When the house was built in 1932 odds are it had NO insulation, single pane windows (no storms), and the 180K boiler was STILL oversized for the heating load. The net efficiency of that 180K boiler was likely 70-75% steady state, maybe less, and retrofitted with a post 1980 flame retention burner might have hit 80%.
Apparently you didn't actually read the
Brookhaven effciency report I had referred to in a prior post, nor looked at the regression curves to ponder just how big the true efficiency hit is at gross oversizing levels. You might also want to download the
FSA Calculator from the
NORA site which uses the data from lab testing several different boilers & types, which would also estimate what your actual operating efficiency would be in comparison to a more right-sized modern oil boiler with heat purge controls. The tool also estimates your heat load, and would tell you fairly accurately how much you'd waste by going with the 150K Columbia vs. a 60K Burhham vs. a 50K mod-con, if you figure out to use the tool correctly. (It's not bug free or particularly user friendly, but it works.)
At your heat load a 50K barely-legal 82% AFUE cast iron boiler would literally never leave you cold enough to be reaching for a sweater any time before the next ice age, and would yield higher comfort and higher average efficiency than a 150K 86% AFUE cast iron boiler. Set it to 72F and forget it, it'll keep up just fine and use less fuel than the 150K boiler would using a deep setback strategy, and it would pretty much hit it's AFUE numbers, whereas the 5x oversized boiler wouldn't come anywhere close unless retrofitted with heat purge controls.
A 76KBTU/hr condensing Vertex hot water heater would still have 40-50K of burner to spare for the hot water load when it's -2F outside (like it did briefly 101.5 years ago), and would use 15-25% less fuel than the 50K cast-iron boiler, and 30-40% less fuel than the 150K Columbia, in rough terms.
Green? I definitely have a green side, but recommending a 50-60K 82% AFUEcast iron beast wouldn't exactly put me in anybody's Super Green club. You actually have enough radiation and a low enough heat load & operating temp to run with a Daikin Altherma high efficiency modulating air-water heat pump, which would cost slightly less to operate than a condensing gas-burner and alsoe with a somewhat lower carbon footprint at MD's grid source average. But spending 20 grand or so just for some marginal green-cred isn't exactly cash-green, eh? ;-) Even for the carb counter Super Greenies there would be much cheaper carbon reduction to be had out there than the upcharge between a cheap right-sized 82% gas boiler and a high-efficiency heat pump. (Don't even get me started on how ridiculous and expensive ground source heat pumps are, and how heavily they are subsidized!)
At a low enough load (and you're getting close) the absolute efficiency matters a lot less. My recommendation here are far more about comfort & upfront cash than it is about any environmental stuff. Seriously- I'm heating my own house with an ~82% efficient burner. But it's a modulating burner operated at or below the design condition heat load during heating-only calls, ramps up to about 2x the heat load on very long showers if there is a simultaneous call for heat, but that's it. It replaced a still functional 4x oversized cast iron boiler, and the comfort levels have improved, not diminished.
With high mass radiation the rads don't change temperature very quickly even with a monster burner behind it, and with a right-sized burner you can narrow the hysteresis on the T-stat to something very small without short-cycling the sucker (not so with the monster-boiler). With an outdoor reset control and a PID algorithm T-stat and continuous flow you can keep the indoor temps at an amazingly tight temperature range (like 0.2 F) if you really wanted to, but that would cost more money. Right-sizing the boiler and long burn times on high mass systems means it never overshoots the setpoint, and the radiators never actually get cool- they stay within a range, and the heat emission is very very steady, even as the boiler fires on & off.
In the spirit of Jim's McLaren analogy, if you only need to get home from the store in 10 minutes to keep the missus happy and you know to a dead certainty you can get there in a cheap & reliable Ford Focus in 6 minutes without snapping your neck, there's simply no point to buying the 5 liter Mustang that could get you there in 2 minutes burning more fuel to no good end. (That is unless you get some thrill out of the roar of the bigger burner, and the extra operating & upfront costs get filed under "entertainment value."
) I personally don't find the roar of a 150K burner all that appealing. YMMV.
I'm dead serious about just how grotesquely oversized a 150KBTU/hr boiler is- it's tough to convince people just how low their heat loads really are, especially when swapping out an oversized behemoth like yours. But to replace it with another oversized behemoth does literally NOTHING for you other than drain your wallet faster. It costs a few hundered more now, and a hundred or so every year thereafter. If you want more comfort, right-size it, (or get at least as close as you can- your heat loads are on the very small end of what's rightly served by a boiler), and spend the few hundred on nicer controls like a PID algorithm thermostat or a outdoor reset controlled mixer or something.