Experiencing a loud banging or knocking sound in your pipes when water is shut off quickly? This is likely water hammer, and the short answer to how many water hammer arrestors you need is: it depends on your plumbing system’s complexity and the number of water-using fixtures and appliances that can cause it. You might need just one or several strategically placed throughout your home.
Water hammer, also known as pipe shock, is a pressure surge that occurs when moving water is suddenly stopped or its direction is abruptly changed. Imagine a train speeding up and then slamming on its brakes – the force of that momentum has to go somewhere. In your pipes, this force creates a shockwave that travels back and forth, causing that distinctive hammering sound. Over time, this can lead to leaks, damage to pipes, valves, and even appliances. Thankfully, water hammer solutions are readily available in the form of water hammer arrestors, which act as pipe shock absorbers.
Fathoming the Science Behind Water Hammer
To properly address how many water hammer arrestors you need, it’s essential to grasp the basic mechanics of how water hammer occurs.
When a faucet is turned off quickly, or a solenoid valve in an appliance like a washing machine or dishwasher closes suddenly, the flow of water stops instantly. The water that was moving forward doesn’t just disappear; its momentum is transferred to the pipes. This creates a high-pressure wave. This wave travels through the piping system until it encounters an obstruction or a change in direction, at which point it reflects and travels back. This process repeats, creating a series of pressure spikes and oscillations that we perceive as the “hammering” sound.
Common Culprits of Water Hammer
Several plumbing fixtures and appliances are notorious for causing water hammer due to their quick-acting valves:
- Washing Machines: Their solenoid valves shut off water flow very rapidly.
- Dishwashers: Similar to washing machines, they use fast-acting solenoid valves.
- Ice Makers: The valve that controls water to the ice maker is often small and closes quickly.
- Solenoid-Operated Sprinkler Systems: When these valves close, they can cause significant surges.
- Quick-Closing Faucets and Toilets: While less common than appliance-related issues, some older or poorly designed fixtures can contribute.
- High Water Pressure: While not a direct cause, high municipal water pressure exacerbates the effects of water hammer, making it more pronounced.
Determining Your Arrestor Needs: A Step-by-Step Approach
Deciding how many water hammer arrestors are needed involves a systematic evaluation of your plumbing system and the potential for hammer-inducing events.
Step 1: Identify Potential Water Hammer Sources
The first step is to walk through your home and identify all the fixtures and appliances that use water, especially those with quick-acting valves.
- Appliances: List all appliances that connect directly to your water supply. This typically includes dishwashers, washing machines, ice makers, and potentially some high-end coffee makers or refrigerators with water dispensers.
- Fixtures: Consider faucets (especially kitchen and bathroom sink faucets), toilets, and shower valves. While some toilets and faucets are designed to close gradually, older or less robust ones can still be a source.
- Outdoor Features: Sprinkler systems, hose bibs (especially those with quick shut-off valves), and even outdoor shower stalls can be culprits.
Step 2: Assess the Severity of the Problem
Is the water hammer a minor annoyance or a significant structural concern?
- Listen: Pay attention to when and where the sounds occur. Is it localized to a specific area or does it seem to propagate through the entire system?
- Frequency: Does it happen every time a specific appliance runs, or is it intermittent?
- Impact: Have you noticed any leaks, dripping faucets, or damage to pipes that could be linked to the banging?
Step 3: Consider Your Plumbing System Layout
The design and complexity of your plumbing system play a crucial role.
- Single-Family Home: In a typical single-family home, you might focus on the most common culprits first.
- Multi-Story Building: In a multi-story building, water hammer can be amplified due to the increased height and potential for more complex pipe runs. The effect of gravity on the water column is more significant.
- Pipe Material and Size: The material and diameter of your pipes can influence how sound and shockwaves travel. Smaller, rigid pipes might transmit the sound more readily.
Step 4: Where to Place Arrestors for Maximum Effectiveness
Arrestor placement is just as important as the number of arrestors. The goal is to install them as close as possible to the source of the water hammer.
- Near Appliances: For appliances like washing machines and dishwashers, the ideal placement is on the supply line directly behind the appliance. This means you might need to temporarily disconnect the supply hoses to install the arrestor.
- Near Faucets and Fixtures: For faucets, especially those that are frequently used or have known quick shut-off issues, an arrestor can be installed on the supply line just before the fixture connects. This can be done at the valve under the sink or behind the toilet.
- Main Supply Line: In some cases, a single arrestor on the main cold and hot water lines near the water heater might help mitigate overall system shock, but it’s often less effective than point-of-use installation for specific appliance issues.
Types of Water Hammer Arrestors and Their Applications
There are primarily two main types of water hammer arrestors, each with its own advantages. The choice can influence how many you might need or prefer.
1. Air Chamber Arrestors (Older Style)
These are essentially vertical lengths of capped pipe installed above the shut-off valve. The trapped air in the chamber compresses when the shockwave hits, absorbing the energy.
- Pros: Simple design, no moving parts, inexpensive to make.
- Cons: Air can be absorbed into the water over time, rendering them ineffective. They need to be periodically recharged by draining the system, which is inconvenient. They can also be bulky.
- Number Needed: If using this type, you’d still follow the placement guidelines, but their declining effectiveness over time makes them less desirable for a comprehensive plumbing system protection strategy.
2. Piston/Diaphragm Arrestors (Modern Style)
These are the most common and effective type of water hammer arrestor. They contain a sealed chamber with either a piston or a flexible diaphragm separating the water from a compressed air or gas charge.
- Pros: Highly effective, long-lasting, and require no maintenance. They are also more compact.
- Cons: Slightly more expensive than basic air chambers.
- Number Needed: This is where the core question comes in. For a modern, quiet plumbing system, you’ll likely want one for each appliance or fixture that exhibits water hammer.
