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Solved problems · Plumbing · Crew answer

Why does my bilge pump run every time someone showers?

Shower sump water pushes into the main bilge through a shared discharge manifold. The usual fix is a dedicated above-waterline through-hull for each shower sump pump.

Symptom

The head sink drains into the shower sump, and a sump pump sends it overboard. The shower sump discharge shares a manifold with a bilge line. When a shower drains, grey water ends up in the main bilge and the main bilge pump starts.

Likely cause

Hair packs into the shared overboard fitting. When the sump pump runs against that restriction, pressure builds in the manifold and pushes past the soft (joker-style) check valve on the bilge branch, so grey water feeds the bilge side. The water is not flowing back into the shower sump.

Stop now if

Water in the bilge is rising faster than the pump can clear it, or you cannot tell where it is coming from. Treat it as a leak first. Any new through-hull needs a proper seacock or fitting and correct bedding; get a yard to do it if you are not sure.

The fix, in short

  1. Clear the overboard fitting and add a hair strainer. Clear the hair and debris from the existing overboard fitting. Hair in that fitting creates the back pressure that forces water past the check valve. Fit a hair strainer on the sump inlet so it does not build up again.

  2. Give each shower sump pump its own through-hull. For a lasting fix, stop sharing the manifold: give each head's shower sump pump its own above-waterline through-hull so it never discharges into the bilge plumbing.

  3. Cap the old manifold port and fix the bilge check valve. Cap or plug the old shower port on the manifold, and replace or remove the worn check valve on the bilge branch. Bilge discharge should not share a line with other drains.

  4. Route the new discharge hose high, with a siphon break if needed. Run the new discharge hose in a loop as high as possible above the heeled waterline. If the outlet can go under when heeled, add a siphon break (vented loop) placed where an occasional drip is harmless.

  5. Test with a shower. Run a shower and confirm the main bilge pump stays quiet unless there is real bilge water. If it still cycles, look for one more tee or cross-connection.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Equipment involved

  • Shower / grey-water sump: Sump box with float-switched sump pump
  • Bilge plumbing: Shared discharge manifold with a joker-style check valve
  • Where it shows up: Common on production boats where shower sump and bilge pumps share one discharge.

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