This flow-thru by-pass whole house humidification unit is low maintenance, cost effective, and is simple to operate. This unit requires good access to a return duct and drain. The humidifier mounts on supply plenum or return. By-pass carries air from supply, through humidifier, to return. Standard control. No. HE220A1019: 12 gallon per day, uses Vista model No. 710 filter No. HE260A1010: 17 gallon per day, uses Vista model No. 714 filter