Water hammer is the sharp, loud banging noise heard in plumbing systems. This sound occurs when a fast-moving column of water is abruptly stopped by the rapid closure of a valve, causing a sudden and significant pressure surge within the pipes. This pressure wave travels backward through the system, creating the characteristic noise and potentially damaging pipes, fittings, and appliances. Water hammer arrestors are mechanical devices designed to absorb this shock wave by providing a cushion, typically of air or gas, that compresses when the pressure spikes. Correct placement of these arrestors is necessary to effectively mitigate the damaging pressure spikes.
Diagnosing the Source of Water Hammer
Effective placement of an arrestor depends entirely on accurately identifying which fixture or appliance is generating the pressure surge. The most common method involves listening for the exact moment the loud bang occurs relative to a specific action. If the noise happens immediately after the toilet fill valve shuts off, or when a washing machine solenoid valve closes, that fixture is the cause.
The nature of the sound can also provide clues about the underlying issue. A single, sharp bang typically indicates a single-event pressure spike caused by a single valve closure. Conversely, a sustained, rattling noise or vibration suggests a more complex issue, possibly involving loose piping or a faulty pressure regulator. Pinpointing the exact moment and location of the noise helps confirm the required proximity for the arrestor installation.
Placement Near Specific Fixtures
Appliances that use solenoid valves, such as washing machines and dishwashers, are the most frequent culprits of water hammer due to their extremely fast closing action. These valves can stop the water flow in milliseconds, generating the most forceful pressure waves. For these high-risk appliances, the arrestor should be installed within 6 to 18 inches of the shut-off valve. Placing the device close to the source ensures it intercepts the pressure wave before it travels through the house plumbing.
Refrigerators with built-in ice makers or water dispensers also utilize quick-closing valves. Despite the smaller pipe size, the sudden stop of water can still create disruptive noise and stress on the supply line connections. The arrestor should be placed directly on the cold water supply line servicing the refrigerator, keeping the device as close as possible to the point of valve closure.
While less common, certain quick-closing faucets, single-lever mixing valves, or thermostatic shower valves can also cause minor water hammer issues. In these instances, the arrestor should be placed in the wall cavity, directly behind the fixture connection point, and attached to the branch line supplying the troublesome valve.
When the source of the pressure surge cannot be reliably traced to a single fixture, or if multiple appliances contribute to the problem, a whole-house approach may be necessary. Installing a larger capacity arrestor on the main water line, usually near the pressure reducing valve or the main shut-off point, offers broad protection. This placement acts as a general buffer for the entire system against overall high pressure spikes.
Installation Methods and Orientation
The physical orientation of the arrestor is a significant factor in its long-term performance, especially for traditional piston-style units. These mechanical devices rely on an air or gas cushion to absorb the pressure wave, which works most effectively when the arrestor is mounted vertically with the piston end facing upward. This upward orientation uses gravity to keep the piston centered and correctly positioned within the cylinder.
While piston-style arrestors benefit from the vertical, upward installation, modern sealed bellows-type arrestors are more forgiving regarding their mounting angle. Bellows units use a sealed, permanent cushion of gas, making them effective even when installed horizontally or facing downward. However, for maximum efficiency and longevity, the vertical, upward position remains the preferred installation method for all types.
Arrestors can be installed either as point-of-use devices, directly attached to the supply line of a single fixture, or as in-line units integrated into the branch piping system. Point-of-use installation is generally preferred for high-impact appliances like washing machines, as it places the shock absorber closest to the source. New construction projects often integrate the arrestors into the pipework behind the wall, ensuring they are positioned correctly before the drywall is installed.
Code Requirements and Routine Care
Plumbing standards often dictate the mandatory inclusion of water hammer arrestors in new construction and major renovation projects. Codes like the Uniform Plumbing Code (UPC) and the International Plumbing Code (IPC) generally require the installation of these devices on supply lines serving quick-closing valves. Compliance with these requirements ensures the longevity and safety of the plumbing system by mitigating excessive pressure spikes that could lead to pipe failure.
Water hammer arrestors are designed to be largely maintenance-free, particularly the modern sealed units, but routine attention can extend their effectiveness. If the characteristic banging noise returns after a period of quiet operation, it often indicates that the air cushion within the arrestor has become depleted or waterlogged. Older, non-sealed air chamber units can sometimes be recharged by temporarily shutting off the main water supply and draining the system to restore the air pocket. Consulting a licensed plumber is advisable if the noise persists or if the installation requires modifications to inaccessible walls.