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A radiator makes a gurgling sound


July 10, 2009
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The supply valve is partially closed.
In a one-pipe steam system, the steam and the condensate share the same pipe. Steam takes up more space and moves much faster than condensate. The steam and the condensate have to be able to get out of each other's way. That's why a supply valve on a one-pipe steam radiator has to be fully opened during operation. The only reason you'd close it would be to service the radiator, or to shut it off completely.
If the radiator gurgles, make sure the supply valve is fully opened.

The supply valve has come apart.
The valve's handle may turn, but the valve's seat may have come loose from the stem. Remember, this is an old system and it's constantly corroding. Check the valve to make sure the parts are still attached. If they're not, repair or replace the valve.

The radiator is pitched the wrong way.
A one-pipe steam radiator must pitch toward the supply valve so the condensate can drain back to the boiler. Over time, however, the radiator may lose that pitch and this can make it gurgle. This is especially true of radiators that sit on wood floors. The supply pipe supports the radiator only on one side. The other side is left to expand and contract against the wood floor. A cast-iron radiator is heavy, and as the years go by it can dig a trench in the floor. If the radiator's air vent spits, the escaping condensate can soften the wood under the radiator's legs and dig an even deeper trench.
Check the radiator with a six-inch level and make sure it pitches back toward the supply valve. If necessary, shim the radiator.

The radiator is sagging in the middle.
Large cast-iron radiators often sag in the middle. This happens because the condensate that lays in the bottom of the radiator is acidic. It slowly weakens the lower push nipples and causes the radiator to bow in the middle. As water builds in the radiator, the radiator gurgles. This is why it's important to check the pitch of the radiator with a six-inch level. A level this size lets you check the pitch from section to section, not just from end to end.
Correct the sag by shimming the radiator in the center. If the push nipples have corroded to the point where they leak, you'll have to replace them (if you can find replacements).
I've heard of people who have successfully repaired leaks in cast-iron steam radiators with epoxy. Success depends on where the leak is and its severity. If you can't repair the leak, you'll have to replace the radiator.

The radiator has too much pitch.
There's a mud leg at the bottom of a steam radiator. That's the place where sediment accumulates over the years. If you pitch the radiator too much, the sediment will slide toward the supply valve and gather there. Although the supply valve seems to be opened, it will be partially closed by the sediment.
Remove the radiator and flush it out.

The radiator air vent is improperly installed.
An air vent lets the air out, but it also the air back in when the steam condenses. If the air can't get back in, a partial vacuum will form in the radiator, and that will keep the condensate from draining. As the condensate builds, it causes the radiator to gurgle.
Check the air vent. Is it installed sideways or upside down? Some people turn the air vent upside down when they don't want any more heat. Check, too, to see if the air vent is installed on an extension pipe. Condensate may build up inside the air vent and keep it from operating as it should.
Remove the air vent from the radiator and check its tongue. The tongue is the short piece of metal that extends from the vent and hangs inside the radiator. Its job is to drain condensate from the vent. If its twisted or broken, the vent won't drain and the radiator may gurgle.