Installing a Whole-house Humidifier

Intro

Whole-house humidifiers typically attach to the main stack of a forced-air furnace. The unit draws its water from a nearby hot or cold water pipe.

Some humidifiers are installed in the return-air duct and add moisture to the air before it is heated. Others employ a bypass duct to move heated air through the humidifier and reintroduce it to the air going to the house.

Installing either system involves cutting a hole in the venting using templates provided by the manufacturer. Be sure to shut off the furnace before installing the humidifier.

Prestart Checklist

Time
About two hours to mark templates, cut holes, and install the humifier

Tools
Felt-tip marker, cordless drill and bit, aviation snips, screwdriver, wire strippers wrenches

Skills
Cutting sheet metal, drilling

Prep
Shut off furnace

Materials
Humidifier unit, vent pipe, sheet metal screws, wire nuts

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Step 1

Using the manufacturer's template and instructions, mark the hole for the humidifier with a felt-tip marker. Drill a starter hole in a corner of the outline, then cut out the hole with aviation snips. Position the mounting plate, drill screw holes, and attach the mounting plate with sheet metal screws.

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Cut hole for bypass vent, Attach bypass vent

What If... My model requires a bypass vent?

If your humidifier requires a bypass vent, use the template to mark the hole for a 6-inch bypass pipe. Drill a starter hole and cut the vent hole with aviation snips. Install a round collar in the hole, securing it with sheet metal screws. Measure the distance between the humidifier air inlet and the collar and cut the vent pipe to length. Install elbows on both ends of the vent pipe and fasten the assembly to the collar and humidifier air intake.

Step 2

Refer to the manufacturer's instructions for the location of the humidistat (most are attached on the cold air plenum). Mark the outline for this unit and cut out the hole.

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Step 3

Install the humidistat mounting plate and attach the unit. Connect the humidistat to the furnace controls following the instructions. If the unit requires a step-down transformer and you haven't yet run the wiring, shut the power off, install the transformer, and wire the humidistat.

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Connect humidistat leads to solenoid valve

Step 4

Using the correct-size wire nuts, connect the leads from the humidistat to the solenoid valve on the humidifier. Insert the wire ends into the nuts (do not twist the ends of the wires) and turn the nuts until they're tight.

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Step 5

If local codes allow tapping water lines directly, install a saddle valve on a water line -- preferably a hot water line -- and run a line to the humidifier, fastening it with the compression fitting. If saddle valves are not allowed, turn the water off, cut the water line, and install a T-fitting.

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