John Mills, Mechanical Genius

Published: November 7, 2009 - by Dan Holohan

Categories: History Lesson


John Mills was one of the great-granddaddies of heating.

He worked with the H.B. Smith Company as a freelance inventor and engineer from 1873 until 1897. He was forever experimenting with heating, and sharing what he knew.

Between 1888 and 1890, he wrote a two-volume book titled Heat, Science and Philosophy of its Production and Application to the Warming and Ventilating of Buildings. It was one of the first serious books about heating and this magnum opus became an important resource for boiler- and steam engineers in the years that followed.

John Mills invented the boiler that bears his name, and Mestek, the successor company to H.B. Smith, still makes the Mills boiler to this day. John Mills came up with the idea that in a tall building, which was to have a one-pipe-steam system, it would be best to send the supply main straight up to the top of the building, then turn it horizontal to go around the perimeter of the attic, and finally downfeed all the radiators so that the steam and the condensate traveled in the same direction. They called this the Mills System and it worked beautifully.

I remember once going to a fancy building on Central Park West in Manhattan where I saw a Mills System that had 12-inch screwed pipe. It was something to marvel at because a 12-inch screwed tee is a lot bigger (and heavier) that you think. I thought about The Great Experimenter that day, and about the men who installed that big pipe to his design. I try to imagine what sort of wrenches those guys owned, where they stood to get the leverage, what size their arms were. Those guys left echoes of themselves in that building. I heard them.

John Mills also experimented with heat loss, and he developed a quick method for figuring out the load in a building. They called it (not surprisingly) the Mills Rule.

Contractors loved the Mills Rule because it was so easy to use. Before long, they nicknamed it the 2-20-200 Rule and I’ll explain why in a minute. Most of the size-it-quickly rules of thumb that followed John Mills’ rule evolved from this method. They were all fine for their time, I suppose, but not so good nowadays because we build better buildings that they did.

Here, consider what sort of windows they used during John Mills’ era. Most likely, they were single-pane, double-hung windows, with leaky sashes. There were no storm windows.

And what about insulation? Do you think they used fiberglass batts back in the day? Do you think they used anything at all inside those walls? Not from what I’ve seen, and not from what I’ve read. The heat loss of a building was much greater back then, and the Mills (2-20-200) Rule was appropriate for that sort of construction. Not so good nowadays.

Here’s what I mean. Take any building and size a new steam boiler using the Mills Rule. The first thing you’re going to do is measure the square footage of all the glass in the building. Once you have that number, divide it by 2 (that’s the 2 in the 2-20-200 nickname for the Mills Rule). Okay, now set that aside for a moment.

Next, measure all the cold surfaces in the building. A cold surface is any wall, floor, or ceiling that doesn’t have heat on its other side. In a two-story house, the first floor walls are cold surfaces if they face outdoors. It’s warm on one side of those walls and cold on the other side. If a wall faces another heated room, you wouldn’t measure that wall for heat loss.

You’d probably measure the ceiling on the second floor of this building because that ceiling faces the attic space (which is unheated), but you wouldn’t measure the ceiling on the first floor of that building because that faces the heated second floor. The same goes for the floor; you’d count it if it was over a cold crawlspace or an unheated basement, but not if it was over a heated basement. Make sense? Good.

Okay, once you’ve measured all the cold surfaces, divide that number by 20 (that’s the second number in the 2-20-200 nickname). Put it on the back burner for a minute; we got one more measurement to make, and this has to do with the air that’s inside the building. The air is constantly changing because of infiltration. Old buildings were drafty (many of them still are). Measure the cubic feet of air by multiplying the length, times the height, times the width of each room. Now add those numbers together and divide the total by 200 (the third number in the 2-20-200 nickname of the Mills Rule).

The grand total you come up with will be the required square footage of Equivalent Direct Radiation (or E.D.R. for short) for the building. One square foot of E.D.R. for steam will give out 240 Btuh when there is 70-degree air on the outside of the radiator, and 215-degree steam on the inside of the radiator. That temperature of the steam is significant because 215-degree steam is steam at about 1-psi pressure, so what the definition of E.D.R. is telling us is you don’t need pressure greater than 1-psi inside the radiator, even on the coldest day of the year.

For hot-water heat, when the average water temperature is 170-degrees, the value of E.D.R. drops to 150 Btuh per square foot. This is because hot water isn’t as hot as steam. The Dead Men used the Mills Rule for both steam- and hot-water systems.

But here’s the problem with the Mills Rule. We’ve upgraded the windows and even the insulation in many of those old buildings. The radiators are now oversized, based on the current heat loss of the renovated building. That can cause money to flow out through those new windows if people are going to be cracking them open so they can be comfortable. If you used the Mills Rule, or a similar shortcut, to figure the radiation for a modern building, you’ll probably wind up with enough radiation to heat three buildings. That’s the problem with using sizing shortcuts from more than 100 years ago. They didn’t keep up with the times, and I think John Mills would agree, were he around today. He’d be using a computer to size his systems. He’d probably be writing the software.

A footnote on Mr. Mills: Sometime in 1905 or 1906 (and this was in Westfield, Massachusetts), Mr. Mills wandered into town, dressed shabbily and looking penniless. J.R. Reed, who ran the H.B. Smith Company in those days saw him and said, “John Mills, I always warned you of this. Didn’t I say that if you kept on at the rate you were going that you would surely scratch a poor man’s pants?” He then gave Mr. Mills a check for $5,000 and said, “You are not going to give this money away or use it for any more experimenting.”

John H. Mills never again appeared in Westfield. He died in 1908, and he probably never stopped experimenting.