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.

By Dan Holohan

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 one.

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).