Plumbing Study Guide – Water Hammer & Water Hammer Arrestors

What Is Water Hammer?

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Water hammer is a sudden pressure surge in plumbing when flowing water is abruptly stopped — for example, when a solenoid valve closes, a washing machine finishes filling, or a toilet shuts off. This surge travels back through piping and can cause:

• Loud banging noises
• Stress on pipes, fittings, valves
• Joint fatigue and potential leaks
• Damage to appliances or fixtures

The longer the run and the faster the water velocity, the greater the force exerted on the pipes, sometimes exceeding static pressure. The last 15% of the valve closure is directly related to the intensity of the increase in pressure. 

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Why Water Hammer Is a Concern

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Without mitigation:

• Pipe supports loosen
• Connections weaken
• Systems fail prematurely
• Fixtures break 
• Valves break 
• Causes unnecessary/ preventative pressure fluctuations within system 

Older DIY air chambers (simple capped pipes) were common but lost air quickly, becoming ineffective. Modern pre-manufactured water hammer arrestors with sealed gas chambers are required. The resulting pressure developed in the water distribution system will be approximately 60 times the normal pressure. 

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Formula: Water hammer shock (PSI)= Flow velocity (ft/sec) X 60 

National Plumbing Code – What It Says

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NPC Section 2.2.10.15 – Water Hammer Arrestors

• Factory-built water hammer arrestors must conform to ANSI/ASSE Standard 1010 – Water Hammer Arresters. 

This means:

• Arrestors must be certified mechanical devices, not makeshift air-chambers.
• They must meet recognized performance requirements (ANSI/ASSE 1010).
• Installation must follow manufacturer instructions and meet NPC pipe and fitting standards.

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NPC Section 2.6.1.14 – Water Hammer Prevention

The Code identifies water hammer as a pressure buildup that should be prevented wherever valves and faucets exist, especially at the end of long runs. It explicitly allows either:

• Manufactured water hammer arrestors, or
• Air chambers were appropriate, but required a specific length if fabricated from pipe. 

Air chambers must be:

• 300 mm to 450 mm in length if the same diameter as the supply pipe
• Shorter if the chamber is a larger diameter.
• Accessible if they have an air valve or a diaphragm type
• Should have a drain port and/ or charge port 

Types of water hammer arrestors:

• Piston (most popular) 
  
  • Utilizes a piston and air chamber to cushion the water shock 
• Diaphragm (not common) 
  

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How Water Hammer Arrestors Work

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Modern arrestors use a sealed chamber with a piston or diaphragm that:

1. Absorbs pressure shock when water flow stops
2. Compresses the air/gas cushion
3. Prevents the pressure spike from reflecting into the system
4. Releases the pressure back into the pipe gently over time, preventing banging, noise, and damage. 

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Where To Install Arrestors

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Water hammer arrestors should be installed:

• As close as possible to the source of the shock (quick-closing valves).
• On both hot and cold supply lines, where valves may shut abruptly (e.g., toilets, washing machines, as required by code).
• For branch lines up to 20 feet, place one unit between the last two fixtures. For branches longer than 20 feet, place two units: one at the end of the line and another at the midway point.

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Design & Sizing Considerations

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Device Standard‍

Water hammer arrestors shall conform to ANSI/ASSE 1010 in accordance with NPC 2.2.10.15. (1).

Location & Quantity

Install arrestors:

• At each quick-closing valve (upstream of the quick-closing valve)
• On both hot and cold lines of fixtures that shut off water abruptly
• At the end of long, dead-end runs

Larger branches with multiple fixtures may require multiple devices to absorb energy adequately. Plumbing design must consider fixture unit loads and flow rates.

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When fixture unit values have been determined, the table below can be applied 

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Water Hammer Arrestors on long piping runs to equipment 

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Note: When water pressure in the line exceeds 65 psi, upsize to the next sized water hammer arrestor. All information provided is based on 10 feet per second or less. 

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Typical Plumbing Install Order

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While water hammer arrestors are not part of conditioning in the same way as water filters or softeners, they integrate into the domestic water distribution:

1. Municipal/Service Inlet
2. Pressure Regulating Valve (if required by design)
3. Distribution Branches to Fixtures
4. Water-hammer arrestors are installed near quick-closing valves.
5. Fixture Supply (e.g., washing machine, toilet, solenoid valves)

They are not part of water treatment systems but protect the system following conditioning, softeners, filters, etc.

