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Understanding Factors Affecting Water Hammer Arrestor Size

Water hammer can be a significant issue in plumbing systems, causing noise, vibrations, and even damage to pipes and fixtures. The sudden stopping of water flow can create a pressure surge known as water hammer. One effective solution to this problem is the use of water hammer arrestors. But did you know that the size of the water hammer arrestor you need is influenced by various factors? In this blog post, we’ll explore these factors and provide insights into how to choose the right size water hammer arrestor for your system.

Water hammer arrestor sizing
Water Hammer Arrestors Sizing

Factors Affecting Water Hammer Arrestor Size

1. Length of Pipe Downstream

The length of the pipe downstream from the valve or faucet plays a crucial role in determining the size of the water hammer arrestor needed. Here's why:

  • Longer Pipes: The longer the pipe, the greater the volume of water in motion. When the flow is suddenly stopped, this large volume of water can create a significant pressure surge, necessitating a larger water hammer arrestor to absorb the shock.

  • Shorter Pipes: Shorter lengths of piping will have less water volume in motion, leading to less severe water hammer effects. Consequently, a smaller arrestor might suffice.

2. Water Pressure

  • High Water Pressure: Higher water pressure can lead to more intense water hammer effects. A larger or more effective water hammer arrestor is required to manage these higher pressure surges.

  • Low Water Pressure: Systems with lower water pressure may not require as large an arrestor since the pressure surges will be less severe.

3. Flow Rate

  • High Flow Rate: When water is flowing at a high rate and is suddenly stopped, the resulting water hammer can be quite severe. Larger arrestors are necessary to handle these situations.

  • Low Flow Rate: Lower flow rates result in less dramatic pressure surges, so smaller arrestors can often be used.

4. Pipe Diameter

  • Large Diameter Pipes: Larger pipes carry more water, which can create substantial water hammer effects. A larger arrestor is needed to manage the increased volume.

  • Small Diameter Pipes: Smaller pipes have less water in motion, resulting in smaller pressure surges that a smaller arrestor can handle.

Sizing Water Hammer Arrestors

Here’s what you typically need to know to choose the right size considering the above factors:

  • Static Water Pressure: The pressure in the system when no water is flowing.

  • Length of Pipe Run: The distance from the point of valve closure to the last fixture or the end of the pipe.

  • Pipe Size: The internal diameter of the pipe.

  • Fixture Unit Count: The number of fixture units or the flow rate at the point of valve closure.

Practical Considerations

  • Multiple Arrestors: For very long pipe runs, it might be necessary to install multiple water hammer arrestors at various points in the system to effectively manage water hammer.

  • Larger Arrestor: If a single, appropriately sized arrestor is insufficient, opting for a larger arrestor can effectively manage higher pressure surges and longer pipe runs.

  • Proper Placement: The arrestor should be placed as close as possible to the source of the water hammer (e.g., the valve or faucet that is being closed).


Choosing the right size water hammer arrestor is essential for protecting your plumbing system from the damaging effects of water hammer. By understanding the relationship between the length of the pipe downstream and other factors such as water pressure, flow rate, and pipe diameter, you can make an informed decision and ensure the longevity and reliability of your system. Proper sizing and placement will effectively mitigate water hammer and keep your plumbing system running smoothly.

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