Thanks Dana. I'm having a hard time finding a recent bill as wife has paid/shredded. Any other alternatives? Additionally, is there any local hvac expert that you trust can make the approrpriate calculations? Frankly, I'm not sure I have all the inputs and would prefer a professional make the proper calculations (without OVER-calculating of course).
Yes, it's 6degrees, Worcester always gets it worse it seems then along the coast.
Don't let an HVAC contractor run the Manual-J. A RESNET rater or a professional engineer who make their reputation on accuracy rather than installing & maintaining equipment will give more accurate numbers. It'll cost several hundred USD though, and it's not really needed unless making extensive changes to the house or heating system. HVAC contractors tend to have thumbs on the scale, using outlandish design temperatures and insanely high air leakage assumptions, anything to avoid getting the 5AM call from an shivering irate customer. They thus calculate a higher-than reality number, and upsize the boiler from there, ending up with something than 2x bigger than what's appropriate.
The other approach is often to measure up the radiation and spec a boiler big enough to run it all at 180F. This too leads to 2x or higher oversizing on most houses.
Since you have a heating history you can use the existing boiler as the instrument to MEASURE the heat load. A few wintertime gas bills with exact meter reading dates and amounts is easier, cheaper and usually more accurate, as long as you didn't turn down the thermostat to 50F and head off surfing in Belize during the billing period, or make extensive used of auxiliary heating such as wood stoves, etc.
Contact your gas provider, or go online- most gas utilities in MA have options for creating a secure account from which you can view recent bills, make payments, etc.
If you can count, add, and use a tape measure you can do as-good or better job than the average HVAC contractor measuring your radiation. There is no rocket science involved.
Most cast iron boilers are oversized for the heat loads of reasonably tight 2x4 framed 2000' houses with some fluffy insulation in the wall cavities & attic and at least storm windows over double hungs, but some are more ridiculously oversized than others.
A typical 2x4 framed 2000' house above 1000' of basement in Hingham will have a design heat load of less than 40,000 BTU/hr at the
99% outside design temperature of +11F (yes it gets colder than that, but only for 87 hours out of a given year. The +11F temp is the 99th percentile temperature bin in Hingham.) Many will come in under 30,000 BTU/hr. If replacing it with cast-iron the biggest boiler that makes any sense would have an output of 1.4x that much. If the boiler is in the basement, and not a garage, size it off the DOE capacity, not the IBR or AHRI net BTU numbers.
For example, say your load calculations come in at 34,000 BTU/hr. The optimal cast iron boiler would have an output capacity of 1.4 x 34,000= 47,600 BTU/hr. That would be enough boiler to cover your Polar Vortex disturbance cold snaps with margin, but run long enough duty cycles during normal weather to run efficiently, and have enough extra to be able to run overnight setbacks during normal weather and still have reasonable recovery times from setback. There are a few cast iron boilers in that range, but most are north of 60K.
If installing a modulating condensing boiler the maximum capacity number doesn't matter as long as it's over 34,000 BTU/hr with a bit of margin. The more important consideration there is that it can modulate low enough in firing rate to run at much lower temperatures to reap the condensing efficiency without short-cycling the burner on/off. With a mod-con there is no need for a 1.4x oversizing, since they can and should be run under "outdoor reset" control to minimize water temperatures, maximizing combustion efficiency, and don't need rapid recovery from setback. With outdoor reset the boiler is programmed to adjust the water temperature up/down in response to outdoor temperature, which results in maximum efficiency and very steady indoor temperatures. That said, most mod-cons will still have
ample capacity for setback recovery, if desired, even though the higher water temperature/lower combustion efficiency of operating it that way burns more fuel than simply dialing in the outdoor reset curve.
Wall hung combi boilers big enough to run a couple of bathrooms often can't modulate low enough to avoid short cycling on a houser your size.