Second-Floor Shower That Dies When the Irrigation Starts
A 3,200 sq ft two-story house is fed by a 1-inch type K copper service 90 ft long to a ¾-inch meter with an integral check valve, then 1-inch copper through a pressure-reducing valve with a check seat set at 55 psi. The purveyor's static pressure at the tap is 125 psi. The owner reports that the second-floor shower drops to a trickle whenever the 8 gpm irrigation zone runs, and that the shower valve has started to drip. A hose bibb at the front of the house reads 48 psi static with everything off and 22 psi while the irrigation and a second-floor tub filler are running. The PRV is 22 years old, has never been serviced, and there is no expansion tank on the 50-gallon gas-fired water heater.
1. Is the pressure-reducing valve required by code, and what do the two hose-bibb readings tell you about where the problem is?
Read the worked answer
Required. Static pressure at the tap is 125 psi. The code caps static pressure in the distribution system at 80 psi, so a reducing valve (or other approved means) must be installed and must be maintained in working order. A valve that is 22 years old with no service record is a maintenance finding in itself.
Reading the data:
- Elevation is not the problem. A second-floor fixture is roughly 10 ft above the bibb, worth about 4.3 psi - nothing compared with the observed collapse.
- 48 psi static downstream of a valve set at 55 psi is already suspicious; a 7-psi discrepancy suggests a drifting valve, a limiter problem, or a bad gauge. Calibrate the gauge first, then measure at test ports immediately upstream and downstream of the PRV.
- 22 psi flowing with only the irrigation plus one tub filler means roughly 103 psi of total loss between the main and the bibb. A ¾-inch PRV passing 8 to 10 gpm should not fall off anywhere near that much; the loss is concentrated at the PRV, the meter, or the 1-inch service.
Diagnostic order: read static and flowing pressure at the street side of the meter, at the outlet of the meter, and at both PRV ports. If the drop is across the meter or service, the fix is pipe and meter sizing; if it is across the PRV, rebuild or replace the valve with one sized for the connected load, not a generic ¾-inch body chosen by pipe size alone. Then re-verify residual pressure at the worst-case fixture with the design flow running.
2. The owner wants the dripping shower valve and the occasional weeping at the water heater's relief valve fixed by 'turning the PRV down.' Explain what is actually happening and what must be installed.
Read the worked answer
Turning the PRV down does not address it, and lowering the system pressure can make the symptom worse elsewhere.
The system is closed. The meter has an integral check valve and the PRV has a check seat, so there is no path for expanded water to push back toward the main. Every time the 50-gallon heater recovers, the heated water expands - on the order of three-quarters of a gallon for a tank that size and a normal temperature swing - into a piping system that cannot accept it. The pressure rises until something yields: the relief valve weeps, the shower valve seat drips, or a supply line or fixture connection takes the strain.
The relief valve is a safety device, not an expansion control device. Replacing it or raising its setting is a code violation and creates the hazard it exists to prevent.
Required correction:
- Install a listed thermal expansion tank on the cold side of the heater, sized for the system's stored volume, the heater capacity and the pressure differential between the PRV setting and the relief setting.
- Or use another approved method of thermal expansion control - but not the relief valve.
- Verify the relief valve is a new, correctly rated device (setting not exceeding 150 psi / 210°F) and that its discharge is full size and gravity-drained.
Then re-check the shower valve: a seat damaged by repeated pressure excursions may need repair regardless.
3. After the PRV is replaced and set at 55 psi, what flowing-pressure acceptance test would you run before signing off, and what result would you expect?
Read the worked answer
Acceptance is a flowing test, not a static gauge reading - the complaint was a flowing-pressure problem.
Test method:
- Open the worst-case demand: irrigation zone at its rated 8 gpm plus the second-floor shower, tub filler and a flushometer-free fixture group, with the hose bibb gauge on a port near the top of the system.
- Record residual pressure at the bibb and simultaneously at a port immediately downstream of the PRV.
- Compare the difference against the expected friction loss for the load; a large difference points to piping, a small difference with low pressure overall points to the valve or the service.
Expected result: residual pressure at the fixture should stay comfortably above the code minimum for fixture supply with the design load flowing, and the static pressure anywhere downstream must never exceed 80 psi. Elevation accounts for only about 4.3 psi to the second floor, so a properly sized valve and service should hold the shower within a few psi of the no-flow reading.
Also confirm at final: expansion tank pre-charged to the PRV setting for the domestic system, relief valve discharge correct, and the PRV tagged with its setting and date of service so the next technician can tell what is upstream.