Four-Story Apartment Building With Chatter and Water Hammer
A four-story, 42 ft tall apartment building in Tampa is served by a 2 in. domestic water service. The utility reports 92 psi static and 78 psi flowing at 25 gpm at the meter. The original 1-1/2 in. pressure-reducing valve at the meter has a 45 psi set point. After five years in service, the maintenance technician measures 68 psi downstream of the valve at no flow. Third- and fourth-floor tenants report water hammer, and the flushometer valves chatter and sometimes fail to flush. A reduced-pressure zone backflow assembly was installed at the meter last year, and there is no thermal expansion tank anywhere in the system. There is no booster pump.
1. What is the most likely cause of the complaints, and how would you verify it before ordering parts?
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Most likely cause: the valve has failed open, not drifted off its set point. A five-year-old PRV set at 45 psi that reads 68 psi downstream at zero flow has a torn diaphragm, a debris-fouled seat, or a stuck disc.
Verification sequence:
- Install calibrated gauges with bleed valves upstream and downstream of the PRV and take static readings, then flowing readings at the same time (open two hose bibbs and a hose stream). A healthy PRV holds its set point at low flow and sags only as flow increases; a failed valve reads high at static and does not control at all.
- Rule out thermal expansion. With the RPZ at the meter the system is closed. Read downstream pressure at 6 a.m., before any hot water has been used, then again after a day with no hot water draw. If the pressure only climbs on heater recovery, the valve is fine and the problem is expansion.
- Check the strainer and confirm no bypass is hung open.
Chatter at flushometers is consistent with an unstable seat and with expansion-driven pressure spikes, so both must be eliminated before anyone buys a valve.
2. If the valve is replaced, what set point and what accessory would you specify, and why? Show the arithmetic.
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Elevation loss: 42 ft ÷ 2.31 ft per psi = 18.2 psi from the meter to the top floor.
Target: a flushometer needs roughly 20 psi flowing at the fixture to work reliably, plus friction in the riser and branch at peak demand, call it 10 psi, plus the internal falloff of the valve itself at 25 gpm. A 1-1/2 in. body is undersized for a 2 in. service and can drop 10-15 psi internally; a 2 in. body halves that.
- Set point of 55 psi: lowest-floor static = 55 psi, comfortably under the 80 psi ceiling at which a PRV becomes mandatory.
- Top-floor static = 55 - 18.2 = 36.8 psi.
- Top-floor flowing = 36.8 - 10 (friction) - 6 (valve falloff) = about 21 psi, adequate for the flushometers.
Accessory: a listed thermal expansion tank is mandatory, not optional. The RPZ plus the PRV create a closed system, and the water heater has nowhere to push expanded volume. Size the tank to system volume and heater input, confirm the water heater T&P relief valve is piped to a safe discharge, and tag the new PRV with its set point.
3. What code and administrative items must be closed out before you consider the job finished?
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- Permit and inspection. Replacing a PRV and altering the domestic distribution is plumbing work. Pull the permit; most Florida jurisdictions will look for the set point on a tag and a flowing pressure reading at the top floor.
- Closed-system protection. Expansion tank installed and supported; water heater T&P unobstructed and piped to within 6 in. of the floor or to the exterior.
- Backflow. The RPZ must be tested annually by a state-certified backflow prevention assembly tester and the report filed with the utility or health department. Replacing a PRV downstream does not change that obligation, and a failed annual test can shut the service.
- Documentation. Send the owner a short letter with the measured static and flowing pressures before and after, the new set point, and the expansion tank sizing. That letter is your defense the first time a tenant complains again.
- Do not over-set the valve to cure a top-floor complaint: pushing the ground floor past 80 psi trades a chatter call for burst hoses and fixture failures.