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Steam valve hissing after radiator is hot - how to fix

My steam radiator performs like a champion, heating up very well each cycle and radiating a splendid amount of heat. It is tipped at a decent angle of say 5 degrees and does not cause any knocking. However with a replaced steam let off valve and the entry manual valve wide open, it hisses well when the cycle starts but then continues hissing even when the radiator is hot. My trouble and strife (wife) is sure that this proves iot isn't working properly and is about to explode any minute, and wont listen to any explanation of how single steam piping works. However, it seems she has a point. A radiator valve shouldn't continue hissing and letting off steam after it is hot, should it? But the valve does not seem to have any adjustment, if that is what is needed. It is new, in fact, a stainless steel valve with a tongue hanging out of the inlet. It is firmly screwed it with tape to add seal. So what do I tell my wife (and the landlady who is trying to fix the system amid plumbers who seem mainly to advise buying big replacements for everything in the system. The only other symptom in the system is that the radiator on the floor beneath hisses and spurts water even when hot and she is trying to cure that problem by fixing the other radiators, without being sure it will help. Is it true that she needs 1) to tilt the radiator below to stop it spurting water? 2) to adjust the valves in the radiators to get them to close properly when the radiator is hot? 3) maybe fix the boiler settings or pipe valve in some way?


  • "Hissing" Vent

    See if you can find out at what pressure the system is running when it is hot.  Residential steam systems shouldn't be running at more than 2 PSI. Higher pressure than normal (normal is below 2 PSI) that can cause the hissing you're are experiencing.

     You may just have a defective valve/vent. This does happen. You could just swap with a new one or swap  it with a vent you know is working, just to test it. Be Careful! Be sure to do this when the boiler is turned OFF!.  Steam burns AREN'T fun!

    - Rod
  • JStar
    JStar Member Posts: 2,752
    Check the whole system.

    Find out where the radiator connects to the main in the basement. If it's the last on the line (along with the floor below you), and there is a faulty or missing main vent, the problem may not even be the radiator vent at all. If the problem is isolated to just these two radiators, I would look into a piping issue near the main.
  • "trouble" sez, "about to blow up"

    get your landlady to buy a copy of "the lost art of steam heating" in the shop here, and a good low-pressure gauge [gaugestore.com-0-3 psi]. this should be mounted on the boiler pigtail, with the pressuretrol, and the original 0-30 gauge should be kept for code purposes. only in this way can you know what the system pressure is, and adjust it  as low as possible. ideally if a vaporstat were installed, only a few ounces would heat the radiators up, using less fuel.

    that is the place to start, and when you know your pressure, then report back here any further symptoms back here for further advice!--nbc
  • jpf321
    jpf321 Member Posts: 1,568
    let's at least get on the same page ...

    the silver thing (maybe looks like a bullet or a chewing tobacco box) halfway up the rad is called a VENT or AIR VENT .. not a steam let off nor a valve.

    the "manual valve" near the floor is called the "INLET VALVE" and should always be FULLY OPEN in 1-pipe steam .. or FULLY CLOSED .. never anything else.

    all rads should be "slightly pitched (tilted)" towards inlet valve .. the does not mean, in most cases, "noticeably pitched" .. only pitched enough to ENCOURAGE water to flow back to inlet valve .. not so much as to MAKE the water flow back ..

    i agree with everyone else, until you are familiar with the pressure of the system, you will not be able to work towards solving your problems... if you have a 0-30PSI gauge, it really should not ever more above 2! .. if you see it at 5, 7, 10PSI .. stop right there .. and fix your pressure .. in fixing your pressure, everything else should be fixed as well. my system runs at 5OZ (less than 0.5LBS) .. other systems on this forum also runs at 1LBS or less ..

    how big of a dwelling is this? single family? multi-family? where are you located? there are plenty of great pros that will set your landlady right .. once and for all.
    1-pipe Homeowner - Queens, NYC

    NEW: SlantFin Intrepid TR-30 + Tankless + Riello 40-F5 @ 0.85gph | OLD: Fitzgibbons 402 boiler + Beckett "SR" Oil Gun @ 1.75gph

    installed: 0-20oz/si gauge | vaporstat | hour-meter | gortons on all rads | 1pc G#2 + 1pc G#1 on each of 2 mains

    Connected EDR load: 371 sf venting load: 2.95cfm vent capacity: 4.62cfm
    my NEW system pics | my OLD system pics
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