Published: November 21, 2014 - by Dan Holohan

Categories: Steam


The basement was as musty as a used bookstore and the shadows thrown by the single 100-watt light bulb hung like old clothes against the walls. I felt as though I'd walked back seventy years in time.

"They used to burn coal," he said as he shot the beam from his flashlight into the gloom.

"I know," I said. "I can tell by the piping."

"The piping around the boiler?" he asked.

"No, the main," I said. "Look at the way it runs completely around the basement." I aimed my light at the three-inch horizontal pipe which girdled the entire basement before dropping into a short wet return back at the boiler.

"What does that have to do with coal?" he asked.

"This is the way they ran mains during the coal era," I said. "The boiler would come on and run for about ten hours without stopping. They had all day to vent air. That's why they ran the main this way. One long pipe, all the way around the basement.

"I never gave that much thought," he said. "Does it make a difference with what we're doing?"

"When we replace the boiler? Yeah, it sure does!" I said. "The old-timers learned this the hard way in the Thirties when they began to convert these jobs to oil."

"What happened?"

"Well, many of them had uneven heating problems. That's because the length of the firing cycle changed," I explained. "That's the main difference between a coal boiler and an oil boiler, you know. The oil burner cycles on and off."

"But what does that have to do with the length of the main?" he asked. I could tell he was getting a bit confused.

"I'll tell you, but you have to promise to use your mind's eye to see what I'm talking about here. If you don't, you'll never get it, okay?"


"Now think your way into the system. Before you start the boiler, all the pipes above the water line are filled with air, right?"


"And as the steam leaves the boiler, it has to push all the air out of the pipes in order to reach the radiators."

"I know that," he said.

"Okay, so how does the air get out?" I asked.

"Through the radiator air vents."

"Correct. And which radiators get hot first?"

"The ones closest to the boiler," he said.

"And which radiators get hot last?"

"The ones furthest from the boiler," he answered.

"So where do you put the thermostat?" I asked.

He started to laugh. "That's always the problem, hasn't it?"

"Yeah, it sure has," I said. "But back in the coal days, it wasn't that much of a problem because the boiler ran all day long. It never shut down. Remember that ten-hour cycle I mentioned before? That made all the difference in the world."

"You mean the radiators had more of a chance of heating all the way across?"

"Yeah, that's why the old-timers ran the main the way they did - one long, girdling main, completely around the basement with short take-offs to the radiators. They knew all the radiators would eventually get hot because the boiler ran for ten hours."

"So what happens with an oil boiler?" he asked.

"Think about it," I said. "The boiler comes on and very quickly makes steam. The steam rushes out of the boiler and quickly enters the main, shoving the air out of the near-boiler radiators. They get hot and shut off the thermostat which is usually somewhere in the middle of the building."

"It shuts the burner off before all the radiators on the main are hot!" he blurted. He was seeing it in his mind's eye.

"Right. The oil boiler cycles on and off. The coal boiler didn't. It wasn't long before the oil men figured this out. They realized they couldn't pipe their new, oil-fired jobs with those long girdling mains. They began to do things in a different way. Instead of one long main, they ran several shorter mains off the steam boiler's header. They sized these shorter mains so the pressure drop through each was more or less equal. It was textbook stuff. Piped this way, they had a better chance of getting the steam to all the radiators on the first firing cycle."

"I've seen systems piped that way," he said, "but I never gave it much thought."

"Those were systems piped with oil in mind," I said. "And they did one other very important thing with those systems."

"What's that?"

"They made sure they used a high-capacity main vent near the end of each of their mains."

"Right at the end?"

"No, they backed each vent in about 15 inches and put it up on a nipple to protect it from water hammer which sometimes hits the end of the main."

"I've seen those, too," he said. "So what do we do with this job?"

"Well, since it was piped for coal, it's probably going to give you a problem when you change the boiler. Remember, the new steam boilers we use nowadays don't contain nearly as much water as the old ones we take out. They go off on low water a lot quicker."

"Tell me about it!" he laughed.

"The new boiler needs a lot of help in getting the air out if you're going to get the steam around this long main before it cycles off on temperature or high-pressure." I traced the path of the main with my flashlight.

"I see what you mean," he said.

"The only way you're going to be successful is to vent the end of this main with several main vents hooked up on a manifold. In fact, If you want it to work even better, add a main vent or two along the run. That's the way the old-timers frequently did it when they convected an old coal job to oil. They couldn't repipe the mains, so the did the next best thing. They vented the heck out of them."

"Can I drill and tap the main for a vent," he asked.

"Yeah, you can," I said, "But remember, the vent will only vent as quickly as the hole in the pipe will allow. If you tap with an eighth-inch hole, you're not going to vent much air. It's better to cut a tee into the line and vent through at least a 3/4" hole."

"That means I have to cut and thread the pipe in place," he said. "That's not going to be easy."

"Who said anything about things being easy?" I asked. We both laughed.

"How come the old boiler didn't need all these vents?" he asked. "I just see one main vent down there." He pointed it out with his light.

"Well, this system would have worked better with move vents. I can tell you that for sure. The reason it's not short-cycling is because of its water volume. You have a lot of water to work with in this old beast."

"I won't have much water in my new boiler," he said, rubbing his beard.

"No you won't. And that's why the problem is going to pop up when you change the boiler. The customer will swear it never happened before he met you, and you're going to left holding the bag."

"That happens all too often in my life," he said.

"There's no reason it should," I said. "Not if you're aware of the piping as well as the boiler when you price the job. Look long and hard at the mains and notice the way they run through the basement. If you see a long, girdling main such as this one, you'll know you're dealing with an old coal-burning system, so make provisions for it."

"You mean by adding main vents?"

"Right. Figure them into your price, as well as the labor for adding those middle-of-the-main vents which help a great deal. Tell your customer what he has down there in his basement. Give him a little history lesson; most people love that. Explain the difference between an oil fire and a coal fire and how the system air reacts to each. Help him to see it all in his imagination. Believe me, when you do this, the building owner's going to know you know your stuff. The next guy who gives him a price will probably miss these details. You're bound to get the job, even though your price may be a bit higher."

"And I'll also keep myself out of trouble," he said.

"That's the best part!" I said.

"Can't I solve this problem with adjustable air vents at the radiator?"

"Nope. The old-timers learned this the hard way, too. Everything I've ever read on old steam systems tells me you can't be successful without those main vents. You have to treat the air in the mains as a separate problem from the air in the radiators. When you do this, things suddenly begin to work. Try it, you'll see."

"I will. But first, I have to go upstairs and give the home owner a little history lesson!" He gave me a smile that could light up even the darkest basement.