How much does size matter?

A 97,000 input will be around an 84,000 output, derate for efficiency.

If you trust the load calc, go with the 125, it will turndown about the same.

That is a pretty simplistic load calculator. I would run another load calc to tighten up that number.

What size is the existing boiler?

How much does size matter?

This question is better asked in private with your significant other.

As far as the boiler size is concerned…

If you do the load calculation based on I=B=R form 1504WH or ACCA Manual J, you will find a much lower requirement. I have a feeling that you will be able to use the WM AB-80H with a 65,000 NET output, and have the most efficient setup. If you don't want to go to the trouble of an actual Load Calculation for your home where you measure the actual amount of doors, walls, windows, ceilings and floors, then you will be guessing that you have something less that 97,000 NET and I would choose the WM AB-120 with the 97,000
NET output. Either way, the boiler will only use the amount of BTUs required to heat the home and rarely fire more than actual load requirements. Where you will waste fuel is at the low end when you only need 6000, BTU on a spring evening and the turndown ratio only goes to 9700 BTU so the boiler will cycle on and off to provide the lower BTU requirement.

I recommend the process detailed in this article for sizing based on actual fuel usage:

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

Need to start a mailing list for your spicy jokes. 'Ed Young: Too Hot For The Wall"

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

I've used a method like this for decades and it works quite well. However, some of thier assumptions are quite off.

Heat loss of a structure is not close to linear. Heat movement through most materials is nearly linear at typical design temperatures. However, air leakage rates increase nonlinearly, so heat loss accelerates as it gets colder. The old ASHRAE charts show the design air leakage rate of a medium "tight" construction as 0.6 air changes per hour when its 50F outdoors. At 0F , the number of air changes per hour increases to 1.1, nearly double the air leakage and at big delta T for that same structure If you have a leaky building, this can really increase heat load dramatically.

Also, the other big error is the efficiency of equipment. A typical 80% gas efficiency atmospheric boiler without a damper that is 3 times oversized is running about 62% efficiency on a typical winter day ( which is about 1/2 of design load). A 3 times power burner boiler boiler tends to fare much better at about 68% efficiency.

This is why I really like running staged boiler heating plants in larger buildings Sized and controlled properly, you can take the typical day 62% or 68% efficiency and bump that up to 80% to 85% with a properly sized heating plant with two smaller boilers. On the buildings I've done this, fuel usage reduction typically runs from about 20% to 60% and you still have very simple long lived equipment that is inexpensive to repair.

I typically do 2 stages, unless I'm trying to stick with residential pre assembled boilers on larger loads. I have used asymmetrical stages, especially in unusual load situations like church structures that are in setback must of the time and only are heating a small space. For control on Hot water systems, I've used Tekmar for staging controls and reset, but its been quite awhile since I've done these. For steam, I use two stage thermostats quite often with orificed two pipe systems. On some steam systems you need both boilers to prime the system and then you can drop to one boiler. An aquastat at the end of the slow to heat main usually can take care of that. R&D controls makes a 2 stage steam control that will drop out the 2nd stage after the temp rises to about 60% of the differential. I probably will be trying one of these soon.

This particular U.S Boiler calc is looking at the home as a whole, measure outside walls, windows, doors. Follow the Procedure paragraph

A more detailed load calc would be room by room. So use the same input data sheet and use the room dimension. A 10X 10' bedroom 8' tall for example.

In a house with significant solar gain this method will underestimate the design load.

Infiltration is always a guess and always was a guess without a blower door test.

Hot water boilers have a built in 15% piping and pick up factor and steam is 33%. So if the boiler selected is the "perfect size" you would still have some extra capacity.

Yes, round to closest square footage

interior walls Insulated or not does not change the rooms load / loss.

It depends on your mood. Do you want it cold, too hot, or as comfortable as possible? Hmm. I would hazard a guess that you want it as comfortable as possible. Crunching the numbers with more than one heat loss program is a good idea.

Years ago, I had to do heat loss calculations by hand. ( Ok, I used a calculator too, I'm not that old.)

Does anyone know where those calculation formulas can be found? Does Slant Fin still have theirs out there? Is there a calc? That can be reliably used?

Here is another option, in an Excel sheet format, try several of the options offered on this thread to you find the one that works best for you.

They are all based on a Manual J formula, some allow you to enter more details and get a more accurate number.

The infiltration will be a guess unless you have a blower door test done. But easy enough with a spreadsheet to change ACH and see how it changes your load number.

Of course even with a blower door test you're still guessing what the infiltration will be on a cold day, it's just a more informed guess.

how does infiltration change? cold day to hot day?

temperature deference and wind direction.

The temperature difference between the inside and the outside creates a pressure differential which drives infiltration. The greater the temperature difference the greater the infiltration.