How to Install Drain Pipes Without a Vent Pipe

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Prevent severe blow-back through your P-trap by installing a mini-vent.
Prevent severe blow-back through your P-trap by installing a mini-vent. (Image: Zedcor Wholly Owned/PhotoObjects.net/Getty Images)

If your sewer has a defective venting system, installing a new sink drain may cause a serious problem. Draining water creates a vacuum and this builds up negative pressure in the household sewer line. If the pressure differential is not equalized, lower pressure in the sewer line sucks water out of the P-trap and releases sewer gases into your kitchen or bathroom. To avoiding the expense of setting up a new vent through the roof, install a mini-vent system. These compact units equalize air pressure on either side of the P-trap and prevent sewer gases invading your home.

Things You'll Need

  • Mini-vent
  • Steel measuring tape
  • Length of 1 1/2-inch PVC pipe
  • Hacksaw
  • PVC adhesive
  • Spirit level
  • Two 1 1/2-inch PVC female threaded locking rings
  • P-trap kit
  • Felt pen
  • Teflon tape
  • 2 pairs of channel pliers

Strip out the existing drain pipes and P-trap from under the sink.

Read the mini-vent installation instructions and familiarize yourself with all the parts that came in the package.

Place the horizontal outlet of the distinctively shaped Y-piece that came in the package against the sewer outlet on the back wall under the sink. Adjust the fitting to ensure that the forward and upward sweeping leg of the Y-piece faces away from the wall and is positioned midway between the edge of the sink bowl and the back wall.

Measure the distance between the Y-piece outlet and the sewer drain pipe protruding through the wall with a steel measuring tape. Transfer this measurement to a piece of 1 1/2-inch PVC pipe. Cut off this short length of pipe with a hacksaw.

Spread a liberal amount of PVC adhesive, following the manufacturer's directions, onto the inner face of the sewer outlet pipe. Coat the outer surface on one end of the short PVC pipe and insert the end into the sewer outlet. Smear a layer of extra adhesive around the circumference of the connection to ensure a watertight joint.

Glue the horizontal outlet of the Y-piece over the short leg of PVC pipe as directed in the installation instructions. Place a spirit level against the side of the forward and upward sweeping leg of the Y-piece. Center the bubble to ensure that the upward sweeping leg remains vertical while the PVC adhesive sets; this should take about 30 seconds.

Glue a 1 1/2-inch PVC female threaded locking ring into the horizontal inlet leg of the Y-piece, using the same gluing technique.

Cut of a second 18-inch length of 1 1/2-inch PVC pipe and glue a 1 1/2-inch PVC female threaded locking ring onto one end.

Slide the locking ring end of this 18-inch pipe over the downward-facing tube connected to the sink drain. Secure the pipe by snugging the female locking ring temporarily onto the pipe for later adjustment.

Assemble a new P-trap kit as follows: Thread the female threaded coupling on the short leg of the L-shaped sewer outlet pipe onto the P-trap outlet. Leave the coupling loose to allow the P-trap to swivel on the L-shaped outlet pipe.

Position the long leg of the L-shaped P-trap outlet pipe against the Y-piece inlet, with the P-trap inlet held vertically. Swivel the P-trap and line it up with the side of the 18-inch length of pipe protruding from the sink drain outlet.

Mark a cut line on the long leg of the L-shaped P-trap outlet pipe with a felt pen so that when cut to size, 1 1/2-inches will slide into the PVC female threaded locking ring on the Y-piece inlet.

Cut the L-shaped P-trap outlet pipe to length. Slide the cut end into the Y-piece inlet but do not tighten the locking ring.

Loosen the female locking ring on the PVC pipe attached to the sink drain outlet. Slide the pipe upward over the sink drain outlet to provide some wiggle room. Maneuver the loose P-trap so its female locking ring lines up with the sink drain outlet.

Slide the PVC pipe attached to the sink drain outlet pipe down and insert about an inch of the end into the P-trap locking ring. Tighten every female locking ring on the installation as much as possible by hand.

Cut a 12-inch length of 1 1/2-inch PVC pipe to act as the mini-vent extension. Prime the inside face of a female threaded adapter and the outer face of the length of pipe with PVC adhesive. Insert the pipe into the threaded adapter; smear a layer of extra adhesive around the circumference of the connection to ensure a watertight joint.

Wrap Teflon tape tightly around the threads of the mini-vent, in a clockwise direction. Pull the end of the tape until it snaps off and smooth the ragged ends into the threads. Screw the mini-vent into the threaded adapter and tighten the joint securely with a pair of opposing channel pliers.

Line up the mini-vent extension with the upward sweeping leg of the Y-piece. Raise the mini-vent as far as possible so it clears the underside of the countertop without fouling the faucet water supply lines.

Mark a cut line on the side of the mini-vent extension so about an inch can fit into the end of the Y-piece. Cut the mini-vent extension to size and glue it into the upward sweeping end of the Y-piece with PVC adhesive. Allow sufficient time for the PVC adhesive to cure.

Open the sink drain pop-up stopper and turn on the cold faucet. Examine all plumbing joints under the sink. If any threaded adapters are leaking, tighten them another quarter turn with a pair of channel pliers to complete the installation.

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