What is a pressure reducing valve?
Updated August 2026
A pressure reducing valve, or PRV, is a spring-loaded brass valve on the incoming water line that knocks high municipal pressure down to a household-safe 50 to 60 psi. City mains often carry 100 to 150 psi so water reaches hydrants and hilltop houses, and code requires a PRV on any home where static pressure at the fixtures would top 80 psi. Inside sits a diaphragm and a spring; the adjusting screw on top changes how hard that spring pushes back against the incoming water.
Where it is
The valve sits within a foot or two of the main shutoff, on the house side, so study the first stretch of pipe after the water enters. Its shape gives it away: a brass bell or dome 4 to 6 inches tall with a hex-headed screw on top and a lock nut beneath it. In cold climates it hangs on the basement wall beside the meter. In Arizona, Nevada, and much of Texas it lives outdoors in the meter box or on a riser near the hose bibb. A thermal expansion tank mounted nearby is a reliable sign that a PRV is upstream of it.
What it does
Fixtures, supply hoses, and appliance fill valves are built for 80 psi and under. Run them at 100 psi and the damage accumulates instead of announcing itself: faucet cartridges wear out in 3 years rather than 10, washing machine hoses balloon at the crimp, the water heater relief valve weeps, toilet fill valves hiss, and pipes bang whenever a valve snaps shut. High pressure also pushes more water through every open tap, which shows up on the bill. PRVs wear from the inside and fail in two directions. A worn seat lets pressure creep upward month after month, unnoticed until something bursts. A ruptured diaphragm does the reverse and leaves the whole house with a weak shower. Service life runs 7 to 12 years.
Maintenance
Thread a $12–20 gauge onto an outside hose bibb twice a year and read it (how often to check water pressure). Take one reading with nothing running, then leave the gauge on overnight. A number that has climbed past 80 psi, or that jumps 20 psi while the house sleeps, means the PRV is losing its grip or the system lacks an expansion tank. Adjust in small moves: loosen the lock nut, turn the screw a quarter turn counterclockwise to drop pressure, retest, then tighten the nut. Budget a replacement every 7 to 12 years (replacement timing), and read what high pressure costs before deciding it can wait another season.
What it costs to replace
A replacement pressure reducing valve runs $50–150 for the valve alone, or $250–450 installed by a plumber.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
What water pressure should a house have?
Aim for 50 to 60 psi at the fixtures. Below 40 psi, showers feel weak and tankless heaters and irrigation zones misbehave. Above 80 psi you are outside code and outside most appliance warranties. Test with a screw-on gauge at a hose bibb or the washing machine cold hookup, which costs $12–20 and takes a minute. A reading between 60 and 80 is fine to leave alone. A reading of 85 or higher is a valve problem, not a fixture problem.
How do I tell whether my PRV has failed?
Compare two gauge readings. Take one during the day, then leave the gauge attached overnight, ideally one with a peak-hold needle. A healthy system holds steady. Pressure that drifts up over several weeks, spikes while the water heater cycles at night, or stops responding when you turn the adjusting screw points at a worn seat or a torn diaphragm. Supporting evidence: a relief valve that drips on the water heater and toilets that run briefly on their own. Why water pressure gets too high walks through the rest.
Do I need an expansion tank with a PRV?
In nearly every case, yes. A PRV turns the plumbing into a closed system, so when the water heater warms 50 gallons and that water expands by roughly half a gallon, the pressure has nowhere to escape and can spike past 120 psi until the relief valve lets go. A 2-gallon thermal expansion tank absorbs it: $40–90 for the tank, $150–300 installed. Most current codes require one wherever a PRV or a check valve exists on the service line.