I think I know what clogged the return line on my mother's steam boiler.

Mr_Tea

So second post here, I still have to follow up on my first from years back. My mother calls me late one night telling me that the heat pipes are banging pretty loud. It was a colder night and I figured maybe it was because of extra steaming. It's a one pipe steam system in an old NYC home, 1920's. She calls me early that morning telling me water is running from a pipe in the basement and there's no heat. In addition she mentions something about filling the boiler up because the sight glass was low. I rushed over and found the main air vent spraying out water. I start to think of what could possibly be wrong and how to debug the problem.

No way should water be spraying from that vent unless the system is full of water. I shut the system off and let it cool for a good half hour. There was a normal water level in the boiler according to the sight glass and opening the boiler pressure relief valve revealed that there was no excess water, the sight glass was telling the truth. I then think to remove the main air vent and see if there's still water in the line, indeed, water gushed out. Okay I then tap on the dry return line and it sounds full of water all the way to the boiler. Open the boiler drain and try to see if that drains the dry return. Nope, after fully draining the boiler there is still water in the pipe at the air vent. I then break the union on the dry return line at the boiler end and sure enough water gushes out and the dry return empties. I then remove a vertical section of the dry return pipe and was stunned to see that it stayed full of water after being fully unscrewed. It was 100% blocked by corrosion. The elbow it connected into was also fully blocked.

I removed the elbow and realized right away what is most likely the culprit: a brass nipple was used between the iron elbow and tee into the wet return. I thoroughly cleaned the elbow with a wire brush after removing the brass nipple. I replaced the nipple as well as the very corroded vertical pipe section and union with all new black pipe and used pipe dope. Worked perfectly and for about a month has been running reliably.

Now that house was switched from oil to gas about 15+ years ago so the boiler was replaced at the same time. I assume the plumber got lazy and used whatever he had on hand which was a huge mistake. I assume this lead to some sort of galvanic corrosion which caused the scale to build. Shame how some "professionals" don't care. I just want to be sure it won't happen again. Then again it will be in another 20 years or so. I've upket this boiler system as well. I tuned it up two years back replacing the vent valves on radiators and again replacing a woefully undersized main vent valve with a gorton No. 1. House heats very nicely.

The Clog

The culprit:

The clogged section was so corroded that it had rusted right through the thread! Amazed it came apart without issue.

The fix. Pay no attention to the coupler, the local mom and pop hardware shop didn't have a long enough piece of pipe and I did not feel like driving to Home depot and wasting time there.

Fred

That brass elbow didn't cause the problem. Anywhere there is a wet return (no matter how short it may be) there will be a potential for a clog, over several years of crud washing out of radiators. You should replace that elbow with a Tee and a plug on one end so you can take the plug out and wash that loop out every year or two.

EdTheHeaterMan

Sometimes the plug will be impossible to remove after only 2 years of service. I recommend that you use a close nipple and a cap. You will get better leverage with the pipe wrench on a cap over a plug every time!

ChrisJ

Before you someone's work you should at least understand what galvanic corrosion is.

That's not it and brass is generally safe to use with steel.

mattmia2

You probably will want to use a brass cap for the cleanout nipple, it won;t freeze in place as badly as iron.

Mr_Tea

Interesting information! it seemed like too much of a coincidence and I never expected that much crud to form in a return line. And the tee trap idea is certainly something to consider.