Determine the condensate return efficiency, if present at all. You can consult the specifications for the boiler system to determine if a condensate return is part of the system and what kind of efficiency it may possess. If you know your system has a condensate return, but cannot determine what level of efficiency it may possess, you may use 50 percent, according to Milanco Industrial Chemicals.
Use this formula:
Makeup water per hour = (Horsepower * 4) - ((Condensate Return Efficiency / 100) * Horsepower * 4)
Looking at the formula, a boiler can convert approximately 4 gallons of water into steam per hour, so multiplying the horsepower by four allows the calculation of the total water used. However, if the system has a condensate return, then a percentage of that will be the condensate return. Therefore, you subtract the percentage of that figure, which corresponds to condensate return. If there is no condensate return, then the condensate return efficiency is zero, meaning nothing is subtracted.
Plug in the condensate return efficiency and 500 HP, which corresponds to your boiler. As an example, suppose the condensate return efficiency is 50 percent:
Makeup water per hour = (500 * 4) - ((50 / 100) * 500 * 4)
Makeup water per hour = (2,000) - (0.50 * 2,000)
Makeup water per hour = (2,000 - 1,000)
Makeup water per hour = 1,000 gallons per hour