Quick Answer:
If pressure dropped only at your shower after replacing the valve, isolate the repaired component, flush the affected lines, and measure pressure before and after the repair point. Those three checks usually reveal whether the issue is trapped debris, a partially closed stop, or a faulty part that needs correction.
Why This Happens
When a shower valve is replaced you introduce new plumbing connections and might disturb mineral deposits, pipe debris, or small pieces of solder/packing. A partial blockage at the valve body or in the short stretch of pipe immediately upstream will drop flow to that fixture while the rest of the house keeps normal pressure. If nearby shutoffs or servicing outside the house were used recently, you can also see localized instability — for example, Pressure unstable after curb stop use and Pressure problems after winterization can show similar symptoms. The most common causes after a valve change are:
- Debris trapped in the cartridge or the valve inlet.
- A stop or packing nut not fully reopened after isolation.
- An incorrectly sized or defective replacement valve.
- Air pockets that need flushing out of the local lines.
Step-by-Step What to Do
1. Isolate the repaired component
Shut the shower service stop or the local shutoff that feeds the valve. If there is no local shutoff, use the house water shutoff but be prepared to drain water from other fixtures. Isolating the repaired component prevents pressure changes in other parts of the system from confusing your checks.
2. Measure pressure before and after the repair point
Use a simple water pressure gauge or a threaded hose bib gauge:
- Measure at a supply point upstream of the valve (for example the nearest accessible supply tee or a nearby fixture) to see the incoming pressure into the repaired section.
- Measure at the shower outlet or an accessible port downstream of the valve to compare pressure.
- Record both values. A large difference indicates a restriction at or inside the valve assembly.
3. Flush affected lines
With the shutoff for the repaired component closed, remove the cartridge or open the valve body per manufacturer instructions if you can do so safely. Then:
- Briefly open the upstream supply while directing flow into a bucket or through a hose to flush out debris. Keep the valve or outlet pointed to a safe discharge location.
- Clear any visible sediment, then reassemble and slowly reopen the local shutoff while checking for leaks.
- Re-measure pressure after flushing to confirm the difference has been addressed.
4. Verify handle operation and full opening
Make sure the valve handle or cartridge is installed so it can reach full open. Sometimes a new cartridge is installed in the wrong position or gauge stops are not fully opened. Check that the handle movement corresponds to the expected full-open position.
5. Test at normal temperature
Run both hot and cold to ensure temperature-related valve components (like mixing cartridges) are not causing reduced flow on one side. Re-check pressures if the problem appears only on hot or cold.
What Not to Do
- Do not assume pressure will self-correct without flushing and inspection. Letting the system run without checking can push debris further into the valve or down the line.
- Do not overtighten fittings or force components; that can damage seats or seals and make a small problem worse.
- Do not bypass important safety features like thermostatic elements in mixing valves unless you understand the implications for scald protection.
When to Call a Professional
Call a plumber if you’ve isolated and flushed the lines and the pressure difference remains, if you find a damaged valve component, or if you’re uncomfortable removing a cartridge or working on pressurized pipe. Also call a pro if leaks appear after reassembly or if the reduced pressure affects multiple fixtures in the same area and you can’t find a local source.
Safety Notes
- Turn off water at the proper shutoff before disassembling the valve to avoid spray and scalding. If unsure, shut off the main supply.
- Protect hands and eyes when flushing lines—debris and hot water can cause injury.
- Use a bucket and towels to control water. Work slowly when refilling lines to avoid pressure spikes.
Common Homeowner Questions
- Why is only my shower affected? A local restriction at the valve or in the short run of pipe to that shower is the usual cause.
- Will running the water for a while clear the problem? Sometimes short flushing helps, but you should isolate, remove trapped debris, and re-measure pressure rather than just waiting.
- Can a new valve be defective? Yes. If flushing and correct installation don’t restore pressure, the valve or cartridge may be faulty and should be replaced or inspected by a professional.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Pressure Loss After Plumbing Repairs.