Inside a Century-Old Steam System: The Mystery of a Rare Float Air Vent
Key Highlights
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Rare hardware sighting: Early Trane float air vent shows how coal-era steam systems managed draft, vacuum, and condensate without modern controls
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Red-flag modification: A thermostatic trap added at the vent outlet signals steam migration caused by failed radiator traps
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Actionable diagnostic tips: Hot spots on returns, water hammer, and uneven heating point directly to which traps have failed open after a century of service
The first thing I noticed was the Swiss chalet. I was getting out of my car in one of the oldest neighborhoods of Cincinnati, built in the late 1800s up on one of the hills that surround the city. The other houses on the street looked more European than American, but this place was full blown straight from the Alps.
The house I finally came to see was a bit more of the French country style. The tech I was meeting had wanted me to look at something there for a while, but couldn’t work out the scheduling. After greeting the homeowner in the kitchen, we all went down to the basement to gather around the object in the photo. It was hanging in the boiler room, which had an Oriental rug on the floor, which I don’t see much.
An Early Float Air Vent
At first, from the initial angle, it looked like a boiler return trap manufactured by the Trane Company, La Crosse Wisconsin, USA, circa early 1900’s. I have two examples of it at the office. I’ve been using the one for years as a training aid for steam heating classes. But when I looked at it from the angle you see, I could tell that it wasn’t a boiler return trap at all.
In fact, it is a very early version of their float air vent for low-pressure steam systems using hand fired equipment, like a coal boiler. Back in the coal days, there wasn’t a safety switch to turn off the burner if the steam pressure was getting too high. Today we use the high-pressure switch or pressuretrol to control the pressure in our steam systems. Back then, they used a draft regulator and a boiler return trap/float air vent.
The draft regulator would close the draft doors to damper the fire to bring the steam pressure down. If the pressure still got too high, keeping the return condensate from entering the boiler, the boiler return trap would come to action and using the steam pressure force the condensate into the boiler. You can read more about it on page 100 of my Field Guide, available as a free download at steamupairoutwaterback.com.
Ghosts of the Coal Era
The air vent portion is straight forward, a nice large orifice to allow rapid venting of the air from the radiators and the end of the steam supply mains. Sometimes in the coal era, there would be a light ball or disc in the air stream to close tight when the coal fire died down. This produced a vacuum in the boiler, which allowed it to continue to make steam at the lower water temperatures as the fire diminished. This vacuum worked well in the days and nights of shoveling coal, but is now completely unwelcome with automatic fired equipment, like gas and oil.
The float function of this vent is to prevent water from being pushed up and out of the system if the steam pressure got too high and the boiler return trap wasn’t keeping up. The float would rise up as the water level rose, until it got high enough to close off the hole in the top that the air is supposed to go through, but not the water. The float mechanism in the Trane boiler return traps I have at the office work with springs, while most of the other manufacturers work with levers. I imagine that the float in this air vent works on springs because of the shape of the body.
Big Red Flag
The novel aspect of the vent at this job is the addition of a radiator trap, piped into the location of the air discharge off the top of the vent. Now a radiator trap is sometimes used on two pipe steam systems at the end of a steam main to allow air from the end of the main to cross over to the dry return and find its way out of the system through the float air vent. But this radiator trap or in some cases an automatic main air vent, is connected where the original vent discharge would have been.
The original float air vent did not have any thermostatic operation, like our current automatic air vents used at the end of the mains. The thermostatic operation closes the vent when steam reaches it, after discharging the air in the system. The old ones would only close when water rose too high in the return lines, or when used to produce a vacuum, which we don’t want to do anymore.
The addition of a thermostatic air vent—or in this case a radiator trap—raises a big red flag in my mind. It usually is added by someone without the knowledge of the system, when steam starts coming out of the vent. Since this is used on two pipe systems with traps at the radiators, there will only be steam at that point in the system if one of the radiator traps has failed open.
Look for the Failed Traps
The steam coming out is a cry for help to fix the trap or traps that have failed after 100 years of service. Failed traps in two pipe systems aren’t hard to find. They show up as hot spots in the return piping, water hammer, or as areas of the building that don’t heat. Whenever you run across an old float air vent, check to see if the disc or ball is under the original brass cap from the coal fired days. If that has been replaced by a vent or trap, get busy looking for a failed trap.
The irony of using a trap instead of a vent is traps do not close for water. In fact, they are designed to pass water. Modern air vents do have a float feature and do close to prevent water from escaping the piping.
About the Author
Patrick Linhardt
Patrick Linhardt is a forty-one-year veteran of the wholesale side of the hydronic industry who has been designing and troubleshooting steam and hot water heating systems, pumps and controls on an almost daily basis. An educator and author, he is currently Hydronic Manager at the Corken Steel Products Co.