41KBTU/hr is a very reasonable heat load estimate at a +9F design temp for a ~2000' house since 1950 if it has storm windows or double-panes, and ANY amount of insulation, but has NO foundation insulation. (It's in fact a bit on the high side. at over 20BTU/ft.- you probably have some low hanging fruit to pluck on weatherization.)
Using 73,000 BTU/hr (>35 BTU/foot) would be INSANE heat load at 9F even with single-pane glass, unless you have several panes missing or sleep with the bedroom windows open!
Steady-state efficiency is one thing, but with oversized boilers you have to properly account for the cycling losses, distribution losses, and standby losses, which in your case are probably considerable since the boiler is probably at LEAST 3x oversized, with no "smart" controls purging heat from the boiler, and it's probably close to 10x oversized for any one zone, leading to WAY more burn cycles that a single-zone system that's 2-3x oversized.
If you have mid or late winter oil bill with "K-factor" stamped on it, that number is the heating degree-days per gallon, and you can use that ratio and your measured combustion efficiency. If the K-factor is say, 4.4 that's 4.4 HDD per gallon, or 4.4 x 24 degree-hours per 0.79 x 138,000, BTU, or:
105.6 degree-hrs/109,020 BTU
Flipping the ratio and doing the division to come up with BTU per degree-hour that's
(109,020.105.6=) 1032 BTU/degree-hour.
Assuming 65F as your approximate heating/cooling balance point (that was base-65 heating degree-days, right?), with a +9F design temp that works out to (65F - 9F= ) 56F heating degrees.
So at 9F the heat load would be 56 x 1032 = 57,792 heating degrees as an absolute upper bound.
Discounting at least 10% for hot water heating (4 people, right), more if you have tub-bathers or folks who take endless showers and it would be on the order of 51K.
Do the math on the real K-factor, if you have it, and know that it's really an upper bound.
As a sanity check you can also use the
NORA FSA calculator (scroll down about mid-page for the link to download the tool), which is based on boiler modeling work done at the Brookhaven National Labs to come up with something, which will also model the as-used efficiency at your oversizing factor. It's pretty good- WAY better than WAG, but not quite as good as a mid-winter K-factor and a tested steady-state combustion efficiency. The tool can take either annual gallons or K-factor as an input but doesn't let you choose a design temp (and it assigns a 99.5% design temp or cooler, much cooler than you really need to use.)
It's not a perfect tool, crashes a lot and sometimes has conversion-error issues, but plugging in 775 gallons/year, 79% efficiency, and 122KBTU/hr out, using Bridgeport CT as the location the FSA tool estimates a design heat load of 25,556 BTU/hr and an as-used efficiency of 53.1% due to idling losses (most of which occur over the shoulder seasons and summer.)
The 25.5K number (13BTU/ft) number actually NOT insane for a house that size if the basement is insulated or mostly below-grade, and the house is reasonably tight. By comparison, the heat load at my house in Worcester MA (2400' of fully conditioned space + 1500' of insulated basement that stays about 65F all winter) comes in the neighborhood of ~35,000 KBTU/hr, @ +5F based on gas usage and efficiency of this (modulating) burner. And this is a 1920s 2x4 framed house with known gaps in the insulation, original single-pane double-hungs with clear-glass storms, and 2x6 rafters for about R20 average attic insulation. MOST houses built since 1970 have better U-factors than this place on both windows and wall/ceiling. While raised-ranches are prone to air leaks and poor insulation on the cantilevers, they tend to have reasonable exterior surface area to floor area ratio.
I'd be truly shocked if the 41K estimate was an undershoot. Most Manual-J type heat loss calcs come in about 15-25% over measured-reality, so that would put you in the low to mid-30K range, or HALF the 73K calculation.
The length of the baseboard, while relevant for heating water temp purposes, is in no way related to your actual heat load. To keep the boiler from short-cycling on zones, the length in the smallest zone needs to be able to emit as much heat as the boiler is delivering, and that will change with output temperature. The WORST thing you could do for a modulating condensing boiler is to oversize the thing, since the lower-bound temperature before it starts short-cycling is determined by the minimum-fire output of the boiler and the amount of radiation on the smallest zone. The lower the water temperature, the higher the condensing efficiency, but if it's short-cycling on zones it'll be throwing away much of the efficiency in cycling losses, and burn it's way into an early grave. The absolute SMALLEST boiler that actually meets the heat load at the 99% outside design temp is going to be the right choice, since that will be the boiler that can reap the most condensing efficiency. The domestic hot water heating can be done as a "priority zone" with no impact on heating-comfort.
My bet is that your house (like almost all 2000' houses in the US) can be heated with the very smallest 50-60K boilers from any of the big players. (You couldn't PAY me to install an oversized combi like the Navien or Rinnai in my house.) The TT Solo 60 woould probably be a good fit, or the ~50K Peerless mod cons (PI-T50, or PF-50). The combis are kludges, with WAY more burner than can run efficiently for your low heating load chopped up into three zones, sized to be able to deliver reasonable hot-water heating. An indirect sized for your biggest tub fill and mod-con sized-optimized for your heating loads is a much more satisfactory system, and will run far more efficiently.
The min-mod fire of the CC125 is about 30KBTU/hr, which would (barely) be OK if your heating system ran as a single zone, but it's g'dawful oversized for 50' of baseboard, and your smallest zone is probably smaller than that. To get 30KBTU/hr out of 50' of baseboard to keep it the boiler from cycling at min-mod takes about ~180F for average water temperature, which is WAY over condensing temps. Running 180F water is fine if you were running an 83% AFUE ast iron pig of a boiler but a total waste of condensing burner. The mini-mod output of a typical 50K mod-con is about 13KBTU/hr, so in a 50' stick of baseboard or 260BTU/ft it balances with ~130F AWT, and you're condensing! And even with 110-120F water, with hysteresis in the boiler's outdoor setpoint control it would have reasonably long burns, and you'd be getting 95% efficiency out of it rather than 85% with the CC125.
But going with any combi smaller than the CC125 would leave you a bit starved on the hot water end in mid-winter, which is exactly why combis SUCK, in most applications. (Even the CC125 doesn't support much more than one high-flow shower this time of year- try starting the laundry when your spouse is in the shower, you'll see!
)
The GV90+4 doesn't have sufficient internal thermal mass to keep from short-cycling on zone calls with your chopped up 160' of baseboard either. Even as a single zoned with it's thermal mass you'd be looking at 150F+ water and ZERO condensing efficiency to keep it from short cycling into an early grave, and at 3 zones it's hopeless without adding a lot of thermal mass. You might get about 85% as-used AFUE out of it, but not more.
Seriously, try to narrow in on the true heat load, and measure the lengths of the baseboard on each zone. You need to be able to match the boiler's output at it's lowest (or only) firing rate to the
smallest zone radiation reasonably, or any condensing efficiency will be eaten up in cycling losses and boiler maintenance.
My house is multi-zoned, and radiation-limited to about 44K out at the temperature I'm running it, and it cruieses through -10F weather without losing ground. It's highly unlikely that your house is lossier than mine, and if it is, it's cost effective to fix it so that it isn't.
What jadnashua said is dead right: "Oversizing is a big waste", but worse than a waste, it's a design problem that has to be worked around. (And there are probably dozens if not 100s of threads on this site describing the necessary workarounds when you step into the oversizing trap.)