What Is Boiler Horsepower?

If you have ever wondered how a boiler can be rated at 500 horsepower without being able to move anything, the answer is that boiler horsepower is not the same unit as engine horsepower. It is not even close — one boiler horsepower is over thirteen times larger.

The Definition

Boiler horsepower measures thermal output — how much heat a boiler can deliver — rather than mechanical work. Modern industrial practice defines it as a thermal output of 33,475 BTU per hour.

The older and more physical definition is the energy needed to evaporate 34.5 pounds of water into steam per hour, starting from water at 212°F and producing steam at 212°F at atmospheric pressure. Both definitions describe the same quantity; the BTU figure is simply easier to work with.

Where the Number Came From

It is a leftover from the age of steam engines. In the nineteenth century, a boiler was sized to feed a steam engine of a given output, so it made commercial sense to rate the boiler by the engine it could supply. The figure was set at the steam needed to run a one-horsepower engine of the era.

Those engines were spectacularly inefficient — converting only a few percent of thermal input into mechanical work. That inefficiency is baked into the unit, and it is precisely why boiler horsepower is thirteen times larger than mechanical horsepower. The number preserves the thermal appetite of a Victorian steam engine.

Modern boilers have nothing to do with steam engines, but the unit survived because the industry had standardised on it. It remains common in North American boiler specification, while much of the world quotes kW or lb/hr of steam directly.

Boiler Horsepower Conversion Chart

Standard industrial boiler sizes with their equivalents:

Our boiler horsepower calculator handles any value in all four units.

What This Means When Sizing

Two cautions matter in practice.

The rating is output, not input. A 100 BHP boiler delivers 3,347,500 BTU/hr of useful heat, but consumes more fuel than that because combustion efficiency is never perfect. At 80% efficiency the burner must supply about 4,184,000 BTU/hr. Sizing gas supply or fuel storage from the output figure will leave you short.

Steam output assumes ideal conditions. The 34.5 lb/hr figure applies at 212°F feedwater and atmospheric pressure. Real installations run colder feedwater and higher pressure, both of which increase the energy needed per pound of steam. Actual output at operating conditions is typically lower than the nameplate figure suggests — a factor known as the "from and at" correction.

The Confusion It Causes

Because the word is identical, boiler horsepower gets mixed with engine horsepower in surprising places — most often when someone converts a boiler rating with a generic unit converter set to mechanical horsepower and produces a number thirteen times too small.

Watch for it in these places:

• Feed pump sizing. The pump motor driving the boiler is rated in mechanical horsepower, often just a few HP on a boiler rated at hundreds of BHP.
• Electrical load calculations. A 100 BHP boiler is not a 100 HP electrical load. Its electrical draw is the burner, fans and pumps only.
• Comparing against chillers. Cooling equipment is rated in tons of refrigeration, a third unrelated scale — see our ton to HP calculator.

Boiler horsepower is one of four different quantities sharing the name. The others are mechanical, metric and electrical horsepower — all covered in our comparison of hp vs bhp vs PS vs kW, and the exact definitions are set out in the exact HP to kW conversion.