What causes water hammer in HVAC piping systems?

• A. Water quality changes
• B. Gradual pressure buildup
• C. Sudden change in fluid velocity
• D. Increase in ambient temperature

Answer: (C)

Water hammer happens when a moving column of liquid is forced to stop or change speed abruptly. The inertia of the water means it keeps moving even as a valve closes or a pump trips, generating a pressure surge that travels through the pipe. In HVAC piping, this transient surge can be strong enough to cause noise, vibration, or damage, so the key factor is the sudden change in fluid velocity, not changes in water quality, a slow, gradual pressure buildup, or ambient temperature shifts. If flow is controlled smoothly—soft-close valves or gradual pump ramp—the hammer effect is greatly reduced.