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Main line steam vent

mfleck | Posted in General Questions on

Not sure if this falls in the bandwidth of this forum, but I thought I’d ask the experts just in case!

I have a single-pipe steam system in my 3-story home. In the garage, there is a main line steam vent right before the pipe goes into the garage ceiling and is no longer accessible. When we got the home 2 years ago, the contractor replaced this valve to help get the radiators steam at the same time. 

today I was poking around in there as the heat was on and held my hand above this valve. I felt really hot hair blowing up on to my hand. It wasn’t hissing or gurgling, but I definitely felt heat that I’d assume should be staying in the pipes. Based on some initial reading, I guess it’s possible that the vent needs to be replaced, or that the pressure in the system is too high for it. 

any advice?


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  1. tommay | | #1

    How old is your boiler? If it is a newer boiler, you may be able to just remove that vent and put a plug in it. Older systems used to run long and hard to build up pressure, so those vents were installed to make the return lines operate a little faster. Newer boilers are more contained and will run less to build up the same pressure. If your individual vents on the radiators are working properly the radiators should heat up quickly so no need for a fast return. If that vent is not shutting off as it should then you will begin to lose water from the system.

  2. walta100 | | #2

    I want you to understand you are living in a house with a bomb!

    There is an enormous amount of potential energy stored in a steam boiler. When you select a steam contractor you have chosen to put the live of your family in his hands and made it his job to make sure the bomb does not explode.
    If for any reason you are second guessing your steam mechanic, it is time to find a new contractor no ands ifs or buts.

    Do it yourself steam repairs are a bad Idea.


  3. tommay | | #3

    Walta, I am sure you realize that steam boilers have low water cutoffs and pressure switches to turn off the boiler if unsafe conditions occur. Residential steam boilers only run on 1/2 pound, 1 pound should be max. This is of course gauge pressure which is on top of atmospheric pressure. Maintaining and testing these switches by occasionally draining the boiler and adjusting the pressure switch to ensure the boiler turns off is definitely a must. Removing and cleaning the pigtail to the pressure switch and flushing out float type lwc should be done as routine maintenance.

  4. walta100 | | #4

    I am not saying steam is not safe. Just that it demands respect as everything inside the system want to escape and become 12 times its current size.

    Anyway you look at it there is a huge pile of stored energy always looking for anyway to escape.

    In my opinion there are a few things better left to professionals and steam heat is one.


  5. mikeolder | | #5

    I wouldn't plug that fitting. I bet its a vacuum breaker which is a glorified check valve..

  6. johns3km | | #6

    If it is a main you typically want to increase your main venting with multiple, large vents. You then want to vent your radiators very slowly. This gets steam quickly to the end of the mains so they can hit the radiators at the same time, heating evenly. Every time you have a heating cycle you need to get all of the air out of the pipes, thus large main vents.

    I have 1 pipe steam as well with almost 50 year old boiler. Phenomenal system. Even, silent heat that requires almost zero electricity.

    I’d recommend Lots of steam heads over there that’ll educate you.

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