Thanks for the quick response. I will not be able to do any energy audits so as far as sizing the unit, I'm on my own. This is because of the lender, not because of me.
Anyways, zip code is 14469, natural gas, the house is a 1240 sq ft. ranch, pumped hot water system, copper plumbing with no insulation. Most of the windows are old, not in the greatest shape. Insulating the attic is on my priority list. I will have to get into the house to crunch some numbers. Do you know of an online calc. that would be best to use?
If you arrive at the size boiler you need, it seems like chances are you will end up either under or over-sized because of the difference each model puts out. Since the manufacturers do not offer a 45k, 50k, 55k, etc model, how do you decide what to go with if your in the middle of two outputs?
Using Rochester's 97.5th percentile design temp you'd be looking at +5F for design temp, or +1F for the 99th percentile. The design temps for Syracuse are similar, so the design temp used for the boiler sizing need not be any lower than 0F. Don't be surprised if somebody uses -5F or even -10F, but interpolate what the heat load would be at 0F or 5F from the BTU/hr number they come up with. (eg: Say they used -10F, and an interior temp of 70F, and came up with a heat load of 60,000BTU/hr. The interior to exterior delta is 70F - (-10F)= 80F. But at 0F outside temp the delta is 70F, so the heat load at 0F is 60,000 x (70/80)= 52,500 BTU/hr. )
To come up with a crude heat load calc we'd need to know how many windows, and do they have (or WILL they have) exterior storms?
Number of doors?
Full basement?
Type of wall construction/siding? (Brick cavity wall, 2x4 timber framed, 2x6 timber framed? etc.)
Type and amount of insulation in walls, attic, floor, basement walls, etc (the "after" picture, not the "as-is right now" values.)
Number of stories?
Online calculators are really crude, and will oversize the sucker completely. If you have the time, Taco has a professional tool available as a freebie download
here, but it may take a bit of experience to know what to enter for data, and when the defaults are way off. The ventilation rates use should be no more than 0.5 air exchanges per hour, and if you plan to do some blower-door testing & remedial air sealing before or during your insulation upgrades you can probably lower that to 0.25 ACH.
Crown boiler has a freebie spreadsheet based heat loss calculator downloadable
here.
Don't be surprised if these tools deliver a number in the ~45KBTU/hr range, or higher, but if this is a 2x4 framed single-story mostly rectangular house with a modest amount of window area (not lots of huge single-pane picture-windows and multiple glass sliding doors etc.) it's fair to say that with modest air sealing and insulation efforts + storm windows as-needed, its unlikely that the true heat load at 0F will exceed ~40KBTU/hr, and may be shrinkable to under 25KBTU/hr, which is getting into the smallest of the 2-plate cast iron boilers, and under the peak output of the smallest modulating condensing boilers. With a mod-con the lowest-fire output is the more important number, and as long as the min-mod of the boiler is half your calculated heat loss (or lower) it will be tweakable to deliver long efficient burns even during the shoulder seasons, even if it's 2x oversized at high-fire.
If your design temp heat load is under 30K you may be a candidate for using a combi-hot water heater (condensing or otherwise) rather than a boiler-proper, provided there is enough baseboard to deliver design-condition heat with 130-140F water. You can figure on at least ~300BTU/linear foot out of the most commonly use baseboard with 140F, so if there's more than 100' it'll probably make it with a combi-heater. You can tweak the temp up if it falls short- with 150F water it'll be delivering better than 350BTU/ft. But let's not get too far ahead- you (or your heating contractors) have some heat load calculating to do first.
If you opt for a simple dumb cast-iron boiler and your calculated heat load comes between sizes, it's generally safe to go with the smaller unit as long as it's output is at least 75-80% of the calculated number, and the smaller unit will deliver higher as-used efficiency. Let's say your heat load calc comes in at 33KBTU/hr, and say, you're looking at the 2-plate Burnhams:
P202: 37K-in, 27K-out
P202X: 50K-in, 37K-out
The bigger unit's output is about 10% higher than your calculated heat load, and the smaller unit is 22% UNDER your heat loss calc. You can get away with undersizing by
22%? How is that?
These calculation methods overestimate reality by ~15-30% as a rule. Worst-case the smaller unit is undersized to the true load by about (22%-15%=) 7%, which means if it stays at your design temp ( 0F) for several hours the house will cool off to (100%-7%=) 93% of the delta between your design temp and 70F interior temp at which it was calculated. In this example that would be 0.93 x (70F-0F)= 65F, which is a perfectly comfortable room temp for sleeping, and since the design temps or lower occur almost exclusively during the pre-dawn hours, that's probably going to be OK even without auxiliary heat. Absolute worst case- say you're going through the cold snap of the century and the high for the day is a frigid five-below, you'd be able to make up the difference with two or three 1500W oil-filled electric space heaters. And as often than not undersizing the boiler by 20% relative to a Manual-J calc comes closer to a true "right sizing", but few HVAC contractors ever want to risk the 5AM phone call from the client, and consciously or otherwise build-in their bias into the calculation inputs (is that really R38 or R30 in the attic? How leaky IS that house anyway.) It doesn't take many tiny weights on the input-scale for a Manual-J to end up 50% oversized relative to reality. Being 50% oversized isn't an efficiency disaster, but you don't want to then increase it another 25% "just to be sure". (In California under Title 24 rules, up-sizing by more than 10% above a Manual-J calculation is a code violation, but it's easy for the contractors to bias it up by that tweaking the inputs to the software, and some will.)
If your calculated heat load in this example turned out to be 30% high compared to the true heat load (it's possible- it happens all the time), your heat load is then 23K, and a 27K-out burner has 17% of margin at design temp- you're good down to -12F with no shortfall! But if you went with the 37K-out unit you'd be at 1.6x oversizing- still not a disaster, but any bigger than that would be slipping down the part-load efficiency curve, and it would under-perform relative to it's AFUE numbers. At 60% oversizing it would keep up at -42F, which may have happened since the last ice age, but probably not in the last century. This is the basis of the "10% max oversizing" rule, and it's a valid one.