How to 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 best 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.

When connecting the drain line, be sure to include a loop in the hose — this acts as a trap that fills with water, ensuring steam only travels into the airstream and doesn’t sneak back toward the unit. As Allison demonstrated on This Old House, any steam that doesn’t get absorbed into the airstream can condense back to water and needs a clear path to drain. Periodically, the unit will discharge old water and replace it with fresh water in the canister, with the old water leaving through the bottom into a copper pipe pitched toward the drain.

Pro Tip: Ross, an HVAC technician featured on Ask This Old House, advises setting the humidistat conservatively: “We’ll set it for 35%. You can make minor adjustments up or down through that first winter. One thing we don’t want to do is play with it too much and turn it up too much, because if you turn up too much, you could actually have a rainforest in your house.”