The two main types of standard humidifiers are the bypass and power fan.

Bypass humidifiers can be installed on either the supply or return plenum and you run a bypass duct to the plenum the unit isn't installed on.

Power-Fan humidifiers can be installed on a vertical run of either the supply or return plenum and uses a fan to distribute humidity into the air stream.

Maximum coverage area means this humidifier will perform well for home sizes up to the square footage indicated, provided that the home is “tight." A "tight" home is typically one that is sealed well around windows and doors and does not leak air to/from the outdoors.

The amount of water vapor that can be delivered into the home, measured in gallons per day (GPD), that the humidifier can deliver into the space.

Power-Fan Humidifiers require 110 AC plug-in power to run the fan, as well as a 24v switch from the furnace board. Bypass Humidifiers only require the 24v switch from the furnace.

A remote sensing bulb is a small (about ¼” round) sensor that mounts outside the home and connects by light gauge wire (18 to 22 gauge, 2-conductor) to the humidifier controller. This sensor provides temperature readings to the computerized controller which allows the humidifier to automatically adjust humidity levels in the home based on outside air temperature.