How to Add a Humidifier to a Forced-Air System

In this video, This Old House plumbing and heating contractor Richard Trethewey explains how to add a humidifier to a forced-air system.

Steps for Adding a Humidifier to a Forced-Air System

1. Screw the humidifier to a wall near the furnace.
2. Cut away insulation from the duct using a utility knife.
3. Position the hole-cutting template onto the side of the duct.
4. Drill a pilot hole through the duct, then cut out the hole with aviation snips.
5. Secure the humidifier’s steam wand to the duct with self-tapping sheet metal screws.
6. Attach steam hose to the wand and to wall-mounted humidifier; secure hose with cable ties.
7. Connect the flexible drain hose to the steam wand and to the humidifier.
8. Run a copper drain pipe from the underside of the humidifier to the sump pump or floor drain.
9. Cut and solder a copper tee fitting into a nearby cold-water pipe.
10. Run new cold-water copper piping to the bottom of the humidifier.
11. Have an electrician run the electrical cable to the humidifier and to the room humidistat.
12. Turn on the water to the humidifier.
13. Install the air-proving switch in the duct.
14. Set the upper limit of humidity on the high-limit switch.
15. Set room humidistat to the desired level of humidity in the house.

Sizing Your Unit: Before purchasing a humidifier, you’ll need to determine the right capacity for your home. As demonstrated on Ask This Old House, the unit size is based on the square footage of your house, its air leakage rate, and exhaust fans like bath fans. A water hammer arrestor on the supply line is also recommended — it prevents banging in the water pipes when the humidifier’s valve closes.

Pro Tip: Ross, an HVAC technician featured on Ask This Old House, advises setting the humidistat to 35 percent initially: “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.”

Why the Drain Loop Matters: When connecting the flexible drain hose from the steam wand back to the humidifier, be sure to create a loop in the hose. This loop acts as a trap that fills with water, ensuring that steam only travels into the airstream and won’t sneak back toward the unit. Any steam that doesn’t get absorbed into the airstream can condense back to water and needs a place to go — the drain hose directs it back to the unit where it can be reused or discharged to the drain.

Materials

• Utility knife
• Propane torch

Tools