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On a one-pipe steam system, each radiator has a small vent on the side, at the end away from the supply valve. It has one job: to let the air out so that steam can come in, then close when the steam reaches it. When it fails, the radiator fails, and people call about the boiler when the fault is a part that costs less than lunch.
How to tell the vent is the problem
- The radiator stays cold while others heat: the vent is stuck closed, or clogged with scale or paint.
- It hisses the whole time the boiler runs: the vent is stuck open, so steam escapes instead of filling the radiator.
- It spits water or drips: the vent is failing, or the radiator is tilted the wrong way and water is collecting at the vent end.
- The radiator takes far longer to heat than the others: the vent is too slow for the size of the radiator or its distance from the boiler.
Replacing a vent
- Let the system cool. Turn the thermostat down and wait until the radiator is cold. A live steam vent burns.
- Unscrew the old vent by hand, turning counterclockwise. If it is stiff, wrap a cloth around it and use pliers gently. Do not force it, because the fitting on the radiator can crack.
- Clean the threads on the radiator with a cloth and a little penetrating oil if they are crusted.
- Wrap the new vent's threads with two turns of thread seal tape, and screw it in by hand until snug. The vent must point straight up.
- Run the heat and check that the radiator heats evenly and the vent closes quietly when steam arrives.
Buy a steam radiator air vent from a known maker. Vents are sold in sizes that vent at different rates, usually marked with a letter or number. A larger radiator, or one far from the boiler, wants a faster vent; a small one near the boiler wants a slow one. Match the old vent if the radiator heated well before; go one step faster if it was always slow.
Balancing the house
If the rooms near the boiler are hot before the far ones get warm, the answer is not to turn the boiler up. It is to fit faster vents on the far radiators and slower ones on the near ones, so that the steam reaches everywhere at about the same time. This is one of the cheapest comfort improvements a steam-heated house can have.
The main vents
There are also larger vents on the main pipes in the basement, near the ends of the runs. They let the air out of the mains, and if they are missing or failed, every radiator is slow. Look for them, and if you find a pipe end with a plug where a vent should be, ask a plumber to fit one.
Painting radiators
Never paint over a vent. If it has been painted, replace it.
When to call a plumber
- A vent fitting on the radiator that is cracked or stripped
- Main vents that are missing or failed
- A whole system that heats slowly or unevenly after the vents have been replaced
- Any radiator that spits water after the vent is replaced and the tilt corrected
Find a licensed plumber near your town, and ask whether they work on steam.