g/plumbing • Posted by u/pipefitter_pat • 1 months ago

Water hammer at 120 PSI in a 12-story commercial building — getting called back constantly

12-story office building, 1970s construction. Building pressure is 120 PSI at the base — way too high, no PRV in the original design. We installed a Watts 25AUB PRV at the main entry 8 months ago, set to 80 PSI. Water hammer complaints continued.

Discovered the solenoid valves on the 7th and 8th floor HVAC water-cooled units are closing hard — quarter-second close time with 80 PSI on a 2" line. Calculated water hammer pressure spike: around 250 PSI.

Added hammer arrestors at each solenoid. Problem reduced but not eliminated. Now looking at slow-close solenoids or a second pressure zone above floor 6. Anyone dealt with high-rise hammer problems? Is a second PRV the right answer?

💬 3 replies

3 replies

Best New Old Controversial

u/pex_pioneer • 1 months ago

Hammer arrestors need to be sized correctly for the pipe volume and pressure — a lot of guys install the same arrestor regardless of line size. For a 2" line at 80 PSI, you need a much larger arrestor than what fits in the ceiling. What size are the arrestors you installed?

u/drain_dave • 1 months ago

Second PRV zone is the correct long-term answer for a building this size. A PRV at grade serving 12 floors is fighting physics — the hydrostatic pressure difference between floors adds about 0.43 PSI per foot, so floors 10-12 are already at lower pressure than floor 1 even with the PRV. Zoning by floor range with separate PRVs is standard practice on high-rises and should have been in the original design.

u/backflow_ben • 1 months ago

Slow-close solenoids are also worth exploring in parallel. Asco and Parker both make 2" slow-close solenoids with 3-5 second close time. At 80 PSI and 3-second close, your water hammer peak drops dramatically. Might be a faster fix than the second zone while you plan the PRV upgrade.