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Steam radiator banging? Why NYC pipes knock, and how it gets fixed

That loud clanking when the heat comes on is called water hammer. Steam turns back into water (condensate) as it cools. If that water gets trapped in a radiator or pipe, the next rush of steam slams into it. It's very common in NYC's older steam buildings, and usually fixable.

Renting? Heat is your landlord's job, not yours. Tell your landlord or super first (in writing, like a text or email, so you have a record). If it isn't fixed, file a complaint with 311 (call 311, use the 311 app or 311 online), and HPD can send an inspector. Don't pay a contractor to fix your building's heat. Our matching is for homeowners, co-op/condo owners and landlords.

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Emergency? Gas smell, CO alarm, fire, sparking or flooding: get to safety and call 911 first.

The usual causes

  • Valve partly closed on a one-pipe radiator, so water can't drain out while steam tries to come in.
  • Radiator pitched the wrong way. Floors settle over the years, and a one-pipe radiator needs to tilt slightly toward the valve.
  • Sagging pipes in the walls or basement that hold pools of water.
  • Boiler problems: water level too high, dirty boiler water that “surges,” or pressure set higher than the system needs.
  • Bad main vents that make steam move fast and unevenly.

Hissing and spitting

A little hiss from the air vent while the radiator heats is normal. Spitting water, or a vent that hisses nonstop, usually means a failing vent or water backing up into the radiator.

What you can check yourself

  1. Open the valve all the way. (On one-pipe steam, never leave it halfway.)
  2. When everything is cool, check the pitch with a level. It should lean slightly toward the valve. Thin wood or composite shims under the far legs are the classic fix.
  3. Make sure the air vent isn't painted over, and that nothing is pushing the radiator out of position.
  4. Note when it bangs (startup, mid-cycle, one room or all), since that helps a pro diagnose it faster.

Don't open, tighten or remove fittings while the system is running. Steam burns.

What a fix typically involves

A heating pro may re-pitch radiators, replace vents, re-pitch or support sagging pipes, skim or clean the boiler, correct the water level, or adjust the pressure control. Bigger pipe work should be done by a licensed plumber. Each building is different, so get a quote that spells out what's included.

Co-ops, condos and small buildings

Pipes that bang through the whole building usually trace back to the boiler or the mains, which belong to the building, not one unit. Owners in co-ops and condos should loop in management, who will want a contractor's certificate of insurance before any work in the unit.

Questions

Is a banging radiator dangerous?

Usually it's noisy rather than dangerous, but strong, repeated water hammer can stress fittings and cause leaks over time. Get it looked at.

Can I just close the radiator valve to stop the noise?

On one-pipe steam, a fully closed valve won't bang, but that room won't heat. A half-closed valve makes banging worse.

Who fixes banging in my rental?

Your landlord. Tell them or the super in writing, and file with 311 if heat problems aren't fixed.

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