I read somewhere that the most prevalent machine in the world is the electric motor. I thought about that for a while and it made sense. Just look around. But then I began to wonder what the second-most prevalent machine in the world is. Turns out it’s the pump! And most pumps are connected to electric motors of one kind or another, so there you go. Throw a rock and you’ll hit a motor. Or a pump. They’re everywhere.  

But that wasn’t always the case, which brings me to a story. Back in the days of gravity-hot-water heating, before we used pumps, a contractor would install big pipes to keep the resistance to flow at a minimum. The boiler would heat the water, which would then flow languidly upward and toward the radiators. It would nudge the colder water that filled the pipes and radiators above the boiler downward, creating a wonderful Ferris wheel of heat. What goes up, shoves what goes down out of the way, and if you’re using gravity, you can rest assured that it will be dependable. Hard to get away from gravity.

If the contractor followed the rules of gravity-heat piping, the big wheel of water that is a hydronic system would turn like a Ferris wheel and the customers would say, “Ahh!”     

But then 1928 arrived, and with it came the simultaneous introduction of the hot-water circulator in both the US and Germany. Homer Thrush introduced his Thrush circulator to America US. Louis Opländer did the same in Germany with a company he dubbed Wilo. Let’s hear it for paradigm shifts.

These new-fangled pumps sped up the water that flowed in gravity systems, and when combined with a flow-control valve, gave the contractor a way to control the flow of hot water between a boiler and the radiators by having one of those new gizmos they called a thermostat start and stop the pump.

Those early pumps were able to move a lot of water against a relatively low resistance to flow because the pipes in a gravity-hot-water system were large. That kept the resistance to flow to a minimum. The Dead Men didn’t need much of what we came to call pump head in those days. They did, however, have to get used to the fact that pump head didn’t refer to the height of the building, but rather to the resistance to flow that the piping offered as the water flowed around the big loop. The height of the building doesn’t matter because the weight of the water going up balances the weight of the water coming down. Think about the motor on a Ferris wheel and you’ll understand how a pump in a closed system really works. It’s not lifting; it’s turning.

And that fact of life was so important to the dead men that they even changed the pump’s name to circulator. They wanted to distinguish what is in a closed system from what’s in an open system. Fire pump, anyone?

Fast-forward 20 years to 1948 or so and we now see packaged boilers. These showed up because boiler manufacturers realized that if they included the circulator and the controls on the boiler, they could make more money while making the contractor’s job easier. The circulators that came with those early packaged boilers all had the ability to move a relatively large flow against a relatively low head. Larger pipes were the norm in those days, and all of those early circulators ran on 1,750-rpm motors. Their flow vs. head performance curves were nearly identical, regardless of the manufacturer.

Which brings us to the 1970s, a time when the price of fuel soared. People became more aware of efficiency, and that led to the birth of the smaller, water-lubricated circulators. These ran at much higher speed, used less power, and cost less to buy. They soon replaced the 1,750-rpm, oil-lubricated circulators that had been on the packaged boilers for years.

And that’s when things got interesting because the little circulators wound up on packaged boilers that ranged in size from small to pretty darn big. And since the boiler manufacturer was providing them, many contractors began to think in terms of one-size-fits-all when it came to the tiny circulators. And that led to problems.

I have this delicious photograph someone sent me years ago. It shows a rather large boiler in an apartment building. A thick pipe leaves that boiler and enters a manifold that feeds 15, 3/4” zones, each with a zone valve.

Here comes the good part:

“Dan,” the sender wrote. “I have a serious problem and maybe you can help. I’m sending you a photo of the boiler and the piping that I installed.”

I looked at the photo again and noted that the circulator was one that Bell & Gossett called Little Red. That circulator was indeed both little and red.

“Dan,” he continued, “when I’m running just a few zones, everything is fine. But when I try to run all 15 zones at the same time, most of them don’t get hot. The tenants are complaining. Do you think the boiler is too small? I didn’t do a heat-loss before I put in the boiler. I sized the new boiler by what was there before. I don’t know what to do now. Help!”

Clearly, he didn’t have enough flow, and since heat travels on the flow like a passenger on a train, he didn’t have enough heat at the radiators. I asked him why he had used that tiny circulator and he told me he had taken it off another boiler because his customer didn’t care for the color red. The contractor didn’t want to waste the circulator so he used it on this job instead. Waste not, want not, right?

I asked him if he remembered that scene in “Jaws” when they realize they’re going to need a bigger boat.


“You’re gonna need a bigger pump.”

He asked how much bigger and we sized it together. And then he asked me who was supposed to pay for that larger pump and I told him that he was because he is the contractor and the contractor is supposed to know more about heating than the people who hired him.

He thought that was terribly unfair.

Not enough flow leads to not enough heat. But too much friction loss through the piping can also lead to not enough heat because too much friction leads to slower flow. Or even to no flow at all.

Here, imagine a large rectangle of pipe that stretches for hundreds of yards. Install that tiny circulator in that rectangle.

Got it?  

Good. Now do you think the water’s going to move in that circuit?

I don’t either. The circulator will sit there with its impeller spinning, but that will be the only thing moving. There’s just too much resistance to flow in that huge circuit. The pressure drop of the circuit will act like two closed service valves, one on either side of the circulator.

Someone will come along and try to purge that circuit. I guarantee that. But purging won’t work for them because this isn’t an air problem; it’s an adventure in hydronic pumping.

Like another example of this? Back in the ‘80s, when hydronic radiant heating was catching the attention of contractors, a guy called to ask what size circulator he needed.

“What sort of job is it?” I asked.

“Radiant,” he said. “I put a thousand feet of three-eighths-inch PEX in a slab. What size pump do I need for that?”

“What’s your longest circuit?” I asked.

“A thousand feet. I just told you.”

“There’s just one circuit?”

“Yeah,” he said.

I told him he was gonna need a bigger pump.

“Maybe call your local Fire Department,” I suggested. “They have really big pumps.”

To which he said, “Huh?”