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!
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
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
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
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.
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.
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
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.
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.
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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