What to Do When a Steam Radiator Bangs in Your NYC Apartment

A banging steam radiator is almost always water trapped where steam needs to pass, so make sure the radiator valve is fully open and the radiator tilts slightly toward that valve, then call a licensed plumber if the noise continues, because the cause is usually in the piping, the vents or the boiler.

Why steam radiators bang

Most Brooklyn brownstones, Manhattan pre-war co-ops and six-story walk-ups in Queens are heated by one-pipe steam. The boiler in the basement boils water, steam rises through a single pipe to each radiator, gives up its heat, turns back into water (condensate), and that water runs back down the same pipe. The system is quiet when steam goes up and water comes down without meeting in the wrong place.

Banging, which plumbers call water hammer, is the sound of steam hitting a pocket of standing water at high speed. One knock at the start of a cycle is common in old systems. Hammering every cycle means water is pooling somewhere. In NYC buildings the usual reasons are these:

• The radiator is pitched the wrong way. Floors in buildings from the 1890s to the 1930s settle. A radiator that was level when it was installed may now tilt away from its supply valve, trapping water at the far end.
• The supply valve is partly open. On one-pipe steam, steam and water share the same opening. A half-open valve chokes the water trying to leave while steam pushes in. Tenants turn the valve halfway to lower the heat, and that is the most common cause of banging in a single apartment.
• The air vent is stuck or clogged. The small vent opposite the valve lets air out so steam can enter. A vent that is painted over, corroded or spitting water keeps the radiator from filling evenly.
• A pipe in the basement has sagged. Steam mains and returns must slope back toward the boiler. A rusted hanger, or a main cut and re-supported during a renovation, creates a low spot that fills with water. This bangs in several apartments on the same riser, not just one.
• The boiler water is dirty or too high. Oil, rust and sediment make the water surge, so wet steam carries water up into the mains. An automatic feeder set too high does the same. Either one makes the whole building bang, loudest nearest the boiler.
• Two-pipe systems: a failed steam trap. Larger pre-war Manhattan buildings often have two-pipe steam with a trap at each radiator. A trap that fails open lets steam into the return lines, where it hammers against the water. One bad trap can make a whole line of apartments noisy.

What you can do in five minutes

1. Open the supply valve all the way. Turn it counterclockwise until it stops. If the room gets too hot, the right fix is a smaller or thermostatic radiator vent, not a half-closed valve.
2. Check the pitch. Put a small level on top of the radiator. If it does not tilt toward the valve, slide a thin shim under the legs at the vent end. Lift carefully: cast iron radiators are heavy and the pipe below is old.
3. Look at the air vent. If it hisses constantly, drips, or is painted shut, it needs to be replaced. A licensed plumber should size it, since vent size controls how evenly the building heats.
4. Listen for where the noise starts. If it starts in the walls or floor before your radiator gets warm, the problem is in the risers or mains, and the super or management company needs to call a plumber.

Do not loosen the union nut at the valve, do not remove the vent while the system is hot, and do not try to bleed a steam radiator. Steam radiators have no bleed valve.

Radiator still banging after these steps? Our dispatch answers 24/7 and can send a licensed plumber who works on steam heat every day.Call 718-748-2727

What a licensed plumber checks

When Nationwide Plumbers gets a banging call in a Brooklyn brownstone or a Manhattan co-op, the technician starts at the radiator and works back to the boiler:

• The valve seat and packing, the radiator pitch and the radiator vent.
• The main vents in the basement. Many buildings have none working, which also slows heat to the upper floors.
• The pitch of the mains and returns, and any low spots left by old repairs.
• The boiler water level, the low water cutoff and the automatic feeder.
• Water quality: a surging boiler needs to be skimmed or flushed.
• Steam pressure: a pressure control set too high makes hammer worse and wastes gas.
• On two-pipe systems, each radiator trap.

See our boiler repair and heating services pages for the full list of steam and hot water work we do.

Co-ops and condos: who is responsible

In most NYC co-ops and condos the shareholder or owner is responsible for the radiator, valve and vent inside the apartment, and the building is responsible for risers, mains and boiler. Check your proprietary lease or bylaws to be sure. If the cause is in the mains or boiler, we give the management company a written report with photos so the board can authorize the repair without a second visit. If the boiler is gas fired, the building's Local Law 152 gas piping inspection is a separate filing on its own schedule.

When to call a plumber

Call a licensed plumber when the banging happens every cycle and the steps above did not stop it; when more than one apartment on the same line is banging (a riser, main or boiler problem); when the radiator bangs and heats only at the valve end; when a vent spits water or steam into the room; or when the water level in the boiler's sight glass is jumping. If you smell gas near the boiler, leave the area and call 911 from outside.

During heat season, October 1 through May 31, NYC owners must keep apartments at 68 degrees during the day when it is below 55 degrees outside, and at 62 degrees at night, according to the NYC Department of Housing Preservation and Development. A hammering steam system usually heats unevenly too, so fixing it early means fewer heat complaints.

Steam radiator FAQ

Call Nationwide Plumbers at 718-748-2727, we answer 24/7.