Need help with BTU Boiler size

[TCP]

Hey guys I'm looking to replace my very very old boiler this year. I've been in the house for a few years now and its time. Looking for some advice on size and model of boiler. Looking to do it myself, I have piping experience so connections arent a problem. Just looking for some direction on what to look for as far as size and brand. Here are the facts :

Fuel - Natual Gas
Heat type - Hydronic hot water / Baseboards
Number of zones - 2
House age is 1965 (Wood frame on slab) - no basement only ground floor, second floor and attic (Ground floor on slab and new wall insulation, Second floor insulation is original to the house and usually mimics outside temps within a day)

Foot Print of house is about 2500 sq ft
Heated living space is about 1750 sq ft
- Of that 1750 sq ft about
-500 sq ft is 10' ceilings
-650 st ft is 8' ceilings
-600 sq ft is 7'3" ceilings
zip code is 11740

How many btu should I get for this house? Brand? Model? Attached Garage is unheated and gets down to like 55-60 degrees when its about 30 degrees outside. I usually leave the boiler room door open and it keeps the garage at around 55-60. Thanks guys let me know if you need more info.

 

[Dana]

If you have a heating history on this, start by running a fuel-use based load calculation using wintertime-only gas bills. That method uses the oil boiler to directly measure the design heat load. If you're willing to share your exact meter reading dates and amounts from December through half-past March, and your thermostat settings I can run those numbers for you here.

Alternatively, run an I=B=R type heat load calculation on the entire house, and we'll take a WAG on the air infiltration numbers. Even if you're keeping it at some other temperature, use +68F (= code minimum) for the indoor design temperature, and +15F (the 99% outside design temp for most of L.I.) for the outside design temperature.

...0r...

Run a Manual-J type tool to come up with the heat load, such as LoadCalc. Be aggressive rather than conservative on air tightness and R-value assumptions or that tool will oversize by 2x. Even with aggressive assumptions it often overshoots by 1.3x or more, but it's one of the better online freebie tools. Some people prefer CoolCalc, but I've seen some pretty strange numbers for that tool when the construction was something non-standard (I think it'll handle 2x4 with fiber insulation just fine) though I haven't used it myself.

The last thing you want to do is install a boiler the same size as what you have, or base it on some silly "xx BTU per square foot" kind of rule of thumb, which invariably results in sub-optimal oversizing that costs more up front, and in lower efficiency and lower comfort for the life of the boiler.

Most 2500' single story slab-on-grades with 2x4 framing and storm windows would come in under 40,000 BTU/hr @ +15F and many would come in under 30,000 BTU/hr. Even though you may only be actively heating 1750' of the house right now, code requires that the system be capable of heating the whole house.

To further assess which boilers might make sense, how many feet of baseboard is there on each of the 2 zones?

Is the part of the house you are actively heating the first floor?

If you have enough baseboard to run a condensing boiler (we need the load numbers and baseboard lengths to know for sure) it is often cheaper to install an inexpensive modulating condensing boiler than a replacement cast iron boiler, and it can be easier to size it correctly. The most important numbers with mod-cons is the minimum modulation levels and the length of baseboard on the smallest (least baseboard) zone, as explained here.

How is your domestic hot water being heated?