How to Install a Backflow Preventer for a Sewer Line in a Home


When a blockage occurs in the main sewer line, sewage or contaminated water can travel back up through the different household water fixtures. To protect the home from contaminated or irrigation water, a device known as backflow preventer, or backflow valve, is attached to the exterior faucet bib in the main sewer line. This valve features a flap that automatically opens to allow sewer waste to pass through, but closes when the waste backs up and travels the other way. Before installing the valve, check with the local authorities for any permits you may be required to obtain.

Things You'll Need

  • Tape measure
  • Marker
  • Handsaw
  • Dry cloth
  • Knife
  • Damp rag
  • PVC primer
  • Brush
  • PVC cement
  • Measure the length of the backflow preventer and record it. Do not count the inlets along the ends of the valve to which you will install the pipes. Mark the length of the backflow preventer on the main sewer line using a marker. Ensure both parallel marks are straight.

  • Position the blade of a handsaw directly above a marked line and slowly slice through the pipe until it cuts into two pieces. Maintain steady pressure when slicing through the pipe, cutting directly along the marked line. Grasp the other side of the pipe firmly with your free hand and repeat the process to cut directly above the other marked line on the pipe. Set the cut section of the pipe aside.

  • Allow the water to completely drain from the cut pipe, to prevent debris or moisture from establishing a secure bond. Wipe the inner edges of the cut pipes with a dry cloth.

  • Scrape away loose burs along the cut edge with a knife. Work slowly to avoid cutting through the pipe. Rub both the edges of the cut pipe with a damp rag.

  • Apply PVC primer around both the outer edges of the pipe with a brush. Spread the primer by 2 inches on each side. Also apply the primer over the inner edges of the backflow valve, ensuring it is completely covered. Allow the primer to cure for the time specified on the label’s directions.

  • Apply a thin layer of PVC cement over the primed sections of the pipes and backflow preventer.

  • Hold the backflow preventer horizontally against the cut sewer pipe. Push each cemented pipe end into the ends of the backflow preventer, ensuring the hinge flap is positioned in the direction the waste travels. If required, reposition the valve so the hinge flap faces the right direction. Wipe away any cement that seeps out through the joints with a rag.

  • Hold the backflow preventer to the pipes for one minute to establish a secure bond. Allow the cement to cure for the time specified on the label’s directions.

Tips & Warnings

  • Wear safety glasses when cutting the sewer pipes.

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