Sizing a replacement boiler

AdamSwapp

I've got a 1922 boiler which might be getting replaced soon (see <a href="http://www.heatinghelp.com/forum-thread/137802/Can-this-boiler-be-saved">http://www.heatinghelp.com/forum-thread/137802/Can-this-boiler-be-saved</a>).  I've calculated the load of the 12 attached radiators.  If my math and per-radiator EDR numbers are correct; it's about 480 square feet of EDR total, or 115,000 BTUH.  Using a pickup factor of 1.5 gives me a required gross size of 175,000 BTU.



There are unused radiators all along the back wall of a detached garage (about 60' from the house), but they've been disconnected for years (and will remain disconnected, as there's a leak in the underground steam line).  The current boiler has a 320,000 BTU burner, which was installed in the 30's when those radiators were in use.



I've had a couple of guys size a replacement unit.  One measured the radiators, then said 250,000 BTU's.  Another says (without measuring the radiators) that we need 324,000 BTU's (Well-McLain LGB-4), because we "need the same steam volume" as the current system and there would be so much less water in the new system.



Again, my calculation of the actual load (and using a generous pickup factor to account for older piping) comes up with something significantly smaller than either of those.  Am I missing something here?



I don't really care what the answer is, but I'd really like to size the boiler correctly if I do need to replace the current one.  This would be enough of a PITA without buying the wrong stuff. ;)



Thanks.

BobC

Sq Ft steam = or slightly greater than EDR

Everything I have seen indicates you size the connected EDR to the Sq Ft of steam rating of a boiler, that includes a 30% pickup factor for piping etc. All of the pipe in the cellar should have a minimum of 1' of pipe insulation on it for best and most efficient operation. If you need heat someplace in the basement pull a hot water loop off the boiler to provide it, that loop would figure into the boilers sizing especially if it's significant.



You should read this article for more information - http://www.heatinghelp.com/article/323/Boilers/1551/Taking-Another-Look-at-Steam-Boiler-Sizing-Methods-by-Dave-Boilerpro-Bunnell



Bob