Boilers are rated by their horsepower, pounds of steam produced per hour, or British Thermal Units. Horsepower is defined as the ability to evaporate 34.5 pounds of water per hour. Large boilers are rated in pounds of steam produced per hour under specified conditions. Maximum boiler load refers to operating at the boiler's maximum rated capacity in horsepower, pounds of steam, or BTUs. Boiler load and variations in load can influence chemical treatment needs.

1. Boiler Ratings and Boiler Load
• The oldest method of rating boilers, still used to rate small boilers, is
by horsepower (hp). One horsepower is defined as the ability to
evaporate 34.5 lbs of water into steam at 212 deg. F and above.
• Large boiler capacity is generally given in lbs of steam evaporated
per hour, under specified steam conditions. Maximum continuous
rating is the hourly evaporation that can be maintained for 24
hours.
To convert horsepower (hp) into lbs of steam:
Multiply hp x 34.5
Example: 100 hp x 34.5 = 3450 lbs of steam per
hour
To convert lbs of steam to hp: Divide steam per
hour by 34.5
Example: 8625 lbs of steam ÷ 34.5 = 250 hp boiler
• Another measure is the BTU (British thermal unit). 33,472 BTU
equals 1 hp
To convert BTU into hp, divide the BTU rating by
33,472
Example: 8,368,000 BTU ÷ 33,472 = 250
• Boiler load - The horsepower, lbs of steam per hour, or BTU is the
rating indicating the maximum capacity of a boiler. When a boiler
operates at its maximum rated capacity, it is referred to as
maximum load. If the load varies from hour to hour, it operates at a
varying load. Load and load variations can influence the amount of
chemicals required for treatment and the treatment controls
required.