Sizing a steam boiler is much easier than sizing a hot water boiler or warm air furnace. When properly sizing a hot water boiler or warm air furnace you need a heat loss calculation but, when sizing a steam boiler you only need to measure the radiators and calculate the square feet of steam for each connected radiator. Many try to size hot water boilers by measuring the radiation but this will always oversize the boiler. Steam is sized by measuring radiation due to needing enough steam to fill the radiation. If you size a steam boiler using a heat loss calculation your boiler may not get enough heat in all rooms due to the boiler steam output being less than the square feet of steam required to fill the radiation. If you do not produce the proper amount of steam volume the steam will condense fast enough all the radiators will not fill with steam.
Sizing a steam boiler is fairly easy. You need to gather a small amount of information. Measure every radiator height from the floor to the top od the radiator, determine if the radiator is a tube style or column style, count the number of columns or tubes per section and count the total number of sections to make up the length of the radiator. When sizing the radiator we have a worksheet that can be printed out and filled out on the job site. Take it back to the off and calculate the square feet of steam. Once you have the square feet of steam, compare your results to the manufacturers available square feet of steam numbers from their color brochure. Do not add anything for piping providing there is insulation on the steam piping in the basement and the steam main does not run through an unconditioned space. The boiler manufacturer will deduct 33% of the amount of steam the boiler actually produces for piping. The balance is what is printed on the color brochure which is available for the connected radiation load.
Let's do a sample radiator. Let's assume we have a column type radiator that would be 22" high, 3 columns and 7 sections long. We will use the chart below to get the multiplier per section and multiply it times the number of sections. We go down the height column of the chart to the height of our radiator. If the radiator is between two sizes most contractors will go up to the next size. When we find our height we go across the chart horizontally to the column that displays our number of columns for our radiator. In this case 22" high and 3 columns we would have 3 square feet per section. Next, you multiply 3 times the number of sections which we have 7. Out total square feet of steam for this radiator is 21. You will do the same for all radiation in the home and total all the radiators together.
Let's assume after we measure and calculated all the radiators we have a total of 390 square feet of steam. We would compare that to the manufacturers brochure square feet of steam figures. In this case we would have to go to the MST396.
You will be able to find the square feet of steam ratings for column and tube type radiators, cabinet convectors and wall type radiators at this link. You may also want to visit our FAQ page for steam systems.
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