The Origin of the Btu

In this episode, Dan Holohan tells the story of Thomas Tredgold, defining the British thermal unit, and why it pays to develop a healthy skepticism.


Episode Transcript

Ever wonder why it's a British thermal unit and not a Polish thermal unit? I mean, why not a Ptu instead of a Btu? I wondered about that.

I already knew that the Brits were the first to come up with just about everything that has to do with hydronic heating. William Cook came up with the idea of heating buildings with pipes filled with steam. He published that thought in a few paragraphs back in 1745. The rest of the article had to do with ways to keep the worms from eating the hulls of wooden ships. I think he needed to focus more.

James Watt was the first guy to put a radiator in a house. It was in his own proper English house, of course, and the year was 1785.

In 1911, The iconic Royal Liver Building, the world's first reinforced concrete building rose on the banks of the Mersey River in Liverpool, England. It had 119,000 square feet of radiant walls, making it also, I believe, the world's first (and certainly largest) hydronic radiant job.

When I was growing up in the heating industry, I read trade-magazine articles by John Woodworth, who worked for the Hydronics Institute. John was a respected engineer and I admired him. Years later, we became friends through my own magazine writing, and when he retired in July 1999, he sent me a falling-apart copy of Thomas Tredgold's, Warming and Ventilating of Public Buildings. The First Edition of this book arrived in England in 1824. The copy John gave me was a Third Edition, published in 1836. He included a note:

Dan, I think you should have this book, rather than have it rot in a Hydronics Institute cabinet.


John Woodworth

For which I will always be grateful. What a treasure this book is! I have many old books in my collection, but this was probably the first written specifically about warming and ventilating public buildings. In it, on page 24 are these words from that long-gone Englishman, Mr. Tredgold:

"In order to compare the effects of different kinds of fuel, some convenient measure of effect should be adopted: not only for the purpose of lessening the trouble of calculation, but also to render it more clear and intelligible. I shall, therefore, without regarding the measures of effect employed by others, adopt one of my own, which I have found useful in this and other inquiries of a similar nature.

"I take as the measure of the effect of a fuel, the quantity, in pounds avoirdupois, which will raise the temperature of a cubic foot of water one degree of Fahrenheit's scale."

What we have there is the British thermal unit, and the answer to my earlier question, Why not a Ptu? It’s British because it came from the mind of Thomas Tredgold, a British railroad engineer who dabbled in heating and ventilating public buildings in his spare time. What's delightful about that definition, though, is the business about a cubic foot of water rising in temperature by one degree Fahrenheit. These days, as you probably know, a Btu is the amount of heat needed to raise one pound of water one degree on the Fahrenheit scale, not one cubic foot.

So why the change?

Mr. Tredgold goes on in his book to give examples of how much fuel it would take to bring a cubic foot of water to a boil from several different temperatures. He explains about latent heat, and how we have to add a lot of that to get water excited enough so that it changes from liquid at 212°F to steam at that same temperature. I keep smiling about that change from one cubic foot to one pound of water, though. I figure it got changed for one reason, and one reason only: Mr. Tredgold assumed room temperature on the Fahrenheit scale. And once he was gone, all bets were off.

But what does it matter? Let's face it; the guy was just making this up as he went along. Once he was a Dead Man, what’s the big deal about changing it? I mean you have to agree that a pound of water is easier to work with than a cubic foot of water, right? And a pound is also a British measurement, and in more ways than one, so what the heck?

And that's how the sacred British thermal unit came to be.

Oh, and since the British had kicked the snot out of Napoleon in 1815, and their empire sprawled so far and wide that the sun never set on the Union Jack, I suppose the Brits felt just fine naming this unit of heat measurement after their grand and glorious country. Hey, they were the victors, and to the victors go the definitions.

The French weren't buying it, though. And neither were the Germans. Which is understandable.

Today, even the British don't use the British thermal unit. We Americans are the only ones hanging on to that made-up unit of measurement. We don't change easily.

But doesn't this sort of inspire you to start a scientific movement? All you have to do is make up a new term and gather a group around it. You can't miss!

Keep in mind, though, that there is nothing sacred about any of this stuff, and that it pays to develop a healthy skepticism about all of it. It also pays to question people when they say things because sometimes they're just going along. They do that because they don't want you to think they don't know.

It's the nature of our species.

Here's a fine example for you. I was doing a steam seminar for a large group of people, most of whom were mechanical engineers. I love and admire mechanical engineers because they are all smarter than I am.

We were serving a buffet lunch at this seminar, and because most of the attendees were engineers, we allowed an extra 15 minutes for the lunch break. We did this because engineers don't make a sandwich; they build a sandwich. The meat and the cheese and the bread must all be in perfect alignment. The mustard (or approved equal. It could be mayonnaise) has to reach to within two millimeters of the crust.

There's a plan and spec for all of this, hence the extra time allotted at the seminar lunch.

Anyway, I'm up there yakking away about the glories of steam heating when I mention that I had been on a problem job with a contractor and we came across a Farquhar Flange.

I paused for a moment. Not one of the engineers said a thing.

"Do you all know what I'm talking about?" I asked. Many heads nodded, which was delicious because I have no idea what a Farquhar Flange is. The name had just popped into my brain and slid down out of my mouth.

"You've seen Farquhar Flanges??" I said.

Heads nodded.

"What color was yours?" I pointed to one of the nodders.

"It was red?" he said.

And there you have it. One of the coolest things I have learned in this business is that when people don't know what's going on, they will never let that stand in their way.

You know the old joke?

First guy says: "You ever drive a Henway?"

Second guy says: "What's a Henway?"

First guy says: "About two pounds!"

Rim shot! Uproarious laughter!

Thing is, in this business, the second guy would just say, "Henway? Sure, I rented one from Hertz last summer."

And the rest of the folks in the room would nod.

Gosh, I love this business.

And I hope you loved that story. If so, please share it with your friends. We all need a good laugh. And subscribe to this podcast. I have many more Dead Men Tales to share with you. Thanks for sharing part of your day with me. I appreciate you. Thanks.

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