Replacement Cast Iron Radiator Help (New or Used)  Heat Loss/BTU Calculations, Etc.
I'm fixing up the heating system in a house I own that had some freeze damage last winter. The boiler and about half of the cast iron radiators in the system are all fine and functional for the last week, but a few radiators are broken. The system was both air tested and water pressure tested.
New and Used Replacements
I'm looking at both used, and brand new cast iron replacement options. What I'm learning quickly is that some of the radiators are oddball sizes. Some are built into a space in the walls so they have some height/width limitations, and some are not accessible from underneath. Others are free standing against a wall and are accessible from below so I can easily move the pipes as needed. There are all sorts of options out there, new market all are pretty much standard as 19" height, 25" height, and 4 or 6 tube. If I go the used route I'll expect to pressure test them and then put them into service if I can find locally.
What are my options? Calculations?
So what are my options? I'm assuming I should do a Heat Loss calculation for each room, radiator btu output calculation, and then see how the existing radiator BTU output matches, and then determine what a size suitable replacement would be, either used or new? Is this what you all recommend?
I see all sorts of calculators, etc. Any tips you can all share to help me get through this would be great. House is a late 1940's cape style house, some insulation but not well insulated overall. Sheetrock/Plaster style walls, vinyl replaced windows, etc.
Any trusted tools I can use for heat loss calculations, and existing radiator btu output? Or someone here who could help if I put it all into a spreadsheet?
Weil McLain Gold Oil Fired Boiler with 2 zones.
Comments

First step is to identify the EDR of the radiators you want to replace.
If the rooms heated properly with the old radiators, then replacing them with the a radiator with the same EDR rating, makes it easy
EDR stands for Equivalent Direct Radiation. Basically, it is the total square foot of surface area of the outside of the radiator. It is the surface that can radiate the heat into the room as room air flows past the radiator.
The basic chart that most of us use to determine the EDR if you don't have the original manufacturer's info is this.
There is a book available that has more detailed radiator information available from the store on this site. The book is called EDR Every Darn Radiator https://www.amazon.com/DRRatingsRadiatorconvectorprobably/dp/099647725X
If you can somehow get replacement radiators that are close to the same rating as the old radiators, then you should be good to go.Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Thanks for sharing this Edward, looks really interesting. I have seen some simple calculations on the new radiators but didn't realize there was a resource out there for old stuff. I may try to use this as a guide and get my numbers as close as I can to then see what is out there for new replacements. If I can find identical old used ones locally that are priced right I may go that route as well.EdTheHeaterMan said:First step is to identify the EDR of the radiators you want to replace.
If the rooms heated properly with the old radiators, then replacing them with the a radiator with the same EDR rating, makes it easy
EDR stands for Equivalent Direct Radiation. Basically, it is the total square foot of surface area of the outside of the radiator. It is the surface that can radiate the heat into the room as room air flows past the radiator.
Since I have no history of this property and have no connection to past people who've lived there I'm uncertain how it heated with the existing setup unfortunately. So doing all of this math should at least help me better understand if what's there is reasonably sized.
Just to be sure I'm understanding those pictures properly, can you confirm if this is correct?
Radiator:
Sections: 9
Height: 23"
Width: 83/4"
Number of Tubes: 5
Following the Tube Type Radiator chart, this shows this radiator puts out 3 Sq Ft per section. 9 sections X3sq ft=27 sq ft of radiation?
Then using a EDR to BTU calculator for 27sq ft it says 6480 BTU with a 20% calculation built in. So far this rough math seems to be making sense using some quick calculators and room size calculations but I'm going to plan to go through and do it accurately.
Any heat loss calculators available online that you recommend? I'm sure most pros have software or a paid option, I'm just a regular joe trying to get by with whatever is available online.
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After some more digging, it looks like my radiators are American Radiator/ARCO's so that makes it slightly easier to figure out existing radiator output. On the used replacements, I've found all sorts of brands, or nonmarked, but following the general EDR numbers it seems like I can get close to knowing what will work, and what won't.EdTheHeaterMan said:
The basic chart that most of us use to determine the EDR if you don't have the original manufacturer's info is this.
Looking at some new radiators, they used Steam at 240 degrees as their baseline, so I'm doing some of that calculating to adapt to 180F water. Some of these numbers and calculations are making my head hurt .0 
If they give you the edr you can compare that directly to the edr of your broken radiators0

Standard conditions for steam EDR are 1 psi steam in the radiator, which is 215° F., and a room temperature of 70° F. This gives a heat output of 240 BTUH per square foot of heating surface.Not sure where the OP is seeing 240° F as a radiator rating temperature, I have never heard of this. That would correspond to saturated steam at 10 psi, which is seldom used in direct radiation.—
Bburd0 
Thanks for the info. So here is one of the new radiator options out there.mattmia2 said:If they give you the edr you can compare that directly to the edr of your broken radiators
Can someone help me do the math?
It states:
NOTE: HEAT EMISSION  BASED ON 240 BTU (STEAM) PER SQUARE FOOT PER HOUR
At 180F it states HEAT EMISSION 170 BTU PER SQ. FT
So a 24 section at 19" tall states 1.6SQ.FT. PER SECTION at 38.4"
So 38.4" X 180F of 170BTU per sq ft= 6528 BTU?
Or, must I do the math between the 240BTU Steam per Sq Ft and covert that down for 180F for an output of 170 BTU per sq ft? If that's the case, that puts me at about a 25%+ loss of BTU per Sq FT based on that initial chart. BUT the sq ft doesn't change so I'm assuming I'm simply using the sq ft per section total and calculating that against 180 degree water at 170 BTU per sq ft to give me that number of: 38.4" X 180F of 170BTU per sq ft= 6528 BTU?
Any tips appreciated if I'm looking at this properly. Its the holiday and I'm going crazy handling everything else right now, but I'd like to get this math straight so I can finalize my calculations.
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