Banging pipes: water hammer, steam, and the other causes

Pipes that bang are telling you something specific, and the moment the noise happens is the clue. A bang when a faucet shuts off is water hammer. A clank as the heating comes on is something else entirely. Here is how to tell them apart and what each one needs.

A bang the moment a faucet or appliance shuts off: water hammer

Water moving through a pipe has weight and speed. When a valve closes suddenly — a washing machine or dishwasher finishing its fill, a single-handle faucet snapped shut — that moving water stops dead, and the pressure wave slams through the pipe. That thud is water hammer.

It is worth fixing rather than living with. Every hit stresses joints, valves and the fittings on appliances, and over time that is how a noise turns into a leak.

How water hammer is fixed

• Water hammer arrestors. Small sealed cylinders installed near the valves that cause it — the washing machine, the dishwasher, a shower. A cushion inside absorbs the shock.
• Check the house pressure. High incoming pressure makes water hammer much worse. If there is a pressure reducing valve where the water comes in and it has failed, the banging often starts suddenly with no other change.
• Secure loose pipe. A run that is not clipped down moves with every hit and bangs against the joists. Clipping it often quiets it even if the hammer itself is still there.
• Old air chambers. Older houses often have short capped pipes beside fixtures that once worked as cushions. They fill with water over the years and stop working. Draining the system down and refilling recharges them for a while; arrestors are the lasting answer.

Ticking or knocking as the heating warms up

A steady tick or creak as the heat comes on, and again as it cools, is piping expanding and contracting where it passes through notches in joists or sits tight against a clip. Annoying, not dangerous, and usually fixed by padding or re-clipping the run.

Clanking and banging on a steam system

On a one-pipe steam system — still common in the pre-war housing of Yonkers and the river villages — banging as the heat comes up is usually water sitting where steam needs to travel. The steam hits it and drives it along the pipe with a crack.

• A failed air vent on a radiator, so it cannot vent and fill properly.
• A radiator pitched the wrong way, so condensate cannot drain back toward the boiler.
• A sagging pipe or return, where water collects in the low spot.
• A boiler water level set too high, pushing water up into the steam mains.

Most of these are adjustments rather than replacements. For other symptoms on the same system, see boiler not working.

Gurgling in hot water radiators

On a hot water (rather than steam) system, gurgling or rushing water in the radiators usually means air in the system. Bleeding the radiators normally fixes it; if it keeps coming back, air is getting in somewhere.

When to call about water hammer or banging

Call if the banging started suddenly, if it is getting worse, if it comes with a change in water pressure, or if it is the steam heat and adjusting the radiators has not helped. Water hammer is a short job once the cause is found; steam banging is boiler repair and radiator work, and usually not the expensive kind.

KBD Plumbing & Heating works across lower Westchester — Dobbs Ferry, Hastings-on-Hudson, Irvington, Yonkers, White Plains and the rest. Licensed and insured, price agreed before work starts, and the phone is answered any hour including holidays: (914) 233-3000.

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