Estimating the Number of Arrestors for Your Home
Let’s break down how to estimate the number of arrestors for different scenarios. The goal is to provide surge protection where it’s most needed.
Scenario 1: The Typical Single-Family Home
In a standard single-family home, focus on the most common culprits.
- Washing Machine: Nearly always requires an arrestor. (1 arrestor)
- Dishwasher: Another prime candidate. (1 arrestor)
- Ice Maker/Water Dispenser: If you have one, it’s a good idea. (1 arrestor)
- Kitchen Faucet: If it’s used frequently and shuts off abruptly. (1 arrestor)
- Bathroom Faucets (especially near toilets): If you hear hammer when flushing or washing hands. (1-2 arrestors, depending on how many sinks are problematic)
- Toilets: Less common, but if a noticeable bang occurs after flushing, consider one on the supply line. (1 arrestor per toilet if needed)
- Shower Valve: If you experience hammer when turning off the shower. (1 arrestor)
Estimated Total for a Single-Family Home: 3-7 arrestors, depending on the number of appliances and fixtures exhibiting the problem. You might start with the appliances and add more if necessary.
Scenario 2: The Multi-Story Building
In a multi-story building, the height of the water column can create more significant pressure variations, and hence more potential for severe water hammer.
- Each Unit/Apartment: Each unit will likely need arrestors for its appliances (washing machine, dishwasher, ice maker).
- Common Areas: If there are shared laundry facilities, each machine will need an arrestor.
- Vertical Risers: In larger buildings, especially those with high water pressure, you might consider an arrestor on the main cold and hot water risers on each floor to help manage general system surges, in addition to point-of-use devices.
- Building-Wide Assessment: A more thorough assessment by a plumber might be necessary, as the scale of the system increases the complexity.
Estimated Total for a Multi-Story Building: This can range from dozens to hundreds of arrestors, depending on the size of the building and the number of units and shared appliances. The focus remains on point-of-use protection for each appliance and fixture.
Scenario 3: Appliance Water Hammer Focus
If your primary concern is appliance water hammer, you can simplify the process.
- Identify all appliances connected to water.
- Install one arrestor on the supply line for each appliance that has a quick-acting valve. This will generally be washing machines, dishwashers, and ice makers.
This targeted approach can provide significant relief for the most common sources of loud water hammer.
Practical Considerations for Installation
Once you’ve decided on the number and placement, consider these practical aspects:
Installing Arrestors on Existing Plumbing
- Shut off the main water supply to your home.
- Drain the lines: Open the lowest faucet in your house to drain the water from the system.
- Access the supply line: For appliances, you’ll typically need to pull them out slightly to access the water inlet hoses. For faucets, you’ll access the shut-off valves under the sink or behind the toilet.
- Install the arrestor: Most arrestors thread directly onto the male end of the supply valve or outlet. Ensure you use pipe thread tape (PTFE tape) for a watertight seal.
- Reconnect and test: Reconnect the appliance hoses or fixture supply lines. Turn the main water supply back on slowly and check for leaks. Run the appliance or fixture to test if the water hammer has been resolved.
Arrestor Sizing
While this guide focuses on how many, it’s worth noting that arrestors come in different sizes (e.g., 1/2 inch, 3/4 inch). Ensure the arrestor’s connection size matches your existing plumbing fittings. The capacity of the air chamber within the arrestor also matters; larger chambers can handle more significant surges. For most residential applications, standard-sized arrestors are sufficient.
When to Call a Professional
While installing water hammer arrestors is a manageable DIY project for many, there are situations where professional help is recommended:
- Complex Plumbing Systems: If you have an older home with potentially outdated or uniquely routed plumbing, a plumber can better assess the system.
- High Water Pressure: If your home consistently experiences high water pressure (above 80 PSI), a plumber can install a pressure-reducing valve (PRV) in addition to arrestors. A PRV can prevent excessive pressure from exacerbating water hammer.
- Persistent Issues: If you install arrestors and still experience significant water hammer, there might be other underlying issues with your plumbing, such as improperly secured pipes or air trapped in the system, that require professional diagnosis.
- Inability to Access Lines: If you cannot easily access the necessary plumbing lines for installation.
Frequently Asked Questions About Water Hammer Arrestors
Q: Can I use just one water hammer arrestor for my whole house?
A: While a single arrestor on the main line might offer some minor relief, it’s generally not sufficient for effective plumbing system protection. Water hammer is often localized to specific appliances or fixtures. For the best surge protection and to eliminate appliance water hammer, it’s best to install arrestors close to the source of the problem.
Q: How do I know if my pipes are damaged from water hammer?
A: Signs of damage include visible leaks from pipe joints or fittings, dripping faucets that weren’t dripping before, loose pipes, or even small cracks appearing in visible pipework. If you suspect damage, shut off the water to the affected area and call a plumber immediately.
Q: What is the difference between a water hammer arrestor and a shock arrestor?
A: These terms are often used interchangeably in the plumbing industry. Both refer to devices designed to absorb the shockwaves (water hammer) in a plumbing system.
Q: Can I install arrestors on both hot and cold water lines?
A: Yes, absolutely. Water hammer can occur on both hot and cold water lines, especially if you have a water heater with a quick-shutoff valve or if appliances draw from both. It’s often advisable to install arrestors on both lines if the problem persists on either.
Q: Do I need arrestors if my water pressure is low?
A: While high water pressure exacerbates water hammer, it can still occur with low to moderate pressure if the shut-off is sudden enough and the system is prone to it. However, if your pressure is consistently low, water hammer might not be your primary plumbing concern.
By following this comprehensive guide, you can accurately determine how many water hammer arrestors you need to protect your plumbing system and enjoy a quieter, more reliable home.