Quick Answer:
If the tub fills slowly while sinks in the same house have normal pressure, the problem is usually a restriction at or near the tub spout or the shower diverter rather than a whole-house supply issue. Start by checking the spout opening for debris, remove the spout if you can, and compare the flow at the shower riser to narrow the cause quickly.
Why This Happens
Most of the time a single weak tub spout comes from one of these local causes:
- Mineral buildup or debris blocking the spout opening or aerator area.
- A stuck or partially closed diverter inside the tub spout that diverts water to the shower and reduces flow from the spout.
- Internal cartridge or valve parts that are partially clogged or failing only on the tub/shower branch.
If pressure issues appeared after work on nearby plumbing, or dropped suddenly, check related troubleshooting guides like Shower low pressure after plumbing repair or Shower pressure suddenly dropped once you’ve confirmed the symptom is local to the tub area.
Step-by-Step What to Do
1. Observe and compare
- Turn on the tub spout and note how weak the flow is. Then open a nearby sink fully and note the strong flow there; this confirms the problem is isolated to the tub/shower branch.
- Turn on the shower (if you have a hand shower or separate riser) and compare that flow as well.
2. Check the spout opening for debris
- Look into the end of the spout for visible mineral deposits, sediment, or a mesh screen. Use a flashlight if needed.
- Try wiping the opening with a cloth or a soft brush to see if flow improves.
3. Remove and inspect the tub spout if possible
- Many tub spouts pull off or unscrew. Turn off water at the nearby shutoff if you have one, or work carefully with water off at the main if not.
- Gently remove the spout following the manufacturer’s method (pull, unscrew, or remove a set screw), then inspect the inside for debris or a damaged diverter.
- Clean out any buildup and test flow before reinstalling.
4. Compare flow at the shower riser
- If the shower riser has good flow while the spout is weak, that points to a problem inside the tub spout, the diverter, or a short section of supply pipe to the spout.
- If both spout and shower are weak, the issue may be the valve cartridge or an upstream restriction and needs deeper inspection.
5. Inspect the diverter and valve
- If your spout has a built-in diverter, check it for mineral buildup or sticking. Operate it several times after cleaning to see if flow returns.
- If cleaning doesn’t help and both spout and shower are weak, consider checking or replacing the valve cartridge (this can be more involved and may require shutting off the house water and basic plumbing tools).
6. Test after each change
- Reattach the spout and test flow after any cleaning or adjustment. Work stepwise so you know which action fixed the problem.
What Not to Do
- Don’t ignore weak tub spout flow — it often points to debris or restriction upstream; if unsure, a plumber is appropriate.
- Don’t force parts that are stuck. Forcing can break threads, fittings, or decorative trim.
- Don’t pour strong chemical drain cleaners into the tub to clear a spout restriction — they are ineffective on spout/diverter buildup and can damage finishes and plumbing parts.
When to Call a Professional
Call a licensed plumber if:
- You removed the spout and cleaned it but flow is still weak.
- The diverter or valve inside the wall appears damaged or you need to replace a cartridge and are not comfortable doing it.
- You suspect a pipe blockage in the wall or near the valve, or you can’t locate the shutoff to work safely.
Safety Notes
- Shut off the water supply before removing the spout when possible. If you must work without a local shutoff, close the main shutoff and drain fixtures first.
- Protect finishes and threads by using soft cloths and the correct tools. Avoid metal tools that can mar chrome or brass.
- If you see corrosion, damaged piping, or are unsure about cutting or disassembling valves, stop and call a professional.
Common Homeowner Questions
- Why does the spout have less flow than the shower? Often because the diverter or the spout opening is partially blocked while the shower riser bypasses that restriction.
- Can mineral buildup be cleaned without removing the spout? Sometimes you can loosen surface deposits with a brush or short soak, but removing the spout lets you clean more thoroughly.
- Will using a stronger cleaner fix low flow? No — harsh chemical cleaners can damage parts and often won’t reach the diverter or valve where the restriction sits.