Loud hissing from steam radiator's air valve

Hello,
I have a one pipe steam radiator system in my condo unit that lets pressure out through the side air valve. There is no banging or clanking, but there is an abnormally loud hissing noise from time to time that's very disturbing. Upon visual inspection of the air valve, I cannot for my life determine where the release hole is, which is very curious. (My intention was to make sure that it was not obstructed.) It is not painted over, either. Any suggestions on how to solve this annoying problem economically would be greatly appreciated!
Cheers, J
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I am a power engineer fourth class and until I retired a few years ago, I operated steam boilers in several buildings which all had two- pipe systems. With the one-pipe system, the steam supply line would be relatively short, and the pipe graded carefully so that the condensate all drains by gravity back to the boiler. I am not familiar with the "side air vent" as you call it, but normally very little air would need to be expelled from a steam system. The fact that lots of air needs to be expelled periodically means that corrosion is taking place in the system. Heat plus carbon dioxide (in the air) plus water equals carbonic acid, which corrodes iron (not good). The fix for the problem is probably in adjusting the controls. Perhaps the boiler is being operated by a computer that allows the system pressure to drop to zero for a set time, then re-starts the boiler. When the pressure drops to zero, air is sucked into the system maybe at the radiator vent. When the boiler cycles On again, the air goes hissing out again at the radiator vent until the steam hits the vent and shuts it. If there's a computer operating the boiler, its not controlling it right. On top of the boiler, see one or more safety relief valves that will release if the pressure were ever to get to 14 psi. There should be an inspection certificate on the wall nearby showing when the boiler needs to be overhauled and inspected again. This is every 2 years for a low-pressure boiler unless the inspector finds an unacceptable amount of corrosion happening. On the side of the boiler, see the High Limit control, which should shut off the boiler if the pressure were ever to get up to 13 psi. The High Limit is a box maybe 4" wide and 3" high with a clear plastic front. There's an adjustment screw on it: Turning the screw "In" will "Increase" the setting at which the control will shut down the boiler. On the top of the control, see a reset button which needs to be pressed or the boiler will not start again. Next to the High Limit, see the Operating Limit, which is a box of the same size, but it has two adjustment screws on it. The bigger screw determines the point at which the boiler will shut off in normal operation. This "Off" pressure is typically 12 psi. The smaller screw determines the differential, that is, the point at which the boiler will start up again. The "On" pressure is typically 9 or 10 psi. Somewhere in a typical system, there is a low pressure alarm. If the pressure were to drop to 8 psi, an alarm would alert Security, who would then phone whoever is On-Call (poor bloke like me). Adjusting these controls should be done by someone who is familiar with them, but if you do it yourself, make sure you know where the toggle switch is on the front of the boiler so that you can manually shut it off if you're getting befuddled with the controls. Near the toggle switch is a dial which controls the rate of firing. Adjust the rate of firing to the bottom end of the scale so that the pressure climbs only slowly while you're adjusting the controls. If air is excluded from the boiler and piping as much as possible, and if the boiler water is appropriately treated with chemicals, your boiler will serve you well for many decades. If you have any other questions, I'd be glad to try to answer them. Sincerely, Brian
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Hi, thanks for this detailed reply, and apologies for my subsequent late reply! I do not have access to my building boiler. I am on the top floor and am now looking to try and replace my air release valve. While the rest of the system might be the culprit, I have to deal with the ill effects, it seems. I'll hunt around on the web for info, but if you have any useful suggestions, I'd be happy to hear them.
Thanks, J
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Get a Dole adjustable and dial in the heat you want, but if yours is bad so the condo and you are wasting money, look for people with open windows. An unbalanced system that is set higher in temp to heat all units can waste 50% I know my building was so bad the gas bill doubled, and you are paying.
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I would strongly suggest that you contact Building Maintenance before you unscrew your air vent. There should be an isolation valve on the radiator and/or in a utility corridor. Even low-pressure steam can fill your condo with scalding steam within seconds. Isolation valves have packing in them that often leaks very slightly. When the valve stem has built-up minerals on it and then you shut the valve, the minerals on the stem tear into the packing, making it leak worse each time the valve is used. So before you turn in the valve, take some fine sand paper and clean the valve stem as best you can, then spray some oil on the valve stem before shutting the valve. On some valves, there is a packing nut that the valve stem runs through. When your project is done, you can compress the packing around the valve stem by slightly tightening this packing nut. To make a good seal between the new (threaded) air vent valve and the existing radiator, on the male thread, put 3 clockwise wraps of pink Teflon tape, then coat the Teflon with pipe dope. Snug it in with a 12" crescent wrench. Cleaver Brooks is a common and well-respected name in boilers in my area (Alberta, Canada). They have a website that probably would be a good starting point in finding quality replacement parts for your radiator.
Sincerely, Brian.
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Looking up I dont think you have replaced air vents, his is one pipe steam, I have no isolation valves on a 1,100,000 btu system, and air vents on my radiators would not screw in with teflon tape, Ive never used it and nobody I know needs to. On valves yes, dope and tape are best, but not vents. You rarely need a wrench on them hand tightening is best so you dont overtighten them and strip them. As far as steam comming in, just do it after a cycle and you will have plenty of time, this year alone I replaced and switched maybe 30 vents taking out leakers and trying to balance the system by different vent sizes. The only issue he could have is stripping threads if he is not carefull.
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Hissing is the last thing it does before it detonates.
Steve
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If you have hissing and no banging, you good to go! Why even bother with it? If anything, maybe the radiator needs to be drained and then realigned to drain back?

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Hello, thanks for the advice. I thought that replacing the air valve should be fairly straightforward, but perhaps I shall learn the expensive way by watching a pro do it the first time ... it sounds like it wouldn't be a bad idea to get a pair of pro eyes on this thing. My building's maintenance guy said he'd come take a look, though it ends up being about 50 USD/hr which I'm obviously not thrilled about. You guys/gals are a lot less expensive. :) Darn radiator hissing just won't shut up.
Thanks again, J
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It could be normal if it stops in a minute or so unless it is letting out steam. Air vents go bad with corrosion and eventualy dont close fast enough. Replace it with the same type, alot of chinese vents are junk and go bad fast. Its not the boiler.
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