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Orifices and gravity hot-water heat

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Published
October 31, 2011
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Gravity hot-water heating is both the simplest and the most complicated system ever to come driving down Hydronic Highway. It’s simple because it has so few moving parts. The water is the only true moving part. Hot water rises; cold water sinks; it’s that simple. But at the same time, it’s complicated because the pipes have to be just the right size for the amount of water that’s flowing, and they have to have the perfect pitch. The fitter has to make sure to remove the burrs from the pipes he cuts, otherwise there could be too much friction presented to the flow. This system is so delicate that a burr can screw you up. Imagine that.

It’s a fine place to learn about modern systems.

Many of the old boilers the Dead Men used on their gravity systems had two outlets, and those pipes were large. The goal was to reduce the resistance to flow so the heated water could rise. Two pipes (and big ones) allow for more flow, and that’s why you’ll see those multiple tappings on those classic, gravity systems.

            You’re also limited to a building no taller than three stories because when the hot water rises, it gives up its heat to the radiators. Beyond three stories, the hot water pretty much stops rising because the water has lost much of its heat. These systems worked by the difference in density between the hot water and the cold water. It’s like a helium balloon. It will go just so high and then stop rising.

            So here’s a good question for you, as long as we’re talking about helium balloons. If you were rising from an old boiler, which floor would you head for first?

Would you go to the first floor, the second floor, or the third floor?

            Think it over. I’ll wait.

            Did you say the first floor? Well, if there was a circulator on the system, the water would definitely go to the first floor first. That’s because water is lazy and will always follow the path of least resistance. With a circulator, that path of least resistance is the shortest route from the circulator’s discharge back to its suction.

            But without a circulator, the path of least resistance is the top floor (think helium balloon). The water climbs the pipe, bypassing the first- and second-floor radiators. That means there are unhappy people in this house, and all because of the other there’s less difference in pressure between the boiler and the top of these old systems than there is between the boiler and the first- and second-floor radiators.

So to fix this, the Dead Men used something quite clever, and they told each other about it, but they never wrote it down so that you and I, stuck here in the 21st Century would know. What they did was add orifice plates to the top-floor radiators. They slipped them into the radiator supply valves’ union connections. An orifice is nothing more than a thin, round piece of metal with a hole poked through its center. It’s there to create pressure drop, and to send the water somewhere else.                                    

            An old-timer once told me that his long-gone father used to smoke Prince Albert tobacco because it came in a can. His dad saved the cans and used a punch and a nail to make his own orifices. His homemade orifices worked, and he told his son about it, but he didn’t tell me. 

Which brings us to Mr. Hicks.

            When F. Scott Fitzgerald wrote The Great Gatsby, he used as his model a couple of the palatial homes on the north shore of Long Island. Just south of where those homes sprawled is a town called Westbury, and in that town there was a small fuel-oil dealer that went by the name of Hicks-Westbury. When I worked for the manufacturers’ rep, I used to call on those good folks. I was about 12 years old at the time, or at least that’s how old I felt when I went to see Mr. Hicks. He grew up delivering oil to those mansions and many other homes in the area. He also serviced all these classic hydronic systems, many of which were of the gravity-hot-water variety.

            My job with the rep at the time was to be the Contractor Boy. I had no direct sales duties. I was just on call to help any contractor who needed a second pair of eyes on any sort of problem job. My main talent (and I didn’t have much else at that age) was that I knew how to use the library. This was way before the Internet. I used the libraries in New York City and on Long Island, and I showed great respect to men such as Mr. Hicks. When I had a question, he would answer it, and I would then share what I learned with the contractor I was helping. It’s amazing what older people will share with you if you just show them the respect they deserve.

            The contractor had installed a new boiler in a big house that had gravity hot-water heat. The new boiler included a circulator, of course. When he started the new boiler, water flowed to the closest available radiators, which happened to be on the first floor of the house, and then back to the boiler. This is because water, as we’ve already agreed, is lazy.

            The contractor figured he had an air problem up at the top floor, and that’s why those radiators weren’t getting hot. He was wrong, but he was also persistent, and he had already made up his mind. We get like that some time. We make up our minds while still in the truck, and then we set out to prove that we’re right, in spite of all the evidence.

            It’s human nature.

 The evidence, in this case, was that when the contractor bled the top floor radiators, he didn’t get any air. He just got water. And the thing is, when you don’t get any air, it ain’t an air problem. It’s a balance problem.

            But I hadn’t learned that yet, in spite of being the Contractor Boy. I just knelt next to those top-floor radiators with that miserable contractor and helped him bleed the radiators, which contained no air. We labored on for hours, with great hope and anticipation, but it was no dice. The radiators would get hot for a while because we  were actually draining the system from the top, but then they’d go cold again. The did this because there was no flow once we stopped draining. And where there is no flow, there is no heat.

            But I wasn’t thinking like that then. We just looked at each other. We did that a lot that day.

            I finally told the guy I’d get back to him and then I went to visit Mr. Hicks. That grand old man sat and listened to my tale of woe and then said, “Did you move the orifices that are inside the upper-floor supply valves to the radiator supply valves on the lower floors?”

            “What’s an orifice?” I asked.

            Mr. Hicks shook his head sadly and said, “Boy, you’re a stupid kid.”

            And I was. But that’s how I learned about what I’m telling you here. It wasn’t an air problem; it was a balancing problem, and those two problems wear the same work clothes.

It’s really tough to tell them apart, especially once you’ve decided that this is a definitely an air problem, even though it ain’t. 

These days, I have other people who sometimes call me stupid, and whenever that happens, I smile and think of Mr. Hicks.

We’re all stupid now and then, right?