Quick Answer:
If pressure falls a lot right after a new PRV was put in, stop and work through the basic checks: confirm the valve is installed in the correct direction, make sure any full-port shutoffs are fully open, flush the lines at a tub spout for 2–3 minutes to clear debris, then adjust the PRV while water is flowing. If that doesn’t restore normal flow, the PRV may be defective/undersized and will likely need replacement.
Why This Happens
Several simple things can cut flow immediately after a PRV change:
- Incorrect orientation: PRVs have an arrow showing flow direction. Installed backwards, they restrict or block flow.
- Shutoffs not fully open: New valves are often installed with isolation shutoffs that must be opened all the way; partial closures greatly reduce flow.
- Debris or packing: Installation can dislodge solder, thread tape, or scale that temporarily clogs the seat or screens.
- Adjustment set too low: The PRV may have been set to a very low downstream pressure at install.
- Wrong size or defective unit: If the valve is undersized for the system or malfunctioning, it won’t deliver required flow even when set correctly.
For cases where pressure is reduced everywhere in the house, see PRV causing low water pressure everywhere for more detail.
Step-by-Step What to Do
1. Confirm arrow direction and correct orientation
- Locate the arrow cast or stamped on the PRV body. It must point in the direction water travels from the main toward the house fixtures.
- If it’s reversed, shut the water off and have the valve reinstalled in the right orientation.
2. Ensure full-port shutoffs are open
- Most PRV installs include upstream and downstream isolation valves. Make sure both are fully open (handle inline with pipe or lever fully rotated).
- Partially open isolation valves can mimic a failed PRV.
3. Flush lines via tub spout 2–3 minutes
- Run a tub or bathtub spout with both hot and cold valves fully open for about 2–3 minutes. This helps clear any debris that might have been dislodged.
- Check other fixtures after flushing to see if pressure returns.
4. Adjust PRV while water is flowing
- With a faucet or the tub running (so you can see flow), turn the PRV adjustment screw or knob slowly to raise pressure. Make small changes and wait a few seconds to see the effect.
- If you have a pressure gauge, monitor downstream pressure while adjusting.
5. If still low, PRV may be defective/undersized
- If orientation is correct, shutoffs are open, you flushed the lines, and a proper adjustment while flowing makes no difference, the PRV may be defective or too small for the household demand.
- Replacement with a properly sized unit or a warranty exchange is the likely next step.
- For guidance when adjustment doesn’t restore flow, see PRV adjustment makes no change.
What Not to Do
- Don’t assume the new PRV is fine—wrong orientation or debris can cripple flow.
- Do not disassemble the PRV while the system is pressurized.
- Don’t make large adjustment turns quickly; raise pressure gradually to avoid surprises at fixtures (scalding from hot water is a risk).
- Don’t ignore upstream isolation valves—leaving them partially closed can fool you into thinking the PRV is the problem.
When to Call a Professional
- There is still low pressure after you’ve checked orientation, opened shutoffs, flushed lines, and tried adjusting the valve.
- You find leaks, damaged fittings, or you aren’t comfortable shutting off or isolating the main line.
- The house requires a larger PRV or multi-zone pressure control and accurate sizing or code compliance is needed.
- The PRV is under warranty and needs replacement—plumbing pros can swap and test safely.
Safety Notes
- Turn off the main before doing any work that requires removing or reinstalling fittings. Relieve pressure before opening the system.
- Be cautious of hot water when flushing or adjusting—run both hot and cold to avoid scalding.
- Use proper tools and eye protection when working on piping. If soldering or cutting is needed, follow safe procedures or call a pro.
- When in doubt, stop and contact a licensed plumber—don’t risk causing a leak or water damage.
Common Homeowner Questions
- Why did pressure drop right after the new valve was installed?
Most often because the PRV is installed backward, an isolation valve wasn’t fully opened, or debris is blocking flow. - Can I safely adjust the PRV myself?
Yes—adjust slowly while a faucet is running to monitor results; if you’re unsure, call a plumber. - How long should I flush the lines?
Run the tub spout for about 2–3 minutes; if pressure doesn’t improve after flushing and adjustment, the valve may need service or replacement.