Hillside PRV, Thermal Expansion, and Irrigation Pressure Drop
A 4,200-square-foot hillside house has 145 psi at the street. The 1-inch copper service enters through a pressure-reducing valve at the meter set to 62 psi, then runs 80 feet to a 3/4-inch main. A condensing tankless water heater with a 1/2-horsepower recirculation pump serves a master shower about 60 feet from the heater. There is no expansion tank. The homeowner reports that the shower goes cold for several seconds whenever the 12-zone irrigation controller opens a valve, and a hose bibb at the lower yard reads 105 psi on a gauge. A plumbing contractor proposes increasing the PRV setting to 75 psi to “improve flow” and adding a larger recirculation pump. The water district requires a backflow preventer on the irrigation service, which is connected upstream of the PRV. The house has copper pipe with some push-fit fittings, and the water heater has a 3/4-inch cold inlet.
1. Diagnose the 105 psi hose-bibb reading and the shower temperature fluctuation. Is the contractor’s proposal appropriate?
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The 105 psi reading is a red flag. Static pressure at a fixture should not exceed 80 psi; a reading that high downstream of a PRV set at 62 psi indicates the PRV is passing street pressure, is installed backward, has debris under the seat, or has failed. The irrigation connection upstream of the PRV is also a clue: the irrigation load may be drawing down pressure ahead of the PRV, but it cannot explain 105 psi at a hose bibb unless the PRV is not controlling. The shower cold slug occurs because a large irrigation demand causes a pressure drop and possibly reverses or starves the recirculation loop, allowing cold water in the hot line to reach the shower. Raising the PRV setting to 75 psi is the wrong direction-it raises pressure on the house and does not restore PRV control. A larger recirculation pump may mask the symptom but will not correct a failed PRV or an upstream irrigation tap. Repair or replace the PRV, verify its setpoint, and correct the irrigation connection so it is protected by a backflow preventer and does not create an uncontrolled pressure drop.
2. What code-required devices are missing or misapplied on this water system?
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A pressure-reducing valve is required where static pressure exceeds 80 psi, and it must be set to control the downstream pressure, typically around 50 to 60 psi. The PRV must be accessible and protected from freezing and debris. Because the PRV creates a closed system, a thermal expansion tank is required at the water heater unless the system has an approved means of accommodating expansion. Without it, heating water can raise pressure until the T&P relief valve opens or fittings are stressed. The irrigation service needs a backflow preventer appropriate for the hazard; a simple upstream tap does not satisfy the requirement and may allow irrigation water to cross-connect to the potable system. The recirculation pump should be controlled by a timer or aquastat, and the return line should be sized and balanced so it does not short-circuit the heater. Verify that the 3/4-inch cold inlet and the main are adequate for the fixture-unit demand and pressure loss; a 60-foot branch to a shower may need a larger line or a dedicated recirculation return.
3. How would you test and commission the corrected system before turning it back over to the homeowner?
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Install a calibrated pressure gauge on a hose bibb downstream of the PRV and record static pressure with no fixtures running. It should be at or below 80 psi and close to the PRV setpoint. Then flow-test the shower while the irrigation controller opens a valve, and watch for pressure drop and temperature stability. Confirm the recirculation loop is operating and that the pump is not running dry or short-cycling. Check the expansion tank precharge against the system pressure and confirm the tank is connected to the cold side without a check valve between the tank and the heater. Test the T&P relief valve discharge line for proper termination. Verify the irrigation backflow preventer is installed, tested if required, and located so it protects the potable supply. Finally, check accessible push-fit fittings for full insertion and any signs of weep, and advise the homeowner that the PRV may need periodic inspection and that the irrigation should not be operated simultaneously with high hot-water demand until the pressure issue is fully resolved.