There are other factors involved in sizing boilers. The incoming water temperature affects the ability to produce steam. Colder water requires more kilowatts to convert to steam. Warmer water entering a boiler allows more pounds of water to be generated. A boiler system having a condensate return tank will act as a "warm" water source and be an energy saver. Condensate water often is 100F warmer than street water or ground water. Care in design is needed to install steam lines and condensate return piping and components. Allowing the condensate tank to be too hot (over 180F) can lead to pump wear and boiler feed problems. Unless you are experienced in this work, it most always is a time and money saver to utilize local steam and boiler craftsman to recommend and install your boiler system.