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Posted by HeatPro on July 10, 2006 at 10:30:25:
In Reply to: Calculating boiler size -- why variable estimates? posted by Ken on July 10, 2006 at 10:01:08:
As steam systems are an early type of central heating, installed in the 1800's and on, the latest generation of plumbers aren't as aware of steam design as is necessary, so mistakes will be made.
A steam boiler must fill the entire steam sytem with steam on each cycle to get all radiators to heat. This means that no matter what the heat loss of the house is today, the steam boiler must have enough steam output to heat all the radiators in the house - or some rooms will remain cold and the boiler will run all the time wasting money if the main thermostat is in the unheated room.
As each square foot of equivalent direct radiation must be hit with steam,
the way to size a steam system is to count the number of square feet of equivalent direct radiation, multiply by 240 btuh for each sq ft EDR, then multiply the total by 1.5 to make sure there is enough boiler output to heat the boiler, pipes and radiation up to 212F from a cold start.
Just taking a heat estimate of the building gives the heat need of the building; but would be a serious mistake if there is (and usually there is) much more radiator than necessary to heat the house. A heat estimate does give a good indication of how much more boiler input there is than necessary to heat the house if you converted to hot water heat.
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