Top-Floor Flushometer Valves Starved After a PRV Was Set for Old Piping
A three-story medical office building. The 2-inch domestic service comes off a 6-inch city main at 78 psi static; night pressure has been logged at 104 psi. A reduced-pressure backflow assembly at the meter is followed by a PRV set at 45 psi to protect the original 1970s piping downstream. The highest fixture is a flushometer water closet whose valve sits 48 feet above the PRV. At peak design flow the riser loses 6 psi and the branch to that closet loses 5 psi. The engineer's basis of design assumes a 15 psi minimum flowing pressure at flushometer valves. The first tenant reports the top-floor closets flush weakly and a solenoid lavatory faucet chatters. The water heater is a 100-gallon gas unit with a T&P valve but no expansion tank, and the meter has a check valve.
1. Compute the flowing residual pressure at the highest flushometer valve and determine whether it complies.
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Work the pressure budget downstream of the PRV:
- PRV downstream setpoint: 45.0 psi
- Static/elevation loss: 48 ft x 0.433 psi/ft = 20.8 psi
- Riser friction at peak flow: 6.0 psi
- Branch friction to the farthest valve: 5.0 psi
Residual at the valve = 45.0 - 20.8 - 6.0 - 5.0 = 13.2 psi
The code minimum for flushometer valves is 15 psi flowing, not static. At 13.2 psi the installation is noncompliant, and the weak flush and chattering solenoid are exactly what a marginal flowing pressure looks like. Note that the upstream meter and RPZ losses do not enter this calculation because they sit upstream of the PRV; the PRV's setpoint fixes the downstream starting pressure.
Do not report a static gauge reading as the answer. A static reading at the top floor will look fine (45 minus elevation) and will mislead everyone.
2. The owner proposes upsizing the riser from 2 inch to 2-1/2 inch. Evaluate that proposal.
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What it buys: the riser friction is only 6 psi. Going up one size on a short riser at this flow might recover 2 to 3 psi. That would put the valve near 15 to 16 psi - technically at the line, with no margin for a dirty aerator screen, a partially closed valve, or a future fixture added on the branch.
What it does not fix: the branch loss (5 psi) is unaffected, the 48 feet of elevation is unaffected, and the 45 psi PRV setting is unaffected. The root cause is the combination of a deliberately low PRV setpoint and a tall elevation, not pipe friction.
Better options, in order:
- Raise the PRV setpoint to about 60 psi, after verifying that downstream piping, water heaters, and fixture-rated maximums (generally 80 psi for fixtures) can take it. Recompute: 60 - 20.8 - 6 - 5 = 28.2 psi, comfortable.
- If downstream piping truly cannot see more than 45 psi, keep the PRV and add a small booster serving the top floor, with the fixture minimum as the design point.
- Only then consider pipe sizing.
Deliverable: a short written recommendation with the pressure budget so the owner sees why the repipe is the expensive non-solution.
3. The water heater has no expansion tank. Explain why one is required here, what failure you would expect, and roughly how much water volume is at stake.
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Why: the meter has a check valve and there is a PRV and an RPZ in the line. Together they close the system - water can come in but cannot push back out to the main. When the 100-gallon heater raises the water from about 50 F to 140 F, the water expands and has nowhere to go, so pressure climbs until something relieves it.
Expected failures: T&P valve weeping or lifting repeatedly, pressure spikes toward the relief setting, water hammer and chatter at solenoid and flush valves, and eventually damage to the heater, the PRV, or the meter. The chatter on the lavatory faucet is consistent with this.
Rough magnitude: expansion from 50 F to 140 F is on the order of 1.5 percent of volume, so about 1.5 gallons on a 100-gallon tank - small in volume, enormous in pressure in a closed piping system.
Key point to state to the owner: the T&P valve is a safety device of last resort, not a method of expansion control. Weeping it weekly shortens its life and leaves the system unprotected. Install a listed thermal expansion tank sized for the system volume and the incoming pressure, and check whether the PRV has an internal bypass or a bypass kit is needed.