The HE220 and the HE260 Bypass Flow - Through Humidifiers provide flexibility in installation location. The ideal location for a humidifier to be mounted is on the warm air supply, if this is not possible due to space restrictions, Honeywell's bypass flow - through humidifiers can also be installed on the return duct. This is because the air will travel from high pressure to low pressure. When the humidifier is installed on the supply duct, the by-pass duct should run to the return. When the humidifier is installed on the return duct, the by-pass duct should run to the supply. Honeywell encourages you to purchase the installation kit, which includes all of the components needed for installation, including the Bypass Duct. The installation kit is part number 32005847-001, and should be available at most home improvement retail stores.