150K is an
INSANE oversizing factor for your true heat load! With an indirect you could probably get along just fine with a 35K boiler!
The
99% outside design temp for Baltimore is about +15F. Assuming a typical heating/cooling balance point of 65F (typical for non-superinsulated houses) that's 50 heating degrees below the balance point.
A typical heating season in Baltimore is about
4600 HDD base-65F
500 gallons over a 4700 HDD is (500/4700=) 0.106 gallons per heating degree day.
A gallon of heating oil has about 138,000BTU, and assuming a steady state combustion efficiency of your oil burner is about 85% (could be, but it's probably lower), that's delivering (0.85 x 138,000= ) 117,300 BTU/ gallon. So per degree-day thats (0.106 x 117,300=) 12,434 BTU per HDD being delivered to the heating system.
With 24 hours in a day, that's (12,434 / 24=) 518 BTU per degree-hour. So with 50 heating degrees at the 99% outside design temp you're looking at a heat load of about (518 x 50=) 25,900 BTU/hr.
And that's probably an overestimate, since the odds of your oil boiler actually delivering 85% isn't high, and we haven't accounted for distribution losses on the system or hot water heating (assuming you're heating hot water with oil.) Odds are pretty good your true heat load is around 22-24KBTU/hr, and there are probably cost effective ways (even with cheap natural gas) to reduce it.
Even a tiny 2-plate 35KBTU-in ~80% efficiency cast iron boiler delivers (0.80 x 35,000=) 28,000BTU/hr, which gives you ~10% margin even if your oil boiler was really running 85% AFUE (not likely) and is probably closer to 30% margin on your space heating load. That's also about the output of a standalone gas hot water heater, and if you zone the indirect as "priority" and size it for your biggest tub-fill you'll have equivalent or better performance of a standalone gas heater.
The
HTP Pioneer is a nice unit, but it's probably way overkill. If you go that route, the smallest of the bunch (PHR100-55) with the 35-100K burner would be an OK match, and is enough burner to deliver the "endless shower" experience. It would never modulate on space heating loads, since it's min-fire output is more than your heat load on the coldest day of the year, but since it's inherently self-buffering you'd be free to microzone the hell out of the place without risk of short-cycling.
Got a link to the Columbia units you were looking at?
It's probably cheaper to hit the same performance point with the AO Smith Vertex (the smaller 76K burner version) or smallest Polaris hot water heater and a plate-type heat exchanger for the heating system loop.
Whether you'd be able to run the system at condensing temps and still deliver the ~20-25K of heat on design day is an open question- depends on the amount & type of radiation you have. If your system has big old-fashioned cast iron radiators, and was installed before the house was insulated odds are pretty good you'd never need more than 135F out of the boiler, and would likely get at least 90% average efficiency out of it. If you need 150F or more out of it odds are you'd be better off with a
2-3 plate mid-efficiency cast iron boiler with internal smart controls & outdoor reset, which should deliver an honest 85% when tweaked in, and a separate indirect. It just depends.
What sort of radiation (total amount & sizes) and how many zones?
[edited to add]
Do you have a mid or late-winter oil bill with a "K-factor" stamped on it? The K-factor number is heating degree-days per gallon, and using the K-factor on a late winter fill-up would be a more accurate way to measure the true heat load. The nameplate BTUs in/out on the existing boiler would also be useful for making that calculation.
Some years are colder or warmer than others, and I know the 500 gallons probably isn't a very precise number either, but most 2x4 framed homes with retrofit insulation and storm windows (or middle-of-the road double-pane replacement) will come in between 12-15 BTU per square foot of conditioned space @ +15F outdoor temps, and almost never over 20 BTU/ft unless it has major air leakage (which can usually be fixed for cheap.) If you have any single-pane windows it's worth installing tight low-E (not cheaper clear-glass) storm windows, which which usually pay back in 5 years or less- faster than the lower-cost clear-glass windows, and make the place a lot more comfortable, both winter & summer.