"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.