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.
About two hours to mark templates, cut holes, and install the humifier
Felt-tip marker, cordless drill and bit, aviation snips, screwdriver, wire strippers wrenches
Cutting sheet metal, drilling
Shut off furnace
Humidifier unit, vent pipe, sheet metal screws, wire nuts
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.
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.
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.