When it comes to replacing boilers We will measure the amount of ft of baseboard and type to determine the BTU output of the new boiler.
New construction example.
manual J*** to figure heat loss in every room. and total loss for house.
design*** baseboard amount of feet needed for that room and each room.
size the boiler to handle the requirments.
unless you have had heating problems in certain rooms due to not enough basebrd.
all baseboards are in all ready so we know roughly the amount of btu out put they will provide.
I would simply provide one to you at signing of agreement and the man J will give you cooling loads as well. To few contractors do load calculations.

The first version of Manual J (heat loss/gain load calculation) was published back in the 1970's. The manual was about a 1/4" thick and the calculation were performed manually without the use of a calculator. Today, Manual J is almost 2" thick and must be performed on a computer using specialized software.

Over the years the industry's knowledge has matured. Unfortunately, most contractors have not.

I am NOT a boiler expert. However, it seems to me that replacing an "ancient" boiler based on the capacity of the "ancient" baseboards is using "ancient" knowledge.

Technically speaking a manual J calculation can and should be done when replacing a boiler. The problem with doing a manual J with an existing and the older the house the more complicated is you have to make an educated guess about the insulation value for the walls, roofs, and windows. The windows aren't all that difficult to determine. The roof insulation is tricky but you can usually see it so that helps.
For example, you can see that the ceiling insulation is 8" fiberglass batts but it 40 years and matted down pretty good. Do you give the full R value for that?
That walls are even more tricky because you can't see anything and the homeowner typically doesn't have a clue.

It also takes time so now you know why heating contractors try to avoid doing the extra work.
Now you can measure all the baseboard rads and easily determine what the maximum output is for all the radiators. There is no reason to install a boiler larger than this number because you can't deliver more heat than this into the house. If you use the boiler for domestic HW than you typically add a little for this.
Now it is possible or even likely that the amount of baseboard is say 25% more than what you need. So if you had an accurate Manual J than you could reduce the boiler size to this number and still heat your house.

Another question is on the coldest day of the year does your boiler run a lot or does it say run for 15 minutes and then turn off for 15 minutes?

A computer program Manual J takes about 1/2 hour, but going to the job site and measuring and guessing about the existing insulation can be time consuming. There are too many people just getting estimates to search for the lowest price and I don't blame the heating companies for measuring the existing baseboard.

Often the baseboard was installed many years ago and could very well be improperly sized, with an emphasis on being too much. It's not uncommon to add 25% as a buffer to the Manual J calculation, but if the installed amount is excessive, you could end up with an oversized boiler with possibly reduced efficiencies or condensating problems.

To get yourself peace of mind, if this is really a problem, have someone do the calculation and pay them. Then call the heating people. That's the ONLY way you're going to get it right.

Thanks folks. I did try to make their jobs easier by providing a detailed floor plan with measurements and information on the insulation, etc in each part of the house. But, oh, well...

In any case, I found a pretty good and thorough-seeming free online calculator and spent several hours doing it yesterday. I came up with a conservative heat loss of 60,000 BTU. (The baseboard calc comes out at 54,000 BTU.)

we were told we should add 30% for the hot water heater zone... does that sound right? If so, that brings us to about 78,000 BTU.

One contractor had spec'd the Buderus G115WS/3, which has a net BTI of 74k and gross BTU of 85k. That seems about right, doesn't it???

What I don't understand is why 3 other contractors spec'd boilers with gross BTU outputs of 112k or so. That seems vastly oversized, no?

By the way, are you supposed to match the NET or GROSS BTU output to your BTU loss number?

NET is what matters.

The rest of the BTUs go up the chimney.

Slightly larger for heat is not that bad, and you are often forced that way since equipment is not available in small size steps.

For example, these are the questions that would be asked for the exterior walls of a house. Two identical houses that face in different directions would have two different losses, because of windows/doors facing different directions.

Walls: (Each exterior wall, on each floor, must be done individually)
Any windows and doors in a wall must be included with that particular wall. A rough drawing of each wall can be supplied, if easier, showing windows and doors. All measurements, in feet and inches for walls, windows, and doors, must be included for the length, width, or height.

What direction does the wall face?
O North
O Northeast
O Northwest
O East
O East Northeast
O East Southeast
O West
O West Northwest
O West Southwest
O South
O Southeast
O Southwest

O A: Wood frame, with sheathing, siding or brick
Wall cavity
O None
O R-11 3 ½"
O R-13 4"
O R-19 5 ½"
O R-27 7 ½"
O R-30 8 ½"
O R-33 9-10"

Select exterior insulation
O None
O R-1.3 ½" Asphalt board
O R-1.8 ½" Beadboard
O R-2.5 ½" Extruded poly board
O R-2.7 ¾" Beadboard
O R-3.6 1 in. Beadboard
O R-3.8 ¾" Extruded poly board
O R-5.0 1" Extruded poly board
O R-8 Sheathing

O B: Masonry, above grade
Select insulation value
O R-5 1"
O R-11 3 ½"
O R-19 5 ½"

Wall construction  no added insulation
O 8" or 12" block
O 8" block and 4" brick

O C: ICF Insulated concrete form, above grade
Select ASTM R-value
O R-12 to R-14
O R-14 to R-16
O R-16 to R-18
O R-18 to R-20
O R-20 to R-22
O R-22 to R-26
O R-26 to R-30
O R-30 to R-36

O D: SIP Structural Insulated Panels
O 4 ½" panel
O 6 ½" panel

Exterior finish
O Stuco or siding
O Brick
O Split logs on both sides

O E: Logs

Circle the log thickness: 6" 7" 8" 9" 10" 11" 12"

O F: Block or brick, extends to 5Â below grade
Select insulation value
O R-5 1"
O R-11 3 ½"
O R-19 5 ½"

Wall construction  no added insulation
O 8 or 12 inch block

O G: Block or brick, extends over 5Â below grade
Select insulation value
O R-5 1"
O R-11 3 ½"
O R-19 5 ½"

Wall construction  no added insulation
O 8 or 12 inch block

O H: ICF, extends to 5Â below grade
Select ASTM R-value
O R-12 to R-14
O R-14 to R-16
O R-16 to R-18
O R-18 to R-20
O R-20 to R-22
O R-22 to R-26
O R-26 to R-30
O R-30 to R-36

O I: ICF, extends over 5Â below grade
Select ASTM R-value
O R-12 to R-14
O R-14 to R-16
O R-16 to R-18
O R-18 to R-20
O R-20 to R-22
O R-22 to R-26
O R-26 to R-30
O R-30 to R-36

Way too many boilers are oversized which means they short cycle on limit which is inefficient. Too many base the size of a hot water boiler on what is there. I usually look at the baseboard but seen many houses with way more baseboard than the house needs so use the Manual J for my sizing.

We took out 2 boilers from a house with infloor radiation. Total 575K input. Replaced with 1 at 175K input. Made a world of difference in comfort and efficiency.

Aunt's house, 335K input 50s vintage. Had 1 bid saying need to put 2 in at 200K each. Ended up with single boiler at 210K input 80%.

Too many boilers and AC are way over sized. Baldloonie is correct that the short cycling when it is too large is very hard on the system. Call the dealers you think you want to use and ask how the hot water affects the heat. I think you'll learn that you system gives you plenty of water for the hot shower. It only takes a minute to warm it up. Don't get a larger boiler. Check the prioritized shower water gpm.