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Posted by DS on August 12, 2007 at 19:20:03:
I agonized over contractors who "only" measured my rads and skipped the J Manual calcs as I've read in numerous forums, including here, that such a step was vital so you wouldn't end up with an oversized boiler. Until I discovered that stem systems are a whole lot different than the FHW variety. Yes. Sizing a boiler properly goes a l-o-n-g way toward maintaining efficiency. What is irrelevant, however, is how the envelope of a building (insulated walls, etc.) affects the size of a STEAM boiler. The size of a steam boiler is determined solely by the ability of the SYSTEM it serves to transfer and condense steam. Simply put, a steam heating system contains a specific amount of radiators and pipes. Those rads and pipes are designed to handle a specific amount of steam for the system to work. Adding or subtracting from that will make the system work less well.
A replacement steam boiler must be sized by adding up all of the radiators in the building, determining their combined direct radiation equivalent, and consulting manufacturer's literature for the closest matching steam boiler. Measuring BTU heat loss won't do that.