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By Tony Oreso

Save money with these DIY steps to install a furnace

By Tony Oreso

The average cost of installing a gas furnace is $2,370, according to However, to install a gas furnace, you do not always need help from a professional. Whether you are replacing a worn-out model or securing your home for the cooler months, these do-it-yourself steps could help you install a gas furnace on your own and save money.

Step 1: Find the right size and brand

Furnaces are available in different sizes and brands. Popular brands include Payne, Trane, American Standard, and Lennox. The size of the unit will depend mainly on the energy needs of your house. The choice ultimately boils down to efficiency, cost of installation, and your budget.

Step 2: Prepare the furnace for installation

Once you get the right unit, do the following:

  • Remove all the loose components in the package, including the users manual.
  • Identify which part of the furnace will connect to the duct. Most units are marked with the appropriate size opening, so look for this information in the installation guide.
  • Cut the required opening on the recommended part. It could be on either of the sides or at the bottom of the part.

Step 3: Put the furnace in the right place

  • Cushion the unit with rubber pads to minimize noise during operation
  • If you place the furnace in the attic, include an alternative drain point in case the main drain is blocked. Besides, consider the actual point in the attic to place the unit to facilitate its maintenance and reduce the amount of noise that may seep into the bedrooms.
  • Decide where to place the exit point of the condensed air. It should slant slightly toward the drain exit.

Step 4: Connect the duct system

  • Seal all connection joints with a duct sealant. Do not use a duct tape for sealing; it is not durable enough. In the absence of a duct sealant, use a metal foil.
  • Connect the vent pipes, which are usually made of PVC. Good furnaces have both inlet and outlet vent pipes. Seal all the joints and ensure the vents slope back to facilitate drainage. Experts recommend a quarter-inch slope for every four feet.

Step 5: Install the gas supply

  • Close the shut-off valve before you release the gas.
  • Check for gas leaks. Use a spray detector and watch for bubbles, which are signs of leaks.

Step 6: Install the electrical components

New furnaces comes with low voltage and line voltage connections, which are rated 240V and 120V, respectively. When connecting the electrical components of a furnace, beware of polarity as new units are polarity-sensitive. Ideally, the connections follow this order:

  • The R terminal in the furnace connects to the R terminal on the thermostat with a red wire.
  • The W terminal on the furnace connects to the W terminal on the thermostat with a white wire.
  • The G terminal on the furnace connects to the G terminal on the thermostat with a green wire.
  • The C terminal on the furnace connects to the C terminal on the thermostat with a blue wire. In addition, a white wire from the C terminal of the furnace connects to the AC unit.
  • The Y terminal on the furnace connects to the Y terminal on the thermostat with a yellow wire. An additional red wire runs from the Y terminal of the furnace to the AC system.

Check that the wiring follows the manufacturer’s instructions before you switch on the device.

Step 7: Join the condensate drainpipe to a drain location

Preferably, the drain point should be located on the floor. The drainpipe should be a three-quarter-inch PVC pipe. Start the unit and test the rise in the air temperature. Perform another check for gas leaks. Finally, check that everything is in line with the manufacturer’s recommendations.

These instructions are mainly for a gas furnace installation. However, the principles here could be helpful during the installation of other types of units.

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