Short definition
A pressure reducing valve (PRV) is a self-regulating valve that lowers incoming water pressure to a stable, safe downstream value regardless of supply-side fluctuations. It’s required by the Uniform Plumbing Code adopted in Washington any time supply pressure exceeds 80 psi at any point. In hilly cities like Seattle, Tacoma, Bellevue, and Spokane, downhill homes often need one — and most repipe quotes include the PRV plus a companion expansion tank as a package.
What it is
The PRV is a brass or bronze valve body containing a spring-loaded diaphragm or piston that responds to downstream pressure. As downstream pressure rises toward the setpoint, the diaphragm lifts and partially closes the valve. As downstream pressure falls, the diaphragm relaxes and the valve opens further. The mechanism continuously adjusts to deliver the target pressure regardless of the supply-side pressure.
A spring-tension adjustment on top sets the target. Residential PRVs are typically adjustable across a 25–75 psi range, with 50–55 psi as the common factory default. The body has a 3/4-inch or 1-inch female pipe-thread inlet and outlet for typical residential service.
A PRV creates a “closed system” downstream. With unrestricted supply, hot-water thermal expansion can flow back upstream into the main; with a PRV in the way, the expanding hot water has nowhere to go. The pressure inside the closed system rises with each heating cycle, and the water heater’s TPR valve eventually weeps. The fix is an expansion tank at the water heater — a small captive-air vessel that absorbs the expansion. Code requires the expansion tank whenever the PRV is installed; most plumbers install both as a package.
Why it matters to a homeowner
The PRV is the answer to high-pressure damage symptoms:
- Water-heater TPR drips during heating cycles.
- Banging pipes from quick-closing valves at higher-than-design pressure.
- Burst washing-machine fill hoses at a frequency above what’s normal.
- Premature solenoid valve failures in dishwashers, washers, and icemakers.
- Toilet fill-valve or angle-stop leaks showing up across multiple fixtures over a short period.
A static pressure test (a $10 hose-bib gauge) tells you whether you’re over the 80 psi threshold. If you are, the PRV install is the standard remediation. Typical 2026 WA cost: $300–$800 for the valve plus installation, plus another $100–$250 for the expansion tank if one isn’t already in place. Existing 15+ year-old PRVs whose downstream pressure varies with supply changes are also typical replacement targets.
When you’ll encounter this term
- A static pressure test reads 90+ psi and the home doesn’t have a PRV.
- A water-heater TPR valve drips and the diagnostic chain leads to over-pressure.
- An insurance underwriter asks for PRV documentation before issuing coverage.
- A repipe quote includes a new PRV plus expansion tank as a package.
- An inspection report flags an existing PRV as past its service life.
Common variants and what a PRV isn’t
- PRV vs. pressure relief valve. PRV is a regulator that sets steady downstream pressure. Pressure relief valve is a safety device that opens at over-pressure to vent. Same abbreviation, entirely different functions.
- PRV vs. backflow preventer (PVB / DCVA / RPZ). PRV regulates pressure downstream. Backflow preventers stop reverse flow back into the supply.
- Direct-acting vs. pilot-operated PRV. Residential PRVs are direct-acting (the diaphragm moves the main valve directly). Commercial and industrial PRVs use pilot-operated designs for higher capacity.
Common failure modes
- Diaphragm rupture. PRV passes mains pressure through; sudden whole-house high pressure. Replace.
- Strainer clogged. Flow drops; in extreme cases the valve sticks closed.
- Spring fatigued. Setpoint drifts over years; downstream pressure trends low.
- Mineral or debris in the seat. PRV won’t fully close; downstream pressure creeps upward over time.
Washington note
WA adopts the 2021 Uniform Plumbing Code under WAC 51-56. UPC Section 608.2 requires a PRV when supply pressure exceeds 80 psi at any point in the building’s distribution system. The rule is statewide and triggered by static pressure measured anywhere in the system.
Common WA neighborhoods where PRVs are standard:
- Downhill of Capitol Hill, Queen Anne, Magnolia, West Seattle, and Beacon Hill in Seattle.
- Lower-elevation Bellevue, Kirkland, Issaquah on the east side.
- Lower-elevation Spokane neighborhoods along the river valley.
- Hilly cities generally where the utility’s hydraulic gradient creates pressure swings between elevations.
PRVs typically last 12–20 years in WA water; cities with chloramine residual (Seattle / SPU) tend to be at the longer end of that range, while harder water (parts of Spokane) at the shorter end.
FAQ
Do I need a pressure reducing valve?
Run a static pressure test at any hose bib. If the reading is over 80 psi, WA’s adopted plumbing code requires a PRV. Below 80 psi, you don’t need one — though some homeowners install a PRV at 75–78 psi for the quieter operation it gives at the upper end of normal. A 24-hour test with a telltale-needle gauge captures nighttime peaks; daytime reading of 75 psi can hide an overnight 110 psi peak.
How much does a PRV install cost in Washington?
Typical 2026 ranges in WA: $300–$800 for the valve plus straightforward installation, with an additional $100–$250 if a companion expansion tank also has to be added (it’s required by code when the PRV creates a closed system). Replacement of an existing PRV runs at the lower end; new installs in cramped spaces or with material transitions run higher.
How long does a PRV last?
Residential PRVs typically give 12 to 20 years of service in WA water conditions. Symptoms of an aging PRV: downstream pressure that drifts (creeps upward) over months, audible chatter at the valve during low-flow demands, or a sudden change in apparent fixture pressure. Replacement is a 1–2 hour job with the water shut off.