How Pumps Can Create Air Problems In Hot-Water Heating Systems
Pump and Circumstance
I am the proud father of four college graduates, and this being the time of year of many graduations, my thoughts naturally turn to higher education, brought to most of us through the School of Hard Knocks, and at great expense. Some of us, however, take the courses, pay the tuition, and won't learn.
Which brings me to pumps and a particular circumstance. I can tell this now that he's gone.
Smokey is currently on the other side of the lawn, but when he was with us, he was magnificent in his thick-headedness. As is the case with some in this trade, Smokey was born with the entire world's knowledge already in his head and didn't feel he ever had to ask for advice. He would take on heating jobs with the total confidence that comes with a thick skull, and when things went south, he would continue to move in the wrong direction because that is the nature of a man who already knows everything. Physics, biology and mathematics mean nothing to such men.
Smokey believed that cigarettes were harmless because his father had had an uncle who had smoked until he was nearly 90 years old, and if it was possible for that man to smoke for so many years without ill effect, then all the doctors in the world were wrong. For Smokey, the exception was the rule, and he was that exception.
Smokey used one match in the morning to get himself lit off and this was only because he had to sleep for a bit at night. If he could have kept last night's last cigarette burning through the wee hours, he would have used its still-glowing butt to get himself going at first light, but no one makes such a cigarette, so he used the one match and sucked smoke all day, as though it was a religious calling.
He asked for my help one day, but in a way that fit his character, meaning that he didn't actually ask for help (men who know everything require no help). He merely called to say that I really needed to see this horrible air separator, which was never able to vent all the air from this heating system that he had just installed because it wasn't as good as the ones they used to make.
"It vents whenever the pumps are running," he said. "It's never finished. I hate it, and you should hate it too. You should write about it in the magazines and tell everyone to stop buying this air separator because it never stops venting. It can't get the job done."
"Where is all this air coming from?" I asked.
"The air separator is making it," Smokey said, and there was not a trace of doubt in his voice. The man was born with his mind made up.
So we got together on the job, which had radiant tubing in a concrete slab. It was Smokey's first radiant job, but that was not a problem because Smokey already knew all there was to know about radiant floor heating. "You put the tubing in the concrete," he said. "You heat the water, and you pump it through the tubing. The building gets hot."
See what I mean? Magnificent.
It was around 9:30 in the morning, and Smokey was bashing his second box of Marlboro's for the day into his left palm before tearing at the cellophane with his teeth. "These pumps suck, too," he said. "I want you to tell people about them. Tell them not to buy them."
"Why?" I asked.
"Look at how many you need to get the job done!" he said.
There were six of them, and Smokey had bolted them together, flange to flange, and on the return side of this cast-iron boiler.
"You get a special on these circulators?" I asked.
"That's how many you need to get anything moving through the tubing," he said. He lit another Marlboro and left the stub of the last one burning on the edge of the boiler.
"How much tubing do you have on this job?" I asked.
"I don't know," he said, "probably more than a thousand feet."
There were no manifolds. The boiler had a 1-1/2" supply, which Smokey had reduced to 3/8" after leaving the air separator (which never seemed to be able to finish the job). The single tube then entered the concrete floor like a deep-space voyager and eventually returned from the concrete on the other side of the boiler, where it connected to an air vent that was the size of a can of Budweiser. The first of the six circulators followed. From there, it was a hydronic daisy chain of pure pressure. It looked like the Six Flags of heating.
"You don't believe in radiant circuits, do you?" I said.
"Wadda ya mean?"
"A thousand feet of tubing, broken up into, say, five 200-foot circuits?"
"Too many chances for leaks," Smokey said.
"You buried more than a thousand feet of three-eighth-inch tubing as one long piece, didn't you?"
Smokey lit another Marlboro, spit a fleck of biomaterial, and nodded. "Yep."
"That's why you need all these pumps," I said.
"I know," he said. "One's not strong enough. You should write about that."
"Oh, I will," I said. "And that's quite a boiler bypass."
Smokey has piped a half-inch copper line from the supply side of the boiler to the suction side of his circulator railroad. There was a ball valve in the line for balance. It was closed.
"Gotta protect the boiler from low temp," Smokey said.
"I'll bet the boiler comes up to temperature quickly."
"Like a rocket," Smokey said. "But you're here for that lousy air separator. Listen to that thing."
It was whooshing, all right. It was nearly as loud as the Budweiser-size air vent on the suction side of Smokey's circulator sculpture. He was staring at the air separator and shaking his head. "Garbage," he said, exhaling a cloud of smoke, which I watched swirl right into the Bud vent.
"Have you ever noticed," I said, "how blowing and sucking sound about the same?"
"Wadda ya mean?" Smokey said.
"I mean how air, when moving quickly through a hole, makes about the same sound, whether it's going this way or that way. Blowing or sucking? Ever notice that?"
"Can't say as I have," Smokey said.
He lit another smoke, blew out, and once again, the Bud vent sucked it right in.
"Do you know about the point of no pressure change? How it's at the place where the compression tank is? And if you pump toward that point with a big enough pump, or with, say, six pumps piped in series, stem to stern like this, you're going to get quite a negative pressure on the suction side of that pump. Or pumps."
"Blow some more smoke that way," I said, pointing to the Bud vent. He did. The vent sucked in the smoke nearly as quickly as Smokey was blowing it out.
"Wow!" Smokey said. "A system after me own heart!"
"Now watch over here," I said, pointing at the air separator. He did. "You won't see the smoke come out because this isn't a hookah."
"You're sucking in air because you're pumping at the point of no pressure change with lots of pumps in series. The combined pump differential pressure is greater than the system's static fill pressure. That's why the vent is sucking. The air is probably eating this boiler. The air separator is trying its best to get rid of the air that the vent is sucking in," I said. "Sucking and blowing."
"I hate that thing," Smokey said.
"Under the circumstances," I said, "I think you should be thanking it."
"I hate it. You're not going to help me get even by writing about this, are you?" Smokey snarled.
"I will when it's time," I said.
"When you graduate," I said.
And now it's time.
Did you like this story and want to hear more? Check out Dan Holohan's What Hydronics Taught Holohan: A Memoir of Life in the Heating Industry.