How to Install a Whole-House Dehumidifier

In this video, Ask This Old House plumbing and heating expert Richard Trethewey shows how to control humidity throughout the house all year round.

How to Install a Whole-House Dehumidifier

1. In the attic, attach four steel hanging bars and the metal pan from the collar ties. Position the pan very close to the air-conditioning unit.
2. Set the whole-house dehumidifier in the metal pan.
3. Cut a square hole in the hallway ceiling and install a new return-air grille.
4. Attach a flexible return-air duct to the port on the end of the dehumidifier.
5. Secure the duct to the roof framing with plastic straps.
6. Cut the duct to length, then attach it to the return-air grille installed in Step 3.
7. Cut a new supply-air duct fitting into the air-conditioning unit.
8. Connect a flexible supply-air duct to the fitting. Attach the opposite end of the duct to the existing air-conditioning duct system.
9. Run a new PVC condensate line from the dehumidifier to the outdoors.
10. Install the dehumidifier’s filter; be sure to replace the filter every two to three months.
11. Mount a humidistat to the wall beside the existing thermostat.
12. Start by setting the humidistat for 35 percent humidity, then adjust as necessary.