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# how to calculate SF of steam

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square feet of external area, also known as "E.D.R." or "Equivalent Direct Radiation".

"E.D.R." also happens to be the name of a book available on this site ("Online Store") which should cover you.

The internal volume of the radiator has less do do with it; it is a factor of pressure (system volume) but ultimately how fast the steam condenses and gives up it's heat is what you need to know: The rate of steam flow.

As you are noting, each radiator has certain distinguishing characteristics but there is (always) more: How they are installed, whether in enclosures, recessed, open to view, all factor into their rated output.

Now, if you are starting from scratch, you would have calculated as accurate a heat loss as you could with the building as it is, taking into account insulation, new windows and so-on. The radiators for each room would be sized by taking the heat loss and dividing by 240. (That is the number of BTU's per hour emitted by a square foot of radiation at 215 degrees in a 70 degree room.)

In all likelihood you have existing radiators and will probably keep them as is, even if the house has been improved. If ambitious you might even reduce them to the heat loss of each room or at least proportional room to room based on the heat loss of each room. (Most folks are not that ambitious!)

Regardless, your EDR number is what is used to size a steam boiler, not heat loss.
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