How to Install a Humidifier in Central Air


The humidity level in your home plays a large part in how comfortable you feel when you're inside. According to the American Lung Association, people feel most comfortable when their in-home humidity level ranges between 35 and 55 percent. If you're handy with tools, you can install a whole-house humidifier into a central air system on your own, easily turning your home into an inviting place to be.

Things You'll Need

  • Humidifier
  • Tin snips, left and right angled
  • Cordless drill
  • 1/4-inch hex screws
  • 1/4-inch hexhead nutdriver bit
  • Tube cutter
  • Small wire nuts
  • 18-gauge 2-conductor thermostat wire
  • 6-inch round duct (starting collar, pipe, elbows or flexible duct)
  • 1/2-inch vinyl hose
  • 1/2-inch pipe clamps
  • 1/4-inch copper tubing
  • Locate the proper spot for your humidifier and the humidistat. To effectively measure the humidity in your home, the humidistat must sit at least 12 inches upstream of the humidifier or the bypass pipe (6-inch duct) on the return-air duct. Most humidifiers mount vertically on either the return or supply duct, which must be perfectly plumb and large enough to accommodate the unit. Plan also for the water line, 6-inch duct and drain line.

  • Remove the cover and take out the humidifier pad. Using a marker, trace the hole in the back of the humidifier, but cut the duct a quarter-inch larger, so the humidifier will mount flush. Level the humidifier, and mount it with screws through the holes in the back.

  • Cut a 6-inch, round hole in the opposing duct, using the starting collar as a guide. Mount the starting collar by folding in the tabs inside the hole, and run the 6-inch pipe between the humidifier and starting collar.

  • Install the included saddle valve on the nearest cold water line. Measure and cut the quarter-inch copper tubing to connect the saddle valve with the solenoid valve on the humidifier. Tighten the compression nuts on each end, using a back-up wrench.

  • Cut the half-inch drain hose to length and run it from the water outlet to the closest drain. Make sure the hose does not kink and stays angled to drain properly. Secure the hose with pipe clamps, if necessary.

  • Cut a small hole in the return-air duct to accommodate the humidistat. Before mounting the humidistat with screws, run the thermostat wire from the humidistat to the humidifier, and then to the furnace wiring control. Run the wire from the humidifier through a small hole in the return-air duct to the back of the humidistat.

  • Wire the humidifier to the 24-volt system in your furnace so that the humidifier gets power only when the heat comes on. Many newer furnaces have humidifier terminals on the control board located in the blower section. You need a hot side and a common. You may have to add a 110-volt to 24-volt transformer, included in your humidifier. The humidistat will open the circuit when it detects the proper humidity, turning off the humidifier.

  • Test your humidifier by setting your thermostat to call for heat and turning the humidistat all the way up. Open the saddle valve to allow a trickle of water to flow over the humidifier pad. Open the damper on the 6-inch pipe to the "winter" setting. Monitor the flow of water through the drain and adjust the saddle valve until the wastewater goes as low as possible. Reset the humidistat to a comfortable setting, usually at 35 percent humidity.

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