The recommended filter change