Quick Answer:
If pressure dropped after installing the new valve, first check the valve supply stops and the showerhead. Most causes are simple: a stop screw or test cap not fully open, a low-flow valve or showerhead, or a blocked line. Confirm the valve is rated for the fixture flow before assuming the valve size is wrong. If stops are restricting flow, call a plumber.
Why This Happens
- Supply stops or test screws left partly closed during installation will limit flow even if the valve and trim look correct.
- Some modern valves or cartridges are low-flow or have internal flow controls; if the valve’s rated flow is lower than the showerhead or your expectations you will notice weak pressure.
- Debris from installation can clog the cartridge or showerhead screen and reduce flow.
- Sometimes the issue is elsewhere in the system: main house pressure, a closed house shutoff, or other fixtures drawing water. See Pressure weak only when multiple showers used and Low water pressure at shower only for related situations.
Step-by-Step What to Do
1. Confirm the house water supply is normal
- Check another faucet on the same floor. If all fixtures are weak, the problem may be house pressure or the main shutoff.
2. Locate and check the valve stops or test screws
- Turn off the shower trim and find the small stop screws or test caps on the valve body (usually behind the trim or access plate).
- Carefully open each stop screw fully—usually turn counterclockwise until it stops. Do this slowly and don’t force it past the stop.
- If your valve has quarter-turn stops, make sure they are aligned to the open position.
3. Confirm the valve’s flow rating
- Check the valve model number on the body or paperwork and compare its flow rating (GPM at given PSI) to the showerhead and your household pressure. A valve designed for low-flow fixtures will limit peak flow.
- If you don’t have documentation, the brand and stamped model number on the valve can help you verify ratings before replacing parts.
4. Inspect the showerhead and minor debris
- Remove the showerhead or hand shower and run water briefly from the valve outlet (with the trim on) to see if flow improves—use a bucket to avoid splashing. If flow is good at the outlet but weak at the head, the showerhead is likely clogged or has a restrictor.
- Clean screens and any flow restrictors in the showerhead if appropriate; many heads have removable restrictors that a homeowner can clean or replace.
5. Look for debris in the valve without disassembling internals
- Flush the valve by briefly turning the supply stops off then on while the trim is removed, using a bucket to catch water. This can dislodge loose debris without taking the cartridge apart.
- If flushing improves flow, reinstall trim and test. If not, stop and evaluate next steps.
What Not to Do
- Don’t modify or disassemble valve internals or the cartridge unless you have the correct instructions and parts—doing so can damage the valve or void warranties.
- If the stops are set to limit flow and you can’t resolve it with the above steps, don’t attempt permanent internal changes yourself—hire a plumber to adjust or replace parts safely.
When to Call a Professional
- If stops are fully open and you still have weak flow after flushing and cleaning the showerhead, a pro can test system pressure, inspect the valve cartridge, and confirm valve sizing.
- Call a plumber if the valve must be replaced or if the job requires removing internal components you’re not comfortable with.
- If you find the valve’s flow rating is below what your household needs, a plumber can recommend compatible higher-flow valves or changes to the system.
Safety Notes
- Shut off the water at the house shutoff before removing trim or working on the valve to avoid flooding.
- Release any remaining pressure by opening a nearby faucet before removing components.
- If you are unfamiliar with the valve brand or configuration, stop and contact a professional to avoid damaging the plumbing or causing leaks.
Common Homeowner Questions
- Why did pressure drop only after installing the valve? Often a stop screw wasn’t fully opened or debris was introduced during installation; it’s rarely an immediate sizing issue.
- Can I remove the cartridge to clean it myself? Not recommended unless you have the exact cartridge instructions and replacement parts; removing internals can cause damage.
- How will a plumber diagnose this faster? A plumber can measure supply pressure, test flow at the valve outlet, and safely remove or replace cartridge parts to find the root cause.