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Steam unit heaters (and what sometimes goes wrong with them)

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Author
Dan Holohan
Published
June 28, 2012
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So there’s this steam unit heater that won’t put out what it’s supposed to put out. It’s wimpy. The guy with the problem is telling me about it and he’s gesturing like he’s swatting bees. “The fan comes on and I get this hot blast, but then it goes cold,” he says. “It’s like half the coil is hot and the other half is, well, meh.”

            “What have you done so far?” I ask.

            “I’ve been raising the pressure,” he says, swatting upward.

            “Of course you have,” I say.

            “But it’s still not working,” he says. “Think I should raise it some more?”

            In case you don’t know, the urge to crank up steam pressure is written into the genetic code of most American heating contractors. It goes back to the days of steam power. Ever notice how you get all drooly when in the presence of a steam locomotive? It’s all that raw power. Contractors have been sending me photos of steam locomotives for decades, even though steam power has nothing to do with steam heating. Equating the two is like going to an electrician for an electrocardiogram.

            “How about if I raise it to nine pounds,” he says. “It’s at seven now.”

            “Tell me about the piping,” I say.

            “It’s steel pipe,” he says. “I don’t use copper on steam.”

            “No, I mean where the pipes are,” I say. “Tell me about the supply and return piping to the heater. And what else is nearby?”

            That’s the thing about steam heating. It’s old and it can be scary, and it’s a system that reaches beyond the spot where the problem appears. If you look at just one part of that system, you probably won’t be able to solve the problem. You have to wander around, and you have to think in terms of basic physics. If you were the steam, what would you do? If you were air, could you get out?

            Don’t let the physics scare you. Most of it is common sense and you already know everything you need to know. Most of it, you learned in grade school.

            For instance: Water can be in three states: solid, liquid and gas. You know that already, right? Water will change temperature when you add or remove Btus. It takes one Btu to raise one pound of water (that’s about a pint) one degree Fahrenheit. When the pound of water reaches 212 degrees, it’s still liquid. We call the heat that it takes to get to that point “sensible” heat because you can sense it with a thermometer. But to make that pound of water boil, you have to add 970 more Btus. We call that “latent” heat because the thermometer can’t see it. And that’s the stuff that heats the building.

            Once you have steam, the only reason you need pressure is to overcome the friction loss the steam suffers as it travels though the piping on its way to the heater. An industry group called The Carbon Club set that standard in December 1899 when they got together at the Murray Hill Hotel in New York City and everyone agreed to use a pipe-sizing chart that would have the steam take a pressure drop of just one once for every 100 feet it traveled through the pipes. This is how the Empire State Building manages to stay cozy warm with just 1-1/2-psi pressure on most days.

            So crank down the pressure. No need for Casey Jones on this job.  

            You know most of this already, right? You probably also know that when water turns to steam it expands quite a bit. The ratio is about 1,700:1 and that’s at zero psi. Once it enters the pipe, the steam will shove air ahead of itself because air is heavier than steam and the two gases won’t mix. The steam plunges the air down the pipe. The air looks for a way out, and if it finds a vent, the heater will get hot and there will be peace in the valley. But if the air can’t get out, the steam can’t get in, and the client will be miserable.

            “So about that supply and return piping,” I say. “What’s going on?”

            “Well, there’s a control valve on the heater,” the contractor says.

            “What about on the return side? What’s there?”
            “There’s a float-and-thermostatic trap,” he says.

            “Where does the pipe go from there?”

            “It goes to a condensate pump,” he says. “And then back to the boiler.”

            That’s a typical setup. You’ve probably seen it many times. But now I’d like you to think like steam and act like air and water. Before the steam arrives, the heater and all the piping leading to and from the heater contain air. Every steam system ever built starts out that way. It’s filled with air and the steam has to push that air out. This happens on every heating cycle. The steam has to shove that air out of its way before it can reach the heater.

            “How does the piping run from the F&T trap to the inlet of the condensate pump?” I ask.

            “It runs down to the floor, across the room, and then it goes up into the receiver’s inlet,” he says.

            I nod.

            Can you see a problem with this?

            When you drop a pipe to the floor and bring it up it forms a water seal, right? In plumbing, you call that a P-trap. You know what a P-trap is supposed to do, don’t you? Its job is to stop gas in its tracks.

            And what is air if not a gas?

            “How does the air that’s inside the heater supposed to get to the vented condensate-pump’s receiver?” I ask.

            “The steam pushes it through the trap and through the return pipe,” he says.

            “What’s the job of a steam trap?” I ask.

            “To trap steam,” he says.

            “So there’s no steam pressure beyond the trap, is there?”
            “Uh, no.”

            “And there’s a water seal in that return line. Air won’t vent through a water seal. And if the air can’t get out of the heater, the steam will just compress it inside the heater, giving you the problem you’re having now.”

            At this point, he smacks himself in the forehead.

            But wait, there’s more. He also has a control valve on the supply side of the heater. When that valve shuts, the steam inside the heater will condense, shrinking in volume by about 1,700 times. Since there’s no way for air to get back into the heater, you wind up with a vacuum inside the heater.

            Have you ever held your finger over the top of a straw and then lifted it straw out of the glass? The water stays in the straw, right?

            You learned that in grade school.           

            “You need a vacuum breaker between the control valve and the heater,” I say. “And you also need a high-capacity air vent on the outlet side of the F&T trap.”

            “I get it,” he says. “That’s to give the air a way out.”

            “Exactly, but that vent also serves another purpose,” I say. “When an F&T trap discharges, you get a bit of flash steam in the return. That flash steam will condense in the pipe on the outlet side of the trap and form a vacuum, and because you have a water seal in that line. The vacuum will slow the flow of water between the trap and the condensate pump and that’s not good. By placing the vent on the outlet side of the F&T trap rather than on its inlet side, the vent will do double duty. It will vent air, and also break any vacuum that forms when the trap closes.”

            “This all seems so simple now,” he says.

            “That’s because you’re seeing it as a system now, and not just as a wimpy heater. Are there any other heaters nearby?”

            “No. Why do you ask?”

            “Because a failed trap in the open position can screw up a nearby heater that has a good trap. Bad traps create backpressure in the return line. Just keep thinking about systems.”

            “So you think I should crank down the pressure?”

            I just looked at him.