How to Determine the Right Size Steam Boiler

Check the gauges on your steam boiler and note the feedwater temperature and operating pressure. On most boilers, an analog thermometer gauge indicates temperature; a sight glass indicates pressure. On some models, a combo gauge indicates both, featuring a scale for temperature and a scale for pressure.

Consult a steam boiler sizing chart, provided online by numerous boiler manufacturers, such as Lattner, online (see Resources). This consists of a chart that converts feedwater temperature and operating pressure to BTUs. Find your feedwater temperature in the vertical column and match it with the operating pressure in the horizontal column. The box where the two measurements meet gives you your BTUs per pound of steam. For instance, if your feedwater temperature is 100 and your operating pressure is 40, your BTU measurement is 1108.

Multiply the BTU measurement by the pounds of steam required by your boiler per hour; contact the boiler's manufacturer, consult the boiler's original operating manual or recruit a professional heating contractor to obtain this information. Divide the sum by 3,413 to determine the required kilowatts.

Divide the number of required kilowatts by 9.809 to determine the horsepower requirements for a steam boiler in your home. Rather than actual physical size, steam boiler size is ranked by horsepower. For instance, a 250-horsepower boiler is “bigger” than a 15-horsepower boiler. Once you have this sum, select an appropriately sized boiler based on its horsepower rating.