How to Vent New Furnaces


All furnaces fueled by natural gas, oil, or propane must be vented to the outside of the building. If a furnace is not vented, deadly carbon monoxide gas will escape into the building. As carbon monoxide builds up, it becomes extremely deadly to people in the dwelling. It is called the "silent killer" for this very reason. Depending on the efficiency of your furnace, it will either be vented into your chimney or directly to the exterior of your building.

Things You'll Need

  • For 90 percent and over furnaces:
  • PVC pipe and fittings
  • PVC glue and cleaner
  • Hole saw
  • Caulk gun
  • Caulk
  • Drill
  • PVC saw
  • Tape measure
  • For 80 percent and lower furnaces:
  • Galvanized sheet metal duct
  • No. 8-by-1/2-inch sheet metal screws
  • Cordless drill
  • Aviation snips


  • Measure the distance on an 80 percent between your chimney and furnace. There should already be an opening in the chimney where your old furnace connected. You will be reusing this opening. Measure the outlet size of the vent on the furnace and check to see the opening in the chimney opening is the same size. For 90 percent and over furnaces, skip to step 4.

  • Make a list of needed sheet metal pipe and fittings. Purchase them at your local home improvement store.

  • Run the vent pipe into your chimney, starting at your furnace. Cut your sheet metal pipe as needed with your aviation snips. Ensure the crimped end of the elbows and pipe face toward the chimney. Each connection should receive at least three screws. The pipe has to pitch up toward the chimney at least 1/4 inch per foot. Secure and hang all pipe.

  • Locate a place on the side of your building that is not near any window if you have a 90 percent efficiency and over furnace that uses a two-pipe PVC venting system. Drill two holes slightly larger than the outer diameter of the PVC. Ideally, the holes should be in a horizontal line and be a few inches apart.

  • Measure the distance from your furnace to the holes you drilled. Multiply this distance by two (you have two pipes). Make a list of pipe and fittings you need. Purchase them at your local home improvement store.

  • Carefully prime and glue the PVC pipe together; start at the furnace and work your way toward the holes. Make the final connection by sliding a pipe from outside the building and connect it to the pipe that's inside. Both PVC pipes must pitch up at a rate of 1/4 inch per foot as they head out the building. Secure and hang all pipes.

  • Consult with the furnace installation manual on how to terminate the PVC pipes outside the building. Caulk around the pipes to make a weather-tight seal.

Tips & Warnings

  • Upgrade your 80 percent furnace to 90 percent or better if possible. Significant savings on your heating bill are likely.
  • Install a carbon monoxide detector in your building. These are inexpensive and a potentially life-saving device. They should be used in any building with gas-fired equipment.
  • Improper venting and pitch on the vent pipe can result in carbon monoxide leaking into the building. This can cause sickness and/or death.

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