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When a steam system floods


Dan Holohan
July 16, 2009
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The couple had lived in the old Victorian for five years and it was their hobby as well as their home. They'd been restoring it and they didn't want to part with the old one-pipe steam system, but it had been acting up lately.

"It bangs all the time now," he said. "It didn't use to, but it does now."

"It started all of a sudden," she agreed.

"Do the radiators heat well?" I asked.

"Not like they used to," he said.

"They heated much better before the banging started," she added.

"When did all this banging start?" I asked.

"Right after the flood," he said without hesitation.

"What flood?" I asked. She looked at him and smiled. He laughed and shook his head.

"The flood that I caused," he said. "Our boiler doesn't have an automatic water feeder. I feed it by hand about once a week. That's what I was doing when the phone rang. I set the valve on a trickle and went to answer it. The call was from my son. He'd had a car accident, nothing serious, but it was enough to make me jump in the car and go see. I forgot the valve was open, and by the time I got back, water was pouring out of the second floor radiator vents."

"I was at work," she said, "The water ruined the ceilings we'd just finished restoring."

"And the next day, the banging started," he said. She nodded in agreement.

"When does it bang?" I asked.

"All the time."

I had a feeling about this one right away because I could see the system filled with water in my mind's eye. Let me tell you a bit about it before I explain where the banging came from.

It was a one-pipe steam system with a gravity return. The house was built in 1914 and while the boiler had been replaced once, the piping was original.

There was a 2-1/2" main that ran from the boiler header across the center of the basement and down to the far end of the house. From there, two two-inch mains broke left and right around the perimeter of the basement. Ten, good-sized, column-type radiators fed off the two mains. The total EDR on the radiators was 485 square feet. I knew that some Dead Man had taken great care with the pipe sizing in this house. The radiation load exceeded what a single 2" main in a one-pipe system should handle at low pressure (that would be 386 square feet EDR), so he split the load and used that 2-1/2" main up the middle of the house.

Half the radiators were on the first floor and the other half were on the second floor. The Dead Man had used the proper size runouts to the upstairs radiators. He'd increased the horizontal runout to each riser by one pipe size to accommodate the steam and the condensate in counterflow. All in all, the system looked pretty good. The worn asbestos insulation bothered me a bit, but the homeowners were planning to get rid of it soon.

We fired the boiler, waited about ten minutes and sure enough, the pipes started to knock. The banging was everywhere at first, and then after the system had been on for a while, it moved to the ends of the mains.

We went upstairs and checked out the radiators. They heated only about halfway. The vents panted, and many of them squirted water. I checked the pressuretrol and found someone had set it to cut in at 2 psi and out at 6 psi.

"Who set the pressure this high?" I asked.

"The guy from the Service Company," the man said. "He told us we needed the high pressure to push the heat into the radiators."

"What do you think of his idea?" I asked.

He shrugged his shoulders. "The guy said it would probably take some time, and then he left. I haven't seen him since."

I cranked the pressuretrol down to where it belonged - cut in at 1/2 psi with a 1-psi differential. I would have set it lower if I could have, but this particular unit didn't go any lower than that. I knew we didn't need pressure to push the steam into the radiator. The steam wasn't getting that far. Something was stopping it.

Here's what I want you to see in your mind's eye. This job has 55 feet of 2-1/2" steel pipe, 138 feet of 2" steel pipe and 60 feet of 1-1/4" steel pipe. The total weight of that pipe is about 960 pounds.

The Dead Man who designed the system supported those pipes with pipe hangers. He knew his pipes would contain steam and air (which weighs practically nothing) and condensate, which is heavier than steam and air, but considering the quantity, doesn't weight that much either.

The Dead Man figured his pipes would have to handle about 121 pounds of condensate every hour. At any given minute, there would be about a quart of water (2 pounds) up in the pipes.

Now think about what would happen if you flooded this system. When you include the radiators, this system can hold 105 gallons of water. You know how much 105 gallons of water weighs? About 875 pounds! That's nearly as much as all the pipe in the house.

How do you think the Dead Man's pipe hangers felt about carrying that extra load? Do you think those mains might have sagged a bit under the strain? Me too.

It was hard to see all of this because of the asbestos insulation, but those mains had sagged quite a bit and that's what was causing the water hammer on start up. Steam mains are supposed to pitch a minimum of 1/2" for each 20 feet of run when the steam and the condensate flow in the same direction. The slight pitch allows the water to drain after each cycle. If the water stays in the mains between cycles, the steam will pick it up on the next cycle and drive it down the line where it will smash into the first available elbow.

So the extra weight did some unnoticed damage here, but that wasn't all that happened. Think about an old cast-iron, column radiator. The inlet is a bit higher than the bottom of the radiator, right? Old-time radiator manufacturers did that for a reason. Since a steam system is an open system, radiator manufacturers realized there would be corrosion taking place as the years went by. They deliberately left some space at the bottom of each radiator for the dirt and corrosion to accumulate.

When he flooded his system, the homeowner freed almost 80 years worth of crud from his radiators. It sloshed down the risers and gathered in the horizontal runouts between the main and the risers. Once there, it did a fine job of blocking the condensate from flowing back to the wet return. A puddle of water collected in each runout and the steam condensed in it before it had a chance to flow up to the radiators. That's why the radiators wouldn't heat all the way across. That's why the air vents were panting. The steam pushed and condensed, pushed and condensed. Raising the steam pressure only made a bad situation worse.

Some of that goop from the radiators also worked its way down into the wet return. That caused condensate to back up into the steam main and created water hammer during mid-cycle. Can you see this in your mind's eye?

The only way we could fix this one was to flush the crud out of the system. We disconnected each radiator, one at a time, and hooked up a hose to each. We broke into the wet return back at the boiler and flushed the system from top to bottom under street pressure. You should have seen what came out.

Once everything was hooked up again, we corrected the pitch, making sure the mains sloped downhill at least 1/2" for each 20 feet of run. All of this was done after the asbestos had been removed, of course. When we were done, the homeowner reinsulated the pipes and we started it up.

The banging and clanging had left, flushed out with 80 years of crud that never should have been disturbed in the first place.

Sometimes there's more to steam heat than meets the eye. That's when you have to use your mind's eye.