My teacher, the late, great Gil Carlson, used to talk about The Law of the Tee, which states: “Whatever goes into a tee must come out.” To which you’re probably saying, “Duh!”

I know. It does sounds obvious and a bit silly, but it’s the hinge pin of most hydronic systems, and that’s why Gil always brought it up. If the water comes out of a tee the wrong way, you’re in trouble.

The Dead Men struggled with that law since the very beginning. The first hot-water heating system, built in 1777 in Paris to warm chickens in a poultry barn, contained mostly elbows. The single heating pipe laced back and forth under the chickens, which lived in small boxes on several floors. The only tee I see in the old pen-and-ink drawing is the one connecting the top of the high point of the heating pipe to the overflow tank, which grew up to become what we call an expansion tank.

The true tee, the one Gil had in mind, showed up later, around the time when the Dead Men started to use radiators. Those tees sent the hot water either through or around a radiator, and I imagine many of those folks learned about The Law of the Tee the hard way.

Time went by and steam became very popular in the United States. The English had thought of it first, but then had the good sense to realize those systems had a habit of exploding, so they let the Americans run with it. This happened at a time when the heating industry was split in two, with some of the Dead Men promoting the less-expensive steam heat, while others went for the much-safer, gravity hot-water systems.

The latter was simple in principle but very complicated in installation because the hot water always wanted to go to the top of the system first. The Dead Men used a bunch of piping tricks to coax the hot water into the radiators on the lower floors, and some of those tricks involved tees. For instance, they’d pipe a first-floor radiator off the run of a vertical tee, whose bull would continue (through an elbow) to the upper floors. Whatever goes into a tee must come out.

The Dead Men made it easier for the hot water to come out of the run of that tee and head for the first-floor radiator first. Simple and effective.

Another challenge of gravity hot-water heating was the horizontal mains in the basement heated from the rounded top of the main to its bottom. That happened because there were no circulators at the time. The water stratified inside the pipe: hot on top, cooler on the bottom.

To deal with this, a company called Phelps invented a tee with two bulls, which I suppose took it out of the realm of being an actual tee, but bear with me. The run of the Phelps tee was in the horizontal main. Knowing the water would stratify in that main, with the hottest water being at the top, Phelps had a bull come directly off the top, allowing the hottest water to rise to the radiator. It set the second bull at 90° to the first bull and positioned it near the bottom of the tee.

Set up this way, the hottest water would rise off the top of the main and the return from that same radiator would reenter the main at the bottom of the tee, where the cooler water was.

My guess is the Phelps tee didn’t work out very well in the field, though. I’ve never seen any on a job, nor have I heard from anyone else who has, but the drawings of them in my antique engineering books tell me the concept was a good one. It’s just that better things would follow.

A chain reaction of ideas

And it didn’t take very long. Mark C. Honeywell, that ambitious plumber from Florida, was patenting all sorts of clever devices in those Fin de siècle days.

One of his ideas was a tee that would work with the stratifying of the water in those horizontal mains. This tee’s run would go in the horizontal main with the bull pointing upward toward the radiator. The trick, though, was the cuff built into the inside of the tee. It was sort of a pipe within a pipe, with a space near the bottom of the tee that the hot water could slide under when there was enough of it.

The hot water would have to completely fill the main from top to bottom before any of the water could slip under the cuff inside Honeywell’s tee. If a Dead Man had a number of radiators hooked up along a main, they would all get the hot water at the same time because no hot water could move from those tees until the whole main was hot from top to bottom. And as soon as the hot water entered the radiators, the cooler water inside the radiators would fall by gravity through a second, identical Honeywell tee on the return side.

It did a nice job of balancing the system in those days. If you looked at this tee from the outside, you would never know there was anything special about it. But now you do.

And when one person has a good idea, more people are bound to do the same. A company called Eureka came out with a tee that looked like it would do well in a modern primary-secondary system, if it wasn’t for the internal baffle.

Eureka’s tee had four tappings, making it, like the Phelps fitting, a questionable “tee,” but it worked well. The run of the tee was in the horizontal main. The flow and return tappings were set next to each other at the top of the tee. The stratified water would move by gravity down the main and the hottest water would rise through the flow tapping and go to the radiator. The cooler water in the radiator would fall by gravity and reenter the Eureka tee through the return tapping.

What made the magic happen was the vertical baffle between the flow and return tappings. When I see this one, I think of the diverter tees that would show up later from companies such as Bell & Gossett and Taco. And yes, I have seen Eureka tees in the field.

My favorite of all, though, was the O-S fitting, and mainly because the name came from the initials of its inventor, Oliver Schlemmer of Cincinnati fame. I favor this tee because I just love saying that guy’s name. Try it. It sounds like you’re biting into a crisp apple. Oliver Schlemmer!

And his tee was the first to really go after the steam fitters with hot-water heat. The O-S fitting was a true diverter tee, which led to the development of one-pipe, hot-water heating, a system that could compete with one-pipe steam. Other diverter tees would follow, of course, and these would change the direction of hydronic heating in the years following World War II, when so many tract houses went up, many of them with convectors fed off of diverter tees.

These days, I think the most special tees are the ones designed to make primary-secondary systems easier for you to install. Taco’s Twin Tee and Webstone’s Pro-Pal Purge Tee are great examples. Both evolved out of the problems contractors were having in the field, just as the older tees did. And that’s my favorite part of this. It pays to listen to the people who do the actual work.


Another wonderful hydronic educational nugget from Dan. Did you learn anything new? Tell us in the comments section below!