|Care & Feeding
of Air Vents
Keeping all air vents
in order helps
and to their fullest.
|In a one-pipe steam system, there’s
just one pipe connecting the radiator to the main steam line;
the steam and the condensate have to share that single pipe. Properly working air vents on main lines also help the system fill quickly with steam.
Do you hear a hisssssssssssssssssss when your heating system comes
on? If your old house is warmed by a one-pipe steam system, that
hissing is probably coming from the air vents on your radiators.
It’s not an unpleasant sound, but it’s also not a normal
The hiss that often accompanies the start of a heating cycle is
the sound of the air moving across the small holes in your radiators’
air vents. Push air through a little hole fast enough and you’re
going to get noise. This noise though is really the sound of the
vent crying out for help. It’s working too hard, and if you
don’t give it attention it will die—and that will cost
you money. Here’s why.
Air Vents in Action
Imagine a crowd leaving an arena after a sporting event. The more
exits they have available, the less pressure there will be at any
one of those exits. Open all the doors and people will leave the
arena in an orderly way. Lock most of the doors and watch what happens.
Everyone starts pushing and things get crazy. The air in a steam
heating system works in a similar way. At the start of each cycle,
all the pipes above the boiler’s waterline, as well as all
the radiators, are filled with air. To heat the radiators, the steam
has to push that air out, and the exits are the air vents. However,
if there are just a few working air vents, the air is going to rush
from them and make a hissing noise.
Here’s how a typical radiator air vent works. On the inside,
there’s a float that will pop up like a cork should water
surge into the vent from the radiator. That helps keep your walls
clean. The float will also respond to heat because it’s partially
filled with a mixture of alcohol and water and sealed at the factory.
Alcohol boils at a lower temperature than water. The manufacturer
heats the float before placing it inside the air vent and that causes
the alcohol to boil and turn to vapor. While the alcohol is boiling,
the manufacturer solders the float closed and then allows it to
cool. When the alcohol/water mixture condenses, it forms a vacuum
inside the sealed float, which causes the flexible bottom of the
float to bend inward, toward the center of the float.
The air vent manufacturer puts the float into the vent casing and
sets the proper distance between the pin that sits atop the float
and the hole in the top of the vent. Steam then pushes the air through
the radiator and out the vent. When the steam reaches the vent,
its heat causes the alcohol/water mixture to boil and the vapor
that’s produced increases the pressure inside the sealed float.
That makes the bottom of the float pop out, driving the pin into
the vent’s hole and stopping the steam from leaking out.
An air vent then is a remarkably simple and reliable device, but
it can still be prone to problems. Since your one-pipe steam system
is open to air, there are flakes of rust inside your pipes that
will always be there because the system is constantly corroding.
If crud from the system works its way into the space between the
pin and the hole, the vent won’t close and steam and water
will escape. Perhaps worse, the faster the air moves past those
flakes, the more likely it will be that the flakes will wind up
inside one of your air vents and clog it shut. When a radiator air
vent gets clogged shut, not only will that radiator not heat properly,
the other vents throughout your system will have to pick up the
slack. They’ll be venting even more air, and that air will
be moving faster than it should. So these vents will be making more
noise. And the faster the vents vent, the better the chances are
that they’ll also get clogged. The more vents you lose, the
worse it gets for those remaining.
Now, take a look at that small piece of metal called the tongue.
It’s right there at the vent’s threaded inlet. The tongue
helps water drain from air vent and back into the radiator. It’s
a simple device, but an important one, sort of like the butter knife
you might stick into a difficult bottle of ketchup to get it going.
If the tongue gets bent, the vent will probably squirt and you’ll
need to replace that vent.
If the steam pressure is too high, it can hold up the float and
keep the pin stuck in the vent hole. That can keep the radiator
from heating properly too, and it’s the main reason why the
pressuretrol on your boiler has a cut-in and cut-out setting. The
fluctuation in system pressure gives the float inside the vent a
chance to drop so that more air can escape from your radiator.
Maintaining Your Vent Inventory
So to go back to the sporting arena analogy, the more exits, the
better crowds can leave the event. In one-pipe steam, add more vents
(or more venting capacity) and the system gets quieter. This is
why it pays to keep air vents in good working condition. Make sense?
Some radiator air vents have adjustable air-release holes. These
help to balance the system since big radiators contain more air
than small radiators, and our goal is to get all the radiators hot
at the same time on the coldest day of the year. You would use the
fastest venting setting (usually the highest number on the adjustment
dial) on the bigger radiators and the slower venting setting on
the smaller radiators.
Some manufacturers offer a line of air vents that have fixed vent
ports, but each vent in the series is faster than the previous one.
Here again, the goal is to balance the overall system by balancing
the release of air from the radiators. Big radiators need to vent
air faster than small radiators.
The radiator’s location in your house has little to do with
the vent you choose. If your system has main vents, the steam will
favor the large pipes over the individual radiators when it first
leaves the boiler because that will be its path of least resistance.
The main vents allow you to very quickly fill all the pipes with
steam so that the steam arrives at the inlet to each radiator at
about the same time. From there, the radiator vents take over, venting
the big radiators quickly and the small radiators more slowly. That
way, everything gets warm at the same time.
This is also why those main vents near the ends of the big pipes
in your basement are so important. Not only do they help balance
the system, they also get rid of the majority of the air that’s
in the piping so that it doesn’t have to leave the system
through your radiator air vents. And that means you won’t
have to listen to it upstairs.
So, make sure your main vents, and all your radiator vents, are
clear of debris. Wait for a day when the system is off and cool
to the touch. Use a wrench to remove the air vents and see if you
can blow air through them. If they’re clogged with debris,
you won’t be able to do this. If your radiator air vents are
spitting, they’re probably clogged with dirt and scale. You
can try to clean them out by removing them (make sure the steam
is off when you do this) and boiling them in a pot of vinegar. Vinegar
is a mild acid that breaks down scale. It doesn’t always work
but it’s worth a try before you go out to buy new air vents.
Dan Holohan is the author of We Got Steam Heat—A Homeowner’s
Guide to Practical Coexistence, available from Heatinghelp.com (800-853-8882; www.HeatingHelp.com).