Whole house filters are used in systems that are either a single canister system or a duel canister system.  Single canister systems are used to control a single water quality issue.  Duel systems are designed to address multiple filtering requirements and sized by the number of persons in a household.  In duel systems the pre-filter is used to remove sediment and a carbon post filter is used to remove unwanted chemicals, VOC's and foul smell or taste.