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How to Install a Whole House Humidifier

Ask This Old House plumbing and heating expert Richard Trethewey shows how a professional would install a whole-house humidifier.

In this video, Ask This Old House plumbing and heating expert Richard Trethewey shows an efficient, low-maintenance way to humidify the air.

Steps for Installing a Humidifier for the Whole House

  1. Disconnect the flexible supply ducts from the sides of the heat pump.
  2. Remove the existing main trunk from the top of the heat pump.
  3. Build a new sheet metal trunk.
  4. Cut three 4-inch-diameter holes into the top of the new main trunk.
  5. Over each hole fasten a take-off fitting with a balancing damper.
  6. Mount a louvered sheet metal elbow to the top of the heat pump, then attach the new main trunk to the elbow.
  7. Attach one 12-inch-diameter take-off fitting onto the side of the main trunk. Fasten a second fitting to the bottom of the trunk.
  8. Cut a 2½-inch hole in the back of the trunk, then insert the humidifier wand. Secure the wand to the trunk with sheet metal screws.
  9. Run a new copper cold-water supply line from the water heater to the humidifier.
  10. Connect a drain line from the humidifier to the existing drainpipe on the heat pump.
  11. Have a licensed electrician run a new 220-volt circuit to the humidifier.
  12. Control the humidifier with a humidistat set to approximately 35 or 40 percent.