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Plumbingaka: hydraulic shockaka: pipe hammer

Water Hammer

In Plain English

The banging sound and pressure shock that occurs in pipes when water flow is suddenly stopped — like when a washing machine valve closes.

Definition

A pressure surge or shock wave caused when a fluid in motion is suddenly forced to stop or change direction, often heard as a loud banging noise in pipes. Water hammer occurs when quick-closing valves, solenoid valves, or washing machine fill valves abruptly stop water flow. It can damage pipes, fittings, and appliances over time and is prevented by air chambers or water hammer arrestors.

Why It Matters in Bidding

Water hammer arrestors are a small line item that estimators routinely miss, yet code and good practice require them at quick-closing valves and appliance connections. Leaving them out creates a scope gap and a callback risk; the sub may add them as a change after award once the spec or inspector flags the omission.

Example

Pricing a multifamily project, the plumbing estimator adds a water hammer arrestor at each washing machine box and dishwasher connection after the spec calls them out, rolling the device cost and a few minutes of install labor into the rough-in line.

Related Terms

ValveSupply LinePressure RegulatorDomestic Water

Frequently Asked Questions

Yes, when the spec or code requires them at quick-closing valves, washing machine boxes, or solenoid-fed equipment. Estimators count arrestor locations during takeoff and add device cost plus install labor. Omitting them is a common scope gap that surfaces as a change order or warranty callback after the system is pressurized.
They review fixture and equipment locations for quick-closing valves, then add arrestors or air chambers at each point per the spec. The cost is small per unit but adds up across many connections in multifamily or commercial work, so accurate location counts keep the rough-in price defensible.
Yes. If arrestors are omitted and pipes bang or fittings loosen over time, the contractor faces warranty callbacks and possible damage claims. Pricing the prevention correctly upfront protects margin and avoids disputes over whether the protection was part of the original scope.

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