Why did my water pressure suddenly get too high?

If your home or building gets high street pressure, it should have a PRV that brings it down. The plumbing code requires one wherever static pressure inside a building would top 80 psi (IPC 604.8). These valves wear out. Springs weaken, diaphragms stiffen and seats collect scale and grit.

Here's the key detail. The code says a PRV "shall be designed to remain open to permit uninterrupted water flow in case of valve failure" (IPC 604.8.1). That's a sensible design, since nobody wants the water to cut off when a valve dies. But it means a failed PRV doesn't fail quietly at a safe pressure. It fails toward whatever the street is delivering. In Phoenix, the city's design manual says pressure zones run at "50 to 100 psi" static, so a failed valve can put a home anywhere in that range.

Signs it's the PRV:

• Pressure is high all day, at every hour you test.
• The reading matches the street, or close to it, when you test at a hose bibb near the meter.
• Faucets suddenly spit and splash, toilets hiss or refill on their own, and pipes start to bang when valves close.
• The valve is old, or someone recently adjusted it.

Our page on the signs a pressure reducing valve is going bad goes deeper, and whether you need a pressure reducing valve explains what the valve is supposed to do.

### Repair, reset or replace?

A PRV has an adjusting screw or bolt, usually on top of the bell-shaped housing. If someone turned it up, turning it back down may be all it takes. WaterSense suggests a setting "between 45 and 60 psi." If adjusting it doesn't change the reading, or the pressure creeps back up, the valve is worn inside.

The code helps here. IPC 604.8.2 says PRVs and their strainers must be installed so parts can be repaired or removed "without breaking a pipeline or removing the valve and strainer from the pipeline." On many valves, a rebuild kit or a replacement cartridge is an option. An old or badly scaled valve is often better replaced. Either way, test the pressure again after the work and keep the reading for your records.

Water expands when it heats. In an open system, that extra volume pushes back out toward the street. But a PRV, a check valve or a backflow preventer on your main line blocks that path and turns the house into a closed system. Now the only place the expansion can go is into the pipes, and pressure climbs every time the water heater runs.

The plumbing code knows this. IPC 607.3 says that where a storage water heater's cold supply passes through "a check valve, pressure reducing valve or backflow preventer, a thermal expansion control device shall be connected to the water heater cold water supply pipe." That device is usually an expansion tank, and the code says it must be sized so system pressure stays within the 80 psi limit. The EPA's WaterSense sheet flags the same problem: installing a PRV "creates a closed system," so check whether thermal expansion needs to be addressed.

Signs it's thermal expansion:

• Pressure climbs after hot water use and after the heater cycles, then falls when someone opens a tap.
• The morning reading is fine but it's high after a long shower or at night.
• The water heater's relief valve drips now and then. See why a water heater pressure relief valve leaks.
• Something just changed: a new PRV, a new backflow preventer or a new water heater, with no expansion tank added.
• An existing expansion tank failed. Tanks have an air charge behind a bladder. If the bladder fails, the tank fills with water and stops absorbing expansion.

### A quick way to check an expansion tank

With the water heater off and cool, tap the tank. The top half should sound hollow. If it sounds full all the way up, or water comes out of the air valve when you press it, the tank has likely failed. The tank is usually on the cold water line near the top of the heater.

Less often, the jump comes from the street. Phoenix's design manual says "operating pressure fluctuations are normal" and the public system "can fluctuate 20 psi higher or lower than the average system pressure." The city's customer page adds that demand "during peak times and during times of little demand" can shift pressure. Main repairs and zone changes can move it too.

Two things to know:

1. 1The city stops at the meter. Phoenix says it "does not regulate the water pressure beyond the meter on the customer's side." If street pressure rose, your PRV is what's supposed to absorb it.
2. 2A working PRV hides street changes. If your indoor pressure followed the street up, the PRV has stopped doing its job, whatever the cause upstream. Have it checked before you chase the city.

A gauge and a bit of patience sort it out.

1. 1Get a gauge with a max needle. A basic hose-bibb pressure gauge with a second "tattletale" needle that stays at the highest reading is the most useful tool here.
2. 2Take a static reading. With every fixture off, read the gauge at a hose bibb or the washer's cold connection.
3. 3Leave it overnight. Check the max needle in the morning. A normal reading with a high max needle points to thermal expansion. A high reading all the time points to the PRV.
4. 4Open a faucet and watch. If pressure drops right away to a steady number, then climbs again after the water heater runs, that's expansion.
5. 5Compare with your neighbors or the city if you suspect the street. A PRV should still hold your indoor pressure steady.

### Why not to wait

High pressure is hard on everything connected to it. WaterSense warns that it "can damage plumbing piping, fixtures, and appliances, and can lead to increased water consumption, premature equipment failure, and risk of leaks." Phoenix's customer page says pressure over 80 psi "can damage your water pipes and appliances." A sudden jump usually means a valve or a tank just stopped doing its job, and it's easier to fix now than after it causes a leak.

Here's the play: take the reading, note when it's high, then have the PRV or expansion tank checked. HQ Plumbing and Air can test your pressure, replace a failed PRV and install or recharge an expansion tank, with free estimates and upfront pricing. If the problem is the opposite, see why water pressure suddenly drops.