Four-Story Apartment Pressure and PRV Review in Nashua
A four-story, 32-unit apartment building is proposed on a city lot in Nashua. The water main is 1-inch type K copper at 65 psi static. The building has a 1-inch meter, a backflow preventer, and a PRV set at 60 psi. The developed length from the main to the top-floor flushometer water closets is 220 feet. The top floor is 42 feet above the main. The flushometer valves require 25 psi flowing pressure. The engineer calculates friction loss in the piping and meter at 12 psi at the peak demand of 55 gpm. The owner reports that in the existing building next door, nighttime static pressure reaches 95 psi. The contractor wants to delete the booster pump and set the PRV to 75 psi to save money.
1. Calculate the available pressure at the top-floor flushometer valves and determine whether the booster pump can be deleted.
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Use 0.433 psi per foot of elevation. A 42-foot rise costs 42 × 0.433 = 18.2 psi. With the PRV set at 60 psi, the static pressure at the top floor is 60 − 18.2 = 41.8 psi. Subtract the calculated friction loss of 12 psi at peak demand: 41.8 − 12 = 29.8 psi flowing. The flushometer valves require 25 psi, so there is a margin of about 4.8 psi at peak demand. The booster pump is not needed based on these numbers, provided the 12 psi friction loss is accurate and the PRV holds its setting. If the PRV is set to 75 psi, the top-floor flowing pressure would be higher, but that does not solve the real issue: the street pressure reaches 95 psi at night, so a PRV is required regardless. The contractor should keep the PRV and verify the calculation with the actual meter and backflow losses.
2. Evaluate the proposal to set the PRV at 75 psi and identify the required pressure-control devices.
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Setting the PRV at 75 psi is within the common 80 psi maximum for plumbing systems, but it leaves little margin and may cause fixture chatter, noise, and shortened valve life. A better setting is usually 50 to 60 psi, which protects fixtures while still meeting the top-floor demand. Because the building has a PRV and a backflow preventer, the system is closed; a thermal expansion tank is required on the domestic water system to absorb expansion from the water heater. The PRV must be accessible, have a strainer if required, and be installed with bypass or maintenance provisions as allowed. The backflow preventer must be tested and maintained under the local program. The inspector should also require water hammer arrestors at the flushometer groups and verify that the PRV is rated for the incoming pressure.
3. What if the nighttime static pressure exceeds 100 psi at the main? How does that change the design?
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If the nighttime static pressure exceeds 100 psi, the PRV becomes even more critical, and the designer must confirm that the valve is rated for the inlet pressure and that the downstream setting protects the system. A single PRV may not be enough if the pressure differential is large; a two-stage reduction or a PRV with a high-pressure inlet rating may be needed. The expansion tank must be sized for the system volume and the maximum heater temperature. The inspector should require a pressure gauge and a means to verify the setting. The booster pump should not be deleted merely because the daytime pressure is adequate; the design must work at both the lowest and highest street pressures. If the calculated top-floor pressure falls below 25 psi at peak demand after accounting for the actual pressure fluctuations, a booster pump or a larger main is required.