Shower pressure weak only when tub used

Shower pressure weak when tub is used at the same time.

Quick Answer:

If the shower feels strong until you run the tub, the most common cause is the tub/shower diverter splitting flow or leaking. Check the diverter while the shower is running, and replace it if water is escaping to the tub spout. If that does not fix it, the issue may be shared piping, a worn cartridge, or a pressure problem that needs a plumber.

Why This Happens

When the tub and shower share the same outlet, a diverter directs water to one outlet or the other. If the diverter is worn or not seating properly, some water keeps going to the tub spout while the shower is running. That reduces pressure at the showerhead because the available flow is split.

Other causes include a partially clogged showerhead, a faulty mixing cartridge inside the valve, or reduced flow from shared piping elsewhere in the house. If you recently changed parts or plumbing, consult information about Low pressure after installing new valve. If the reduction only happens when another bathroom is running, see Pressure weak only when multiple showers used.

Step-by-Step What to Do

1. Confirm the pattern

  • Run the shower alone and note the pressure.
  • Then run the shower and the tub diverter as you normally do (or run the tub faucet) to see if the shower drops only when the tub is active.
  • Try the shower with other fixtures off to rule out house-wide demand.

2. Inspect the diverter while showering

  • With the shower on, watch the tub spout. If water is trickling or flowing from the spout at the same time the shower is running, the diverter is leaking.
  • If you have a pull-up tub spout, try pushing it fully in and see if it seals. If it won’t hold, the spout diverter likely needs replacement.

3. Test the showerhead and valve

  • Remove the showerhead and run the shower briefly. If flow improves, clean or replace the showerhead.
  • If removing the head doesn’t help, the mixing valve cartridge could be faulty or clogged. Many valves reduce flow when an internal part is failing.

4. Replace the diverter if flow is splitting

  • If you confirmed water is going to the tub while the shower runs, replace the tub spout diverter or the valve diverter assembly depending on your setup.
  • Common approaches:
    • Unscrew or pull off a slip-fit tub spout and swap in a new one with an integrated diverter.
    • Replace the internal diverter cartridge if your valve uses one. Shut off water before working on the valve.
  • If the diverter is old and corroded, replacement usually restores full shower flow.

5. Re-check after repairs

  • After replacing the diverter or cartridge, test the shower alone and while using the tub to ensure the issue is resolved.
  • If pressure is still low, note whether it is low all the time or only when another fixture runs—this helps diagnose shared piping or supply problems.

What Not to Do

  • Don’t ignore diverter leakage; if water splits consistently, a plumber is appropriate.
  • Don’t force parts that are corroded—snapping metal can leave shards or damage fittings.
  • Don’t attempt valve cartridge replacements without shutting off the water or without a basic understanding of your valve type.
  • Don’t assume low pressure is only cosmetic—persistent flow splitting or sudden changes can indicate failing parts that will worsen over time.

When to Call a Professional

  • If replacing the visible tub spout diverter does not stop flow splitting, the diverter inside the valve or the cartridge may need professional replacement.
  • Call a plumber when the plumbing layout is complex, the valve is behind finished walls, or you’re unsure which valve parts match your fixture.
  • If multiple fixtures lose pressure at once, or if you suspect a leak in shared piping, a licensed plumber can diagnose and repair supply issues safely.

Safety Notes

  • Always shut off water to the fixture or the house before removing the valve cartridge or working on the valve body.
  • Be careful with hot water—run cold water first to avoid scalding when testing.
  • Use proper tools and avoid excessive force on fittings to prevent damage.

Common Homeowner Questions

  • My shower pressure is low even when the tub isn’t used—what then?
    Check the showerhead for clogs and the valve cartridge; consider a separate troubleshooting path for constant low pressure.
  • Can I replace the tub spout diverter myself?
    Yes, many tub spouts are simple to swap, but turn off water if you need to access the valve or cartridge.
  • How urgent is a leaking diverter?
    It’s not an immediate emergency, but fix it soon—ongoing flow splitting wastes water and can signal parts that will fail later.

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