When must an approved mechanical water hammer arrestor be installed?

An approved mechanical water hammer arrestor must be installed as close as possible to quick-acting valves to effectively mitigate the impact of water hammer. Water hammer occurs when a liquid in motion is forced to stop or change direction suddenly, leading to shock waves that can damage pipes and fixtures. By positioning the arrestor close to these valves, it can absorb the shock before it travels further through the plumbing system, thereby preventing or minimizing noise and potential damage. This proximity allows for quick response to the pressure changes created when a valve opens or closes rapidly, ensuring the arrestor can function optimally.

The other options do not provide the same level of effectiveness in controlling the water hammer phenomenon. For instance, placing the arrestor far from the valve or at the main shut-off valve would not adequately address the immediate pressure fluctuations caused by the quick-acting valves. Similarly, installing it near the water heater does not align with the need for immediate dampening of pressure changes that occur at the valve locations.