Boiler Horsepower (BHP)

What Exactly is Boiler Horsepower (BHP)?

By Luke Wonnell

In the United States, the output capacity of steam boilers is expressed as Boiler Horsepower (BHP) whereas hydronic boilers are typically sized based on their input capacity in Btu’s/hr.  There are definite differences between these two methods, but it is possible to convert between the two units of measure.

1 Boiler Horsepower (1 BHP) is defined as the capacity of a boiler to boil 34.5 lbs of liquid water completely to steam in one hour at atmospheric pressure conditions. This is sometimes abbreviated as “F&A212” which means “from & at 212⁰F” or the boiling temperature of water at atmospheric pressure.

Let’s take several common Boiler Horsepower sizes and convert to equivalent hydronic boiler capacities:

As you can see above, the conversion from BHP to Btu/hr isn’t neat and tidy – you end up with lot of pesky digits which doesn’t make for appealing marketing literature.

Let’s flip this process and convert common standard efficiency hydronic boiler sizes to equivalent BHP:

Finally, let’s convert high-efficiency condensing boiler sizes to equivalent BHP:

Notice how standard efficiency hydronic boilers offer fewer BHP than their high-efficiency condensing boiler counterparts even at the same input capacity. This is because the increased efficiency of condensing boilers allows them to produce more hot water with a given amount of fuel than standard efficiency hydronic boilers.