A whole-house humidifier mounts on the furnace or its ductwork and adds moisture to the warm air as it moves through the house. The job runs in a fixed order. Stick to that order and check each step before the next one starts.

Before you open anything

Confirm the furnace model, the plenum or duct location the manufacturer specifies, and the clearance around the mounting spot. Turn off power to the furnace at its switch and at the breaker. Shut off the water to the line you will tap. Read the installation sheet that came with the unit and set out every part it lists, including the bypass pieces, the water valve, the drain fittings, and the control wire.

Mount the unit to the plenum

Make the opening the template calls for, set the unit flush against the metal, and seal the seams with the sealant or tape the instructions name. A loose mount leaks air and moisture into the cabinet. Check that the bypass damper opens and closes freely before you fasten it. Owners who are comfortable with sheet metal and sealing can do this step. If the furnace housing or the plenum is unfamiliar, have a licensed heating contractor do the mounting.

Run the water line

Tap the cold supply with the fitting the kit specifies, add a shutoff valve so you can service the humidifier later, and route the supply line so it is protected from freezing wherever it runs. Open the supply slowly and check every joint for seepage before you go on. A water line at a furnace is a plumbing job in many places, so confirm the local rules and hire a licensed plumber if you are not sure of the connection.

Route the drain

Run the drain line from the unit to a floor drain or an approved drain point, with a downward slope the whole way. Check that water can leave the unit without pooling in the cabinet. Keep the drain end where you can see it, so you can confirm the flow during the test.

Connect the control and the humidistat

The control wire from the furnace powers the water valve and the humidifier call. Follow the wiring diagram exactly and label each wire at both ends. Mount the humidistat away from supply registers and out of direct draft, so it reads the room air and not the duct air. Connections inside the furnace belong to a licensed electrician or heating technician in most places. Owners can label, route, and inspect the wire run.

Test run

Restore power, open the water, and call for humidity from the humidistat. Watch the water valve open, the pad or cylinder wet, and the drain run. Check the joints, the drain, and the cabinet for leaks after the first full cycle. Then record the humidistat setting you chose. Judge the change in the room after a day of running, not after an hour.

After the install

Keep the installation sheet with the furnace papers. Note the pad or cylinder replacement interval from the manual where you will see it. Write the shutoff location on the furnace door so the next person knows where to close the water.