The origins of the British thermal unit
Why not a Ptu?
Ever wonder why it’s a British thermal unit and not a Polish thermal unit?
I sure did, but then I have way too much time on my hands.
I mean the Brits were first with just about everything that has to do with hydronic heating. William Cook came up with the idea of heating buildings with pipe 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 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 sq. ft. of radiant walls, making it 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. Woodworth was a respected engineer and I admired him. Years later, we became friends through my own magazine writing. 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:
“I think you should have the enclosed book, rather than have it rot in a Hydronics Institute cabinet.
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 here is the British thermal unit and the answer to my earlier question, why not a Ptu? It came from the mind of Thomas Tredgold, a British railroad engineer who dabbled in heating and ventilating public buildings. What’s delightful about that definition, though, is the business about 1 cubic ft.of water rising in temperature by one degree Fahrenheit. These days, as you well know, a Btu is the amount of heat needed to raise 1 lb.of water one degree on the Fahrenheit scale, not 1 cubic ft.
So why the change?
Mr. Tredgold goes on in his book to give examples of how much fuel it would take to bring 1 cubic ft. of water to a boil from several different temperatures. He explains about latent heat and how we have to add a lot of it to get water excited enough that it changes from liquid at 212° F to steam at the same temperature. I keep smiling about changing 1 cubic ft. to 1 lb. of water, though. I figure it got changed for one reason, and one reason only: 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’s under the grass quilt, what’s the big deal about changing it? I mean you have to agree that 1 lb. of water is easier to work with than 1 cubic ft. of water, right? A pound also is a British measurement, 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 nothing is sacred about any of this stuff, and it pays to develop a healthy skepticism about it. It also pays to question people when they say things because sometimes they’re just going along. They do it 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. 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 must reach to within several millimeters of the crust. Or an approved equal, such as mayo.
There’s a plan and spec for it, hence the extra time allotted.
Anyway, I’m up there yakking away about the glories of steam heating when I mention I had been on a problem job with a contractor and we came across a Farquhar Flange.
I pause for a moment. Not one of the engineers says 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 and out of my mouth.
“You’ve seen these?” I said.
“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 it stand in their way.
You know this old joke?
First guy: “You ever drive a Henway?”
Second guy: “What’s a Henway?”
First guy: “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.
This article was originally titled “Why not a Ptu?” in the January 2016 print edition of Plumbing & Mechanical.