Heating the Central Park Arsenal

There’s a beautiful historic building in New York City’s Central Park called the Arsenal. It was constructed between 1847 and 1851 and designed to look like a castellated fortress because it was to be a place where they would store munitions. In this episode, Dan Holohan tells the tale of the Arsenal, its heating system, and how it has served the city through the years.


Episode Transcript

In New York’s Central Park, there’s a building across a walkway from the Zoo that has been there longer than the park.

It is The Arsenal. When I was a boy, we lived not far from there and my parents would take my big brother and me to watch the bronze animals in the Delacorte Music Clock twirl on the hour. That magical clock is right there by the zoo, and very near The Arsenal. This is one of the delicate pleasures of growing old. We can stand in a place and remember a sweet time that will never return. The day I visited The Arsenal as an old man I listened to the clock and remembered. As a boy, The Arsenal was just a building to me, like all the other buildings in Manhattan. I didn’t even realize it was old. It was just there. But now I see it as a place where I left a bit of myself one January day. And that makes it precious to me.

The State of New York built The Arsenal between 1847 and 1851 and designed it to look like a castellated fortress because it was to be a place where they would store munitions. There wasn’t much else around it. But then came the idea of a central park in Manhattan, a place where people could enjoy nature, regardless of their social standing, so the city laid claim to the building and, in 1857, it became a police precinct. Twelve years later, it became the temporary home of the Museum of Natural History, as the now-iconic museum was rising on Central Park West.

Years passed and the Arsenal became, in turn, a menagerie for P.T. Barnum, an art museum, and also home to New York City’s Weather Bureau. The Parks Department took up residence in 1914 and set out to repair the damage done to the stone exterior by the burning of coal in so many boilers. In 1934, it became the home of the new, citywide Parks Department under the direction of Robert Moses, who set out to transform the City of New York, by building bridges and tunnels, parkways and expressways, and so much more.

It’s not just a building, The Arsenal. I knew that when I visited as an old man. And that’s one of the nice things about growing old. Age brings with it an understanding and an appreciation of so many things. I was there because a friend at the Parks Department asked me to take a look at the steam heating system that was misbehaving. I smiled. Who could say no to that?

The Arsenal is one of only two Parks Department buildings that take heat from the district steam system run by Consolidated Edison. High-pressure steam enters from under Fifth Avenue and goes immediately through two pressure-reducing valves, piped in series. These drop the steam pressure from well more than 100-psi to a just few pounds, which is the correct pressure for the building’s cast-iron radiators. The problem, though, was that it was taking two hours for steam to reach the radiators once it had passed through the PRV station, and that had a lot of people very upset. Those who were getting heat were getting too much heat. They opened the windows. Those who weren’t getting heat were complaining. Wouldn’t you?

So we began at the PRV station with the simple question that is always worth asking when it comes to steam: If I were air, could I get out? And then we got nosey. We got a bright flashlight and a couple of guys who work in the building every day. We followed the pipes. We climbed ladders. We looked into drop ceilings. We smiled and apologized as we disturbed people in their meetings. They watched as we shone the light. We asked them if they were comfortable, if their radiators made noise, if they had any particular problems. They gave us clues and we paid attention.

Some of the radiators were mounted on the ceiling. That’s not unusual with a system such as this one. Steam doesn’t know up from down. It’s a gas. It’s looking for a way out. It’s looking for an air vent. If a room’s heat loss is, say, 10,000 Btuh, and the radiator can deliver 10,000 Btuh, it doesn’t matter where in the room that radiator is. And that’s why we often find steam radiators screwed to the ceiling. And it gives the tenants more room for their furniture.

And each of these radiators has a thermostatic radiator trap. A trap is nothing more than an automatic valve that is sensitive to temperature. Steam pushes the air down the pipes like a plunger. Steam and air have different densities so they can’t mix. The air goes through the radiator and through the steam trap and then into the condensate-return pipe. The trap closes on temperature; the steam gives up its latent heat to the radiator, and condensate forms. The trap opens and the pressure differential across the trap moves the condensate into the return line. The whole process is a study in simplicity.

So why was it taking two hours for the steam to reach the radiators?

As we were walking, we kept asking that important question: If I were air, could I get out? We got to the end of each steam main and we saw a large float & thermostatic trap. That was as it should be because the return lines from the radiators are right next to the steam mains. Without the end-of-main traps, steam would cross over into the return lines and that would put pressure on both the supply- and return pipes. The system would stop working because air would be trapped inside the radiators. It’s all about pressure differential. Where there is no difference in pressure, there can be no movement of fluids or gases. It’s that simple.

We walked on, shining the light and following the condensate return lines. We got to the ends of those lines and saw more float & thermostatic traps. “They’re not supposed to be there,” I said.

“But they’ve always been there,” one of the men who works there every day said.

“There are steam traps on the radiators and F&T traps at the ends of the steam mains,” I explained. “That’s as it should be and that is enough. There’s no steam pressure beyond those points because the traps are working.”

“I don’t get it,” he said.

“If you were air, could you get out?”

“I’m not sure,” he said.

“See it in your mind’s eye,” I said. “The steam supply and condensate return lines are both up at the ceiling. They’re practically level, pitching only one-inch in 20 feet. The F&T traps at the ends of the steam mains are only a few inches higher than the totally unnecessary F&T traps at the ends of the condensate return lines. Since there’s no steam pressure beyond the end-of-the-steam-main traps, there’s no difference in pressure to move the condensate though those unnecessary traps at the ends of the condensate return line. They stay closed. So the pipes between the traps fill with water. Air won’t vent through water, will it?” He shook his head no.

So they got some tools and removed the unnecessary traps. It didn’t take long. They turned on the steam and it was at every radiator 10 minutes later. The building was balanced and quiet. They lowered the steam pressure even more and things got even better. That’s the piece of me that I left there. My knowledge. I gained that knowledge by reading, and watching, and paying attention to everyone and everything.

Before we left that day, my friend took me into the boardroom. The Parks Commissioner was having a meeting. My friend excused us and told them I was fixing their heating system and wanted to see the map. They smiled. I smiled. And then I walked to the map of Central Park that was nearly as wide as that long wall. Fredrick Law Olmsted, the designer of the park, had drawn it by hand. It was his proposal for a central park in the City of New York, the place where I was born, the place where I played as a boy. This was such a wonder to me. It was beautifully colored paper, come to life. The whole park. On paper. Drawn by hand.

The following summer, I was in Central Park with The Lovely Marianne. We were watching the seals play in their pool. I turned around and looked at The Arsenal. There had been a steam main vent that someone had installed outside the building, near one of the upper-floor windows. It had been spewing steam on the day that I visited. It had no business being there, but I figure someone had gotten frustrated along the way and tried this. Some days you can get to the point where you’re willing to try anything. And that includes punching holes through stone to install main vents outside the building. “They got rid of the main vent,” I said, pointing upward.

“What are you talking about?” Marianne said.

“Up there. There used to be a steam vent sticking out through the wall.” I pointed again. “And someone double-trapped all their return lines. That’s never a good idea.”

She gave me that look that she gives me when I get like that. She didn’t bother looking up for the missing main vent. “It’s almost eleven,” she said. “Let’s go listen to the clock play music.”

“And watch the animals dance,” I said.


I hope you liked that tale. If so, please subscribe to our podcast, and join us again next week. Thanks.

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