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:

BHP Lbs. of Steam / Hr         (OUTPUT) Equivalent BTU/Hr (OUTPUT) Equivalent BTU/Hr (INPUT) @ 85% EFFICIENCY Equivalent BTU/Hr (INPUT) @ 95% EFFICIENCY
1 34.5 lbs/hr 33,479 Btu/hr 39,387 Btu/hr 35,241 Btu/hr
10 345 lbs/hr 334,788 Btu/hr 393,868 Btu/hr 352,408 Btu/hr
50 1,725 lbs/hr 1,673,940 Btu/hr 1,969,341 Btu/hr 1,762,042 Btu/hr
100 3,450 lbs/hr 3,347,880 Btu/hr 3,938,682 Btu/hr 3,524,084 Btu/hr
250 8,625 lbs/hr 8,369,700 Btu/hr 9,846,706 Btu/hr 8,810,210 Btu/hr
500 17,250 lbs/hr 16,739,400 Btu/hr 19,693,412 Btu/hr 17,620,421 Btu/hr
1000 34,500 lbs/hr 33,478,880 Btu/hr 39,386,823 Btu/hr 35,240,842 Btu/hr

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:

Hydronic Boiler Capacity (INPUT) Hydronic Boiler Capacity (OUTPUT) @ 85% EFFICIENCY Equivalent Steam Boiler Capacity (BHP)
500,000 Btu/hr 425,000 Btu/hr 12.7 BHP
750,000 Btu/hr 637,500 Btu/hr 19.0 BHP
1,000,000 Btu/hr 850,000 Btu/hr 25.4 BHP
1,500,000 Btu/hr 1,275,000 Btu/hr 38.0 BHP
2,000,000 Btu/hr 1,700,000 Btu/hr 50.8 BHP

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

Condensing Boiler Capacity (INPUT) Condensing Boiler Capacity (OUTPUT) @ 95% EFFICIENCY Equivalent Steam Boiler Capacity (BHP)
500,000 Btu/hr 475,000 Btu/hr 14.2 BHP
750,000 Btu/hr 712,500 Btu/hr 21.3 BHP
1,000,000 Btu/hr 950,000 Btu/hr 28.4 BHP
1,500,000 Btu/hr 1,425,000 Btu/hr 42.6 BHP
2,000,000 Btu/hr 1,900,000 Btu/hr 56.8 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.