What Exactly is a British Thermal Unit (BTU)?

What Exactly is a British Thermal Unit (BTU)?

By Luke Wonnell

Oh the Imperial system… home to such gems as hectares, stones, bushels and pints. Personally I’m a fan of that last one, especially when served cold.  The Imperial system is very finicky: 12 inches to a foot, water freezes at 32⁰F, boils at 212⁰F, and the water used to fill a pint glass weighs 1.08lbs. Meanwhile the metric system is so neat and tidy: 100 centimeters to a meter, water freezes at 0⁰C, boils at 100⁰C, and the water used to fill a 1meter x 1meter x 1meter box weighs exactly 1000 kg – amazing!

Since we all deal with heating equipment here in the United States, we’re used to seeing BTUs on product rating labels and literature, but what exactly is a British Thermal Unit (BTU) anyway? Well, the technical definition is the amount of thermal energy needed to raise the temperature of 1 lb. of water by 1⁰F, but for a better hands-on example, it just so happens that striking a match is about 1 BTU! Imagine striking a match and holding it under a pint glass filled with water (about 1 lb.) until it burns out – I think we can agree the match will be able to increase the temperature of the water by 1⁰F.

Now let’s continue this example by relating common boiler capacities to their equivalent #MATCHES/Hour, #MATCHES/Minute and #MATCHES/Second:

Hydronic Boiler Capacity # Matches / Hour # Matches / Minute # Matches / Second
300,000 BTU/Hr 300,000 5,000 83
500,000 BTU/Hr 500,000 8,333 139
750,000 BTU/Hr 750,000 12,500 208
1,000,000 BTU/Hr 1,000,000 16,667 278
1,500,000 BTU/Hr 1,500,000 25,000 417
2,000,000 BTU/Hr 2,000,000 33,333 556
3,000,000 BTU/Hr 3,000,000 50,000 833
4,000,000 BTU/Hr 4,000,000 66,667 1,111

Even a relatively small commercial boiler (300,000 BTU/Hr) would be the equivalent energy of striking 83 matches every second!  Just remember the next time you strike a match to start a campfire or light a fire cracker you are holding 1 British Thermal Unit in your hand!