat load of 70K at any realistic outside design temp with a K-factor of 9.5 would mean the efficiency on the oil boiler would be something like 150%- ain't happenin'.
Tell you what- download the
NORA FSA calculator, and let the Broohaven Nat'l Labs boiler model guesstimate your actual heat load using your boiler & indirect size/characteristings along with your 9.5 K-factor, provided there is a listed city with a similar local climate to yours (or even a colder location.) My guess would be that even using their 99.5% design temps you're under 45K, since even the smallest oil boilers are going to be 2x oversized for what your actual heat load is.
Also, since you're heating those (30 minutes x 2gpm) 60 gallon showers and/or 40 gallon daily tub-fills with the oil boiler, that 514 BTU/degree-hour constant is on the high side of the actual space heating load, unless you're using woodstoves or other serious auxilliary heat to offset oil use. You are high-volume hot water users compared to the US average, unless those are soaker-tub fills are regularly "a bath for two". ;-) The indirect should be sized for the tub-fills, the boiler for the true heat load, and if you're looking for the endless shower experience you can get it with a 50K boiler plus a drainwater heat exchanger (as my teenager proves fairly often.)
Like many other fin tube,
Petite 7 puts out ~600BTU/hr @ 180F AWT. At 22' with ~15,000BTU/in you're looking at ~200F boiler-output to actually balance, and not all of the combi-boilers will actually go that high. But if your real heat load is roughly half your 70K number (probably is), and those 14K zones are really 7K, you can still get design-day heat out of the fin-tube with 140F water, and a condensing HW heater like a Vertex or Polaris with a plate-type heat exchanger might be more appropriate if you like very even room temps and long cycles.
Alternatively you'd probably do OK with a noo-skool 85-86% 50-60K-out cast-iron beastie with built-in outdoor reset that is also tolerant of low return water temps ( like the smallest Burnham ES2, ESC, or Series 3 or similar. ) The new generation cast iron boilers have smart controls to limit short cycling even when under-radiated making use of the boilers mass to deliver a temperature hysteresis around the outdoor-reset setpoint, and internal plumbing & control to tolerate even 110F return water temperatures. They run about ~$2K-USD, a bit less expensive than the modulating combi boilers, yet deliver similar comfort when used with outdoor reset. You can spend the difference in cost on a drainwater heat exchanger for enhanced showering performance. The indirect is (apparently) already big enough for your tub fills provided you set it's temp high enough.