First full heating season in a prewar one-bedroom in Jackson Heights. My living room radiator hisses and spits a little water from the small valve on the side. I turned the knob all the way closed and now the top half is cold but the bottom half is still warm. Am I supposed to bleed it like a baseboard? Or is this a call-the-super situation? The noise is worst at 6 a.m., right when the heat kicks up. I don't want to ruin the wood floor.

Don't go twisting the little silver valve on the side unless you know it's the air vent. That's not a bleed screw like hot water baseboards. If you closed the supply all the way, you just made it unbalanced. Open it back up. That hissing usually means air is leaving through the vent, which is the system doing its job. If it's spitting actual water, the vent may be old and stuck open. They're cheap and easy to swap, but in a rental I'd ask the super first. Half cold top / warm bottom is kind of normal for steam; the top of the radiator will get last.

OzoneParkOllie, you're almost certainly on a one-pipe steam system, not hot-water baseboard, so do not bleed it. That little valve on the side is an air vent. When steam rises from the basement, it pushes air out through the vent, which is why you hear the hiss. Once steam reaches the vent, the vent's internal mechanism should close and the hiss stops. If it keeps hissing all cycle or spits water, the vent is either clogged, stuck open, or the radiator is tilted the wrong way.

Open the supply valve all the way and leave it open. In a one-pipe system, the same pipe carries steam up and condensed water back down, so a half-open valve can trap water and cause banging. If the top half stays cool longer than the bottom, that's not unusual. The hot air rises and the upper section lags. If the vent is working correctly, you should eventually get heat across the whole radiator. If it never gets hot at the top, the vent may need replacement, or the radiator may not be pitched correctly.

In a rental, I'd put in a request with the super before swapping parts yourself. Ask them to check the air vent and the pitch. Many prewar buildings have decades of paint on those vents, which reduces airflow. It's a five-minute fix for someone with the right vent.

Thanks both. I opened the valve back all the way this morning. It still hisses, but I don't see water. The floor under the valve has a small old stain. I checked the top and it slopes away from the pipe? Maybe a little. Should it slope toward the pipe? I read something about a coin under the feet but not sure which side. Also the next room's radiator never hisses at all but gets hot -- is that one broken?

The silent one might have a different vent or it might be painted shut. If it gets hot, it's fine. My bedroom one never makes a peep but the living room one sounds like a bus air brake. You can replace the vent with a slower one for less noise, but again, rental super. Slope should be toward the shutoff pipe so water drains back, not away. Sometimes old floors settle and you need shims. Don't use coins under it unless the super does it.

Flatbush_F is right about the silent radiator. I've lived in three steam-heated buildings, and each radiator behaves a little differently. The silent one may have a smaller vent opening or may be closer to the boiler, so air leaves quickly and quietly. If the radiator gets hot from one side to the other and doesn't bang, it's fine. Don't assume a quiet radiator is broken.

The one that spits is the one to watch. Check the slope with a small level, not coins. The radiator should pitch slightly toward the supply pipe, usually a quarter to a half inch over the length of the radiator. If it slopes away, water pools inside, and steam pushing through that pool can spit rusty water out of the vent. Sometimes the building settles and a leg drops. In a rental, ask the super to add a shim or adjust the pitch.

I once had a radiator where the vent pointed down and old water had clogged it. The super swapped in a new vent and the spitting stopped. Also, if the vent has been painted over, that can block the small opening and make the radiator heat unevenly. You can chip paint off the vent carefully only if you're allowed, but if the paint is old, the super should handle it.

Update: I put a small level on top and it's actually slanting toward the pipe? Maybe I was looking wrong. So that's good. The hiss is just the valve then. It's still loud enough to wake me. I texted the super and he said 'they all do that' and that's it. Is there a temporary thing I can do at night? I don't want to close the valve again. Could I put a towel around it? Or is that a fire risk? Also the paint on the vent is chipped.

Do not wrap a towel around a steam vent. It's hot and wet and you'll just make a moldy rag. If the super says they all do that, you can ask him to swap the vent for a quieter one. If he won't, most people either get used to it or run a white noise app. My old vent spit so bad I put a small ceramic dish under it until it got replaced. Not pretty but saved the floor. Don't close the valve; that can cause banging and worse.

A towel is a risk. The vent needs to discharge hot air and a little moisture to let steam enter. If you block it, the radiator can't vent, which leads to water hammer and uneven heat. A small ceramic dish under the vent is fine as a temporary catch, but don't cover the vent itself.

Since your level shows a pitch toward the pipe, the spitting is probably the vent failing. In a one-pipe steam system, there are replaceable air vents with different sizes. Some are adjustable and can be set to release air slower, which can quiet the hiss. If the super says 'they all do that,' he may be technically right about the hissing, but spitting water is not something to ignore. I'd tell him the vent is spitting enough to damage the floor and ask for a replacement.

In most rentals, the super will swap the vent because it's cheap and fast, but they don't like tenants touching pipes. If he won't, you can ask if he's okay with you getting an adjustable vent. The morning noise you hear is common because condensate from the previous cycle sits in the returns overnight, and the first big steam push of the day moves through cold pipes. It can be louder when the boiler first fires. Once the whole building is up to temperature, you may notice less hissing later in the day.

That makes sense. I'll keep the level where it is and leave the valve open. I might mention the vent to the super again next time he's in the building. The dish idea from Flatbush_F is good for now. Thanks oldyork_otis for explaining the morning thing; I thought I was going crazy.