Quick Answer:

Your tub filling slowly while a nearby sink runs normally usually means the restriction is local to the tub fixture or its valve, not the whole house supply. Start by comparing flows and testing hot versus cold; many problems come from a partially closed valve, a blocked tub spout or a cartridge/diverter issue. If you need to read more about cases where a single fixture is affected, see only one faucet has low pressure.

Why This Happens

Plumbing branches and fixtures often have separate internal parts. A sink and a tub near each other can still use different valves, cartridges, or spouts. Common causes include:

• Partially open or sticky shutoff valve feeding the tub.
• Mineral deposits or debris in the tub spout or cartridge.
• Diverter or cartridge blockage in a tub/shower combo that limits flow to the tub spout.
• Problems after maintenance or a water shutoff—air and debris can reduce flow at some fixtures (low pressure after turning the water back on).

Step-by-Step What to Do

1. Compare tub flow vs sink

Run the tub and the sink one at a time on the same water temperature and observe. Note how many gallons per minute feel like they come out (fast, moderate, or barely a trickle). This confirms the issue is localized to the tub.

2. Test hot vs cold

Check both hot and cold separately. If only one temperature has low flow, the problem is likely in the hot- or cold-side cartridge, valve, or supply line, not the entire fixture.

3. Inspect the tub spout for debris

Turn off the water to the tub, remove the spout or check the outlet if removable, and look for sediment or scale buildup. Clean gently with a cloth and a mild vinegar solution if needed. Reinstall and test.

4. Check the valve opening fully

Locate the tub shutoff valves (often behind access panels). Open them fully and move the handle through its range a few times. A valve that’s only partially open or has sediment can choke flow.

5. Consider cartridge blockage

If the fixture uses a cartridge or valve assembly, mineral deposits or debris can restrict flow even with the handle in the full-open position. Removing and inspecting the cartridge may be necessary; if you’re not comfortable doing that, see the “When to Call a Professional” section below.

What Not to Do

• Do not drill out internal parts of the valve or cartridge—this can cause permanent damage and major leaks.
• Do not crank the handle aggressively—forcing it can break the stem or strip splines, making repairs much costlier.
• Do not ignore a sudden pressure drop—sudden loss of flow can indicate debris, a closed valve, or a developing leak that should be checked promptly.

When to Call a Professional

Call a plumber if:

• You find the cartridge or valve is blocked but are unsure how to remove or replace it safely.
• There’s evidence of a leak behind walls or under floors, or you can’t locate an access panel to the valve.
• You opened valves or removed parts and the problem persists, or you’re uncomfortable performing the inspections. A pro can test pressures, flush the lines, and replace cartridges or valves correctly.

Safety Notes

• Always shut off the appropriate water supply before removing spouts, cartridges, or valves.
• Protect floors and surfaces—place towels to catch water and wear gloves if using cleaning solutions.
• If you’re unsure which valve controls the tub, shut off the main water supply before working and drain taps first to avoid accidental flooding.

Common Homeowner Questions

• Q: Why does the tub have low pressure but the sink is fine? A: The tub likely has a local restriction—valve, cartridge, spout, or diverter—while the sink’s supply is unaffected.
• Q: Could sediment from recent plumbing work cause this? A: Yes. Sediment after a shutoff or work can clog cartridges and spouts; flushing and cleaning usually helps.
• Q: Is this urgent? A: If it’s just slow flow, it’s not an immediate emergency, but sudden drops or signs of leaks should be checked right away.