How to: Plumbing Vent Bow Loop

How to: Plumbing Vent Bow Loop. Plumbing vent bow loops, also known as Chicago loops, are used most frequently in kitchen island sinks, where access to the plumbing vent is not easily available. A loop of plumbing drains and vents is tied to a pipe under the floor that then runs to the main vent. This arrangement allows you to use the home's vent stack, which allows air into the drain. Without a source of fresh air, the negative air pressure inside the drain would prevent water from moving along the pipe.

Vent bow loops connect island sinks to vent stacks.

Things Needed

  • PVC pipe (various lengths)
  • T-fitting
  • Hole saw, 2-inch
  • 3 fittings, 45-degree
  • PVC cement
  • PVC solvent coupler

Step 1

Slip one end of a PVC tube onto the end of the P-trap under the sink. Turn the nut on the tube clockwise to connect it with the P-trap.

Step 2

Connect a T-fitting to the other side of the PVC tube, so that the two openings on the T-fitting point up and down.

Step 3

Place another section of PVC pipe into the bottom of the T-fitting and run the pipe to the drain pipe, which is likely located under the floor. You may have to use a 2-inch hole saw to cut through the floor to access the drain pipe.

Step 4

Place a 12-inch section of PVC pipe into the top of the T-fitting, followed by a 45-degree fitting. Place a 6-inch section of PVC pipe into the 45-degree fitting, followed by another 45-degree fitting. Place one more section of 6-inch PVC pipe into the fitting and then attach one last 45-degree fitting to that pipe. When completed, the pipe will have bent 180 degrees and is not parallel to the T-fitting.

Step 5

Insert a section of PVC pipe into the last of the 45-degree fittings that is long enough to reach the vent stack pipe under the floor. Again, you may need to drill a 2-inch hole through the floor to access the vent stack pipe.

Step 6

Connect the PVC pipe to the vent stack by inserting the pipe into a clean-out plug or other opening along the pipe, and securing with a PVC solvent fitting. Swab the fitting with PVC cement prior to fitting over the vent stack pipe. The loop is now connected to both the drain and the vent stack.


  • Always consult local plumbing codes prior to beginning work on a vent bow loop, as some municipalities prohibit the practice.


  • This Old House: Bow Vent
  • YouTube: Plumbing an Island Fixture Vent/Bow Vent