# Water hammer in household plumbing: causes and protection

> Why pipes bang when a tap closes, how water hammer damages a washing machine or water heater, and how to protect the plumbing with an arrestor.

Web page: https://watertrade.com.ua/en/articles/article/zashchita-ot-gidroudara · Other languages: [UA](https://watertrade.com.ua/articles/article/zashchita-ot-gidroudara.md), [RU](https://watertrade.com.ua/ru/articles/article/zashchita-ot-gidroudara.md)

*September 2, 2026*

![Diagram: pressure wave in a pipe after a valve is shut quickly and a water hammer arrestor next to the valve](https://watertrade.com.ua/uploads/articles/water-hammer.jpg)

Water hammer is a sudden pressure spike that happens when the flow in a pipe stops abruptly. Water is heavy, and its whole mass slams into the closed valve. For a fraction of a second the pressure can be several times higher than normal. You hear it as a bang or knock in the pipes.

## What the diagram shows

1 — the pressure wave travelling along the pipe, 2 — a water hammer arrestor (a small diaphragm tank), 3 — a valve that was closed quickly.

## Where it comes from

- Ball valves and single-lever mixers that are shut quickly.
- Solenoid valves in washing machines and dishwashers — they close instantly.
- A powerful pump stopping without a check valve or with a faulty one.
- Air in the pipes and excessive mains pressure.

## Why it is dangerous

Every bang stresses joints, flexible hoses, mixer cartridges and diaphragms. Sooner or later this ends in a leak — most often a flexible hose under the sink or behind the washing machine bursts.

## How to protect the system

1. **A water hammer arrestor** — a small tank with a diaphragm and 0.16–0.5 L of air. The air absorbs the shock like a spring. Fit it close to the source: on the branch to the washing machine or dishwasher, next to a fast-closing valve.
2. **A pressure reducing valve** at the inlet keeps 3–4 bar in the house instead of 6–8 bar in the mains.
3. **A pressure tank** in a pump system acts as an arrestor for the whole house.
4. **A habit** of closing taps smoothly.

> Mount the arrestor upright, air chamber on top, as close as possible to the valve that causes the shock.

Compact water hammer tanks are in our [pressure tanks](https://watertrade.com.ua/en/catalogue/gidroakkumulatory) section.

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