One-pipe system, steam radiator air vent question

[ALex265]

Sorry for a question that will seem very basic to many...

I have one-pipe steam heating system. Should a radiator air vent ever let out very hot air (or steam)? It happens like this: shortly after the boiler starts, air vent lets some air out, radiator gets warm, air vent closes. If the boiler keeps working for a longer time (~30 minutes) and the radiator gets really hot all the way, the air vent seems to open again and starts letting out very hot air until the boiler shuts off. Is the air vent malfunctioning?

Thank you

[larobj63]

I would say that it is, especially if you ever get actual steam coming out.

[ALex265]

larobj63, thanks for your reply. Just for my education, can you give me an idea of what kind of malfunction might that be? I mean, if the vent was clogged, it wouldn't close at all, or would close inconsistently, but this one closes promptly when steam hits it. It seems to open again when the temperature inside radiator gets really high (or stays high for a relatively long time).

[larobj63]

Well... It would be pure speculation, but I'll give you a thought at least, based on what I know about steam traps. And, we'll talk in general terms because there is more than one type of trap...

Steam traps generally work in such a way that when heat hits the trap, something expands and pushes a little steel piece into an orifice, closing it. This design exhausts warm air, but keeps hot steam. The something that expands can either be a bellows, or a parafin wax.

Now, this something that expands may only be really effective when it is hotter than it's surroundings, because the expanded medium is pushing something closed, but working in an environment that needs to stay the same size, or it won't push anymore. In other words, I suspect that when the entire trap is hot, like after a 1/2 hour of full heat, the expandable medium is no longer effectively pushing the oriface closed, because whatever the expandable medium is incased in is also really hot, and slightly expanded.

Now, when the bellows or wax (or trap in general) was new, it might have worked. But after hundreds of heat cycles, it is failing when the whole trap gets hot.

Food for thought at least. Whatever the reasoning, the symptom of a failed trap is letting steam go by, so....

[ALex265]

Now, this something that expands may only be really effective when it is hotter than it's surroundings, because the expanded medium is pushing something closed, but working in an environment that needs to stay the same size, or it won't push anymore.Yes, this sounds reasonable, thanks.

[ALex265]

Out of curiosity, bought a new vent, same brand and size, and it did exactly the same thing. Wondering if this can be caused by the size of the vent hole being too big for the radiator, but don't see how this works...

[heaterman]

Time ton stop guessing and call a steam hydronics expert before you scald yourself or loved ones

[ALex265]

Time to stop guessing and call a steam hydronics expert before you scald yourself or loved onesThanks for the advise, I will do that.

However, can you explain how and under what circumstances a new air vent can work like that? I searched the Internet, and it's nothing, like that can never ever happen.

[threadcutter]

Sorry for a question that will seem very basic to many...

I have one-pipe steam heating system. Should a radiator air vent ever let out very hot air (or steam)? It happens like this: shortly after the boiler starts, air vent lets some air out, radiator gets warm, air vent closes. If the boiler keeps working for a longer time (~30 minutes) and the radiator gets really hot all the way, the air vent seems to open again and starts letting out very hot air until the boiler shuts off. Is the air vent malfunctioning?

Thank you

On one pipe steam systems the radiator vents open to release cold air from the radiators and close when hot steam makes it into the vent. When the steam condenses in the radiator, it cools. The cooler air enters the vent and the vent opens allowing air to exit and fresh steam to enter the radiator. This open and close process takes place many times throughout the normal heating cycle.
If your vents make loud noises (whistling or sounds like the radiator is panting) or steam is found coming from the vent you have a problem. The issue could be a bad radiator vent ,a main line vent failure, piping issue, steam pressure set too high, or to an over sized boiler. In any case you need a qualified Tech to look at your situation. Poorly operating steam systems can raise fuel consumption considerably. Excess steam pressure can be down right dangerous.
Steam systems are remarkably simple but extremely finiky.
Dont mess with this yourself.

Good luck

[deux]

You can buy them with a dial to set the room temp. They have a spring that works against the temp regulator. They do work.

The odd hiss on a standard steam vent, on the first start of the heating season, is ok. Not if the boiler has ran for a few days though.

They are bellows / heat active. Dirt really messes them up. So does carry over from high TDS. You did check TDS? The hole size has nothing to do with it.

[ascj]

Sorry for a question that will seem very basic to many...

I have one-pipe steam heating system. Should a radiator air vent ever let out very hot air (or steam)? It happens like this: shortly after the boiler starts, air vent lets some air out, radiator gets warm, air vent closes. If the boiler keeps working for a longer time (~30 minutes) and the radiator gets really hot all the way, the air vent seems to open again and starts letting out very hot air until the boiler shuts off. Is the air vent malfunctioning?

Thank you

This normally caused by the steam pressure being over the rated PSI of the vent. Have your boiler checked out by a qualified contractor. THAT KNOWS STEAM.