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humidity caused by steam radiator?

Jim Bennett
Jim Bennett Member Posts: 607
The inside of a steam system is a moist environment. When venting air, it will contain some of this water.

Make sure you have properly sized and functioning main vents in the basement.

If the system is venting the main through the radiator, this could aggravate the problem.
Jim Bennett


  • bklynick
    bklynick Member Posts: 23

    Do steam radiators increase the humidity in a room?

    I have two main risers (front and back). The longest run goes to the back of the building where it connects two radiators on the 1st floor, one on the 2nd floor, and one on the 3rd floor.

    The problem I have is the 3rd floor bedroom (furthest radiator in the building) has a problem with condensation forming on the window sills when the heat is going.

    This is the only room with a condensation problem in the whole building. There is a "D" size air vent connected now - a "C" just wasn't good enough, but the "D" is perfect.

    Any ideas?

  • David Efflandt_2
    David Efflandt_2 Member Posts: 24

    The air vent should just vent air (not steam). Is it, or the stem of the shutoff valve venting any steam?
  • bklynick
    bklynick Member Posts: 23

    No steam leaks anywhere - but if you keep your hand over the air venting out, it will get a little moist. No water drips or anything on the air vent though. Is that normal? If there is "wet steam" being produced from the boiler, could the air that's being vented be very humid?

  • Get rid of the cold surface...

    THe WINDOW appears to be the problem here, not the steam heating system. If you replace the window with a good window, or somehow insulate it, it will stop sucking heat, the radiator will have to work significantly less, and the dew point will not be seen by the beneficial humidity...

    Do you have a Thermostatic Radiator Valve on this radiator?

  • bklynick
    bklynick Member Posts: 23

    At this point I'm trying to determine the source of the moisture - out of 18 windows (all the same make), only the two in the 3rd floor back bedroom have this issue. A window (same size) right outside the bedroom door is nice and dry as well. FYI, the water forms on the window aluminum window sill, not the glass.

    The bedroom has a Maid-O-Mist D air vent. The radiators on the same riser each floor below have a TRV. I replaced two very badly corroded main vents in the cellar (this is my first heating season at the building) with Gorton 1's. I feel a small amount of air from the main vents when the boiler starts up, and I've noticed a small bead of water from the air hole on the rear main vent last time I checked, but not evidence of anything more than that.

    So do I rule out the radiator as the source of moisture in the air?

    I appreciate the help, thanks!
  • Jim Pompetti
    Jim Pompetti Member Posts: 552

    The only time I have seen moisture around a window ,is went the windows are in poor condition. How bad do the windows leak.
  • joe lambert_2
    joe lambert_2 Member Posts: 61

    It happened with my new windows too. I caulked around the trim and between the trim and the window and the problem has subsided.
  • Randy_7
    Randy_7 Member Posts: 23

    If the humidity is 40% or less (check with a gauge) then high humidity is not the problem. Aluminum, especially in older windows without a thermal break, is a notorious conductor and the condensation on the metal is proof. Even if humidity is not high I'd still seal the windows tightly each winter with clear plastic sheeting and/or caulking, until the budget allows new windows.

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