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Maintenance & Lifecycle

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• Modern sealed arrestors are maintenance-free if installed correctly.
• They should be accessible for inspection and replacement.
• Older pipe air chambers are no longer recommended because they gradually lose air and become ineffective, requiring repair or replacement with modern devices. 

Inspect arrestors if system noise returns; failed arrestors may need to be replaced.

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 Installation Best Practices

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• Install with accessible fittings (no buried or hidden locations).
• Follow the manufacturer's instructions for orientation and connection.
• Use compatible fittings and pipe materials.
• Avoid makeshift air chambers where code requires certified devices.
• Isolate arrestors in accessible boxes or panels if behind walls

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  Common Field Problems

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Water hammer velocity example calculation

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If the velocity of the water in a piping system were 30 feet per second, what would the resulting water hammer pressure be? 

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30 (ft/sec) X 60= 1800 PSI 

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Example Layout (Branch Line Longer Than 20 ft)

• Pipe size: 3/4" branch line
• Length of branch: 40 ft (12 m)
• Fixture: Washing machine (quick-closing solenoid valve)

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Water hammer arrestor placement

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Primary arrestor — near the quick-closing valve

• Install the primary arrestor within a few feet of the fixture connection.
• Install arrestors on both hot and cold supply lines to the washing machine.

Secondary arrestor — mid-run of the branch

• Since the line exceeds 20 ft, install another arrestor about halfway down the branch.

If the branch has 20 fixtures, for example, the placement of the water hammer arrestor is as follows: 

The 1st water-hammer arrestor is located between the last fixture and the second-to-last fixture on the branch. The 2nd water-hammer arrestor is located halfway along the fixtures. In this scenario, outlined with 20 fixtures, the second arrestor would be located between the 10th and 11th fixture. Both water hammer arrestors would be size “B.”

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Mock C of Q question

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A commercial building has three flushometer water closets connected to a 1″ branch line that is 12 m long from the main distribution pipe. Each flushometer valve is a quick-closing device. Loud hammering occurs when the valves close.

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According to the National Plumbing Code, what is the correct method to control water hammer in this installation?

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A. Install one water hammer arrestor at the water meter

B. Install a minimum of 2 water hammer arrestors on the branch line serving the flushometer valves

C. Install expansion tanks near the flushometer valves to absorb shock 

D. Install only 1 water hammer arrestor on the branch line serving the flushometer valves

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Correct Answer

B — Install water hammer arrestors on the branch line serving the flushometer valves

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Step-by-Step Reasoning with Code References

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1. NPC requires protection from water hammer

2.6.1.6. Water Hammer

Water distribution systems shall be designed and installed to prevent excessive water hammer.

Quick-closing valves present

Long branch line

Hammer occurring

Protection required

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2. Flushometer valves are high-risk for hammer

Flushometer valves:

• Close quickly
• High flow rate
• Causes strong pressure shock.

These are among the most common exam examples that require arrestors.

Because the branch is long (12 m = 39.36 ft):

• A shock wave travels through the pipe.
• An arrestor must be installed on the branch.

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3. Location of arrestors

Good practice (and exam expectation):

• Install an arrestor near quick-closing devices.
• Install on the branch line serving fixtures.
• Additional arrestors may be required on long runs.

This satisfies the intent of:

NPC 2.6.1.6 — prevent excessive water hammer

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4. Why the other answers are wrong

A — Arrestor at the meter

Too far from quick-closing valves

Does not control local shock

C — Install expansion tanks near the flushometer valves to absorb shock 

Does not eliminate hammer, only good for thermal expansion. 

D — Install only 1 water hammer arrestor on the branch line serving the flushometer valves

Branch is longer than 20 feet which require additional arrestors. 

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Quick Summary – What You Must Know

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• Water hammer is a real issue and can damage plumbing if not mitigated.
• The NPC recognizes water hammer and allows it to be prevented with arrestors or air chambers. 
• Arrestors must conform to ANSI/ASSE 1010 as per NPC 2.2.10.15.(1). 
• Install near quick-closing valves and ensure accessibility.
• Modern arrestors are preferred over simple air chambers.
• Confirm installation with manufacturer instructions and NPC compliance.