Vent, Valve & Volt
Shower drips when off, or the tub spout runs with the shower on

This one needs a licensed pro

Likely cause: a restriction or installation issue (stacking)

Water from the showerhead while only the tub spout should run is what Moen calls stacking. It is usually a restriction or the way the spout was piped.

Why this happens

Moen says water coming out of the showerhead while the tub spout runs points to either a restriction in the system or an installation issue. Its checks include an obstruction in the spout, the pipe between valve and spout (Moen calls for only 1/2 inch copper or galvanized there, not PEX or CPVC), a spout 6 to 11 inches below the valve, only one elbow from the tub port to the spout, and the valve installed with UP facing up. Most of those are behind the wall.

What to do right now

  1. Remove the tub spout and check it and the pipe end for debris or a blockage. Clear anything you find and test.
  2. With the spout off, look at the pipe coming out of the wall. If it is plastic (PEX or CPVC) rather than copper or galvanized steel, Moen says it needs replacing.
  3. On some two-handle models, Moen says the flow director may need to be reversed. Check your model's guide.
  4. If the spout is clear, call a licensed plumber about the piping between the valve and the spout.

What to tell the plumber

  • The showerhead sprays when only the tub spout should run.
  • What pipe material you saw behind the spout.
  • Whether the valve or spout was recently replaced.

Build the full list: what to tell the plumber.

Start this troubleshooter over

References (every fact above comes from one of these pages)
  1. Moen: Water comes out of the showerhead when the tub spout is used

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