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.
