Sizing a Steam Boiler
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. 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, as it needs enough steam to fill the radiation. If you size a steam boiler using a heat loss calculation, your boiler may not provide sufficient heat in all rooms because its steam output is less than the square footage of steam required to fill the radiation. If you do not produce the proper amount of steam volume, the steam will condense fast enough, and all the radiators will not fill with steam.
Sizing a steam boiler is easy. You will need to gather a small amount of information. Measure the height of each radiator from the floor to the top. Determine whether the radiator is a tube style or column style (as pictured on the worksheet). Then, count the number of columns or tubes per section and the total number of sections required to achieve the radiator's length. When sizing the radiator, we have a worksheet you can print out and fill in 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 manufacturer's available square feet of steam numbers from their color brochure. Do not add anything for piping as long as 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 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. We will show a column-type radiator that would be 22" high, three columns, and seven sections long. We will use the chart below to get the multiplier per section and multiply it by 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 larger size. Once we determine our height, we move horizontally across the chart to the column that corresponds to the number of columns for our radiator. In this case, 22" high and three columns, we would have 3 square feet per section. Next, you multiply 3 times the number of sections, which is seven. Our total square feet of steam for this radiator is 21. You will do the same for all the radiators in the home and total them together. That's it, do not add any piping into your calculation.
If the steam piping is uninsulated, you must insulate it. The cost of operating a steam system with uninsulated piping is much higher than with insulated piping. Uninsulated piping can also lead to noisy steam operation, characterized by banging in the pipes and radiator vents, which pushes water out of them.
Let's assume that after we measure and calculate all the radiators, we have a total of 390 square feet of steam. We would compare that to the manufacturer's brochure's 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. To size a boiler, you need the charts. You can view them here. You can download a free Excel steam boiler sizing tool. You will need the Size Steam Form above to gather all the needed information. Transfer the information to the Excel spreadsheet, and you will know what size steam boiler you need.
Download Excel file here
Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, and installations should be reviewed on a per-job basis. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property, or bodily injury from applied information found on this website, as it should be reviewed by a professional.