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four radiators suddenly stopped heating

Dearest steam enthusiasts,

I have an unusual 1906 K-M-C vacuum/vapor system. That means each radiator works like a one-pipe steam system but it has no air vent. Instead there are these little air return lines that lead to a central air vent in the basement. Pictures: <a href="http://www.flickr.com/photos/niftyc/sets/72157608801023873/">http://www.flickr.com/photos/niftyc/sets/72157608801023873/</a>

Four radiators in my house have suddenly stopped heating up and I can't figure out why. I'm hoping I can give you my thought process here and maybe it will spark some ideas.

Each of the problem radiators WILL heat up if I open the union on the air return line. I think that means the problem is venting. The main vent is working as the other radiators are heating. That means that the air return lines aren't working somewhere between the main vent and the problem radiators. The central air return line that goes around the perimeter of the basement seems to be working, too -- otherwise the problem would be more widespread than these four.

At first I thought they were blocked and I opened every union near the problem radiators when the system was cold. I used a shop vac to try moving air through the lines -- but the lines seem clear!

The four radiators that are not heating are near each other and I suspect they may be same air return line. They are all on the second or third floor -- the first floor radiators all work fine. One clue (?) may be that some of the problem radiators do get a little warm very briefly at the steam supply pipe when the system first fires, then they quickly go cold again.

There are five little air return lines on that side of the basement that feed into the main air return line from the floors above. These must serve about seven radiators on three floors. It is very hard to match them up exactly because the lines join together inside the walls and floors and the routes are not logical.

My next big guess as to what is happening is that there may be a first floor radiator that is pressurizing the "problem" air return line with steam. If there is a failed open F&T trap on a first floor radiator as it heats up it would push steam into the air returns, equalize the pressure, and cause those four upper radiators to quit.

The problem in my theory is that I have working valves on the steam supply lines of some of my radiators. So I shut the valve on the 1st floor radiator that looks like it is connected to that air return line. The radiator is now cold and I don't see how it could be pressurizing the air returns if I shut the valve. I was so sure this was the problem! Drat.

I thought I might look in the air return lines for steam while the system is firing but there are no unions near the problem areas except at the radiators. When I open them up I don't see any steam from the return line (but the steam would presumably be farther down at the 1st floor anyway).

Then I thought I could feel the air return lines to see if they have steam in them -- they would be hot! But some service person in the past modified the path of an original steam main and put it right on top of part of the air return line in the basement. They are touching and there they are not insulated. That means it is impossible to tell if the air return line is hot from steam or not -- it would get hot because it is touching part of the steam main. Grrrr.

I'm not really sure what to do next. I was thinking that maybe it is a different first floor radiator with a failed trap. I don't think any other first floor radiators are connected to the problem area but the route of the air return lines is pretty mysterious to me right now. I might try to figure out where the air return lines run so that I can link the upper floor radiators to the lower floor radiators on the same return lines. Then I can look for failed F&T traps. However, I don't really know how to do that without opening up the walls.

I was thinking of trying to buy one of those pipe sensors that tells you what is inside a wall if that will work but I have no experience with them. Would that work?

I'm sort of spinning my wheels now on this one. Does the above give anyone any other ideas about what I might try?

I suspect I brought this problem on myself by posting a blog post elsewhere where I bragged about how well my steam heating system was working! Arrrgh!

I really really appreciate any of your advice. This board has saved my bacon in the past.

This discussion has been closed.