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Venting Radiator at End of Main

Jack_48 Member Posts: 2
The problem is with a small foyer steam heat radiator at the end of one branch of the main. With the vent on, it does not heat at all and the riser is cold back to the main and I believe the main is cold back to the tap for the the preceding riser, which is still hot and that radiator works fine. With the vent off, the foyer radiator heats and spits steam through the vent hole.

I changed the vent with something similar to existing vent, with the same unfortunate effect (no heat with the vent on). The replacement valve was manufactured by Invensys - package label states it is a general purpose valve with a Dole 1933 angle vent, to be usd in system that requires pressure in excess of 2 psi to max of 15 psi.

As I probably should have said earlier, this is a one-pipe steam system in a 1920's house. The foyer radiator is on two- or three-foot riser at the end of one branch of the steam main. This branch, about seven feet in all, also feeds a first floor radiator and a second floor radiator. The opposite branch is much longer, perhaps 35 feet, and feeds many more radiators. There are no vents on the short branch though there is a vent at the end of the long branch. Both branches are nicely slanted and the foyer radiator is tilted toward the riser connection. I tried turning off the radiator at the intermediate point on the short branch - it also cooled back to the main, not just to the shutoff valve, but had no effect on the foyer radiator.

A perhaps related fact is that the first floor is generally a little too cold and the upstairs a little too warm, with about a five-degree difference between them.

Any thoughts or suggestions?


  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    Water is backing up somewhere

    and that's what makes it spit when you take the vent off.

    Look for a pipe that's not pitched properly. If you don't find this, the return from the end of that branch is probably plugged.

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  • Jack_48
    Jack_48 Member Posts: 2

    Yes, maybe so. Though this is the high end of the main, I suppose there might be some kind of build-up in the branch connection to the riser into the supply valve of the foyer radiator. I wonder if, instead, there is just not enough pressure to consistently reach the foyer radiator with the vent attached. Would it be a waste to try a quicker vent?
  • World Plumber
    World Plumber Member Posts: 389

    If I understand you post you have a vent that requires a min of 2psi??. Single pipe systems are designed to work on ounces of pressure.
    Are you getting water at any other vents? It may be it was venting fast enough with the vent off to pull water back if your right on the end. Try a varivent then you can adjust it to achive the venting rate you need to get the proper heat to the room. Make sure there are no sags in the pipes and put a level on the radiator and make sure it pitches back to the valve.
  • alang
    alang Member Posts: 35
    You need a vent at the end of the short main

    If the radiator gets hot with the vent off, then you have enough pressure the get the steam where it needs to go - you just don't have enough venting to move the air out of the radiator before the thermostat is satisfied. You stated that there is no venting on this short main (you called it a branch, but because it has sevaral radiator take-offs it's acting like a main) and that's probably the root of this problem. The air is being forced to the end of this main and has no way to get out except for this vent on the foyer radiator. Radiator vents are sized to vent radiators, not main lines!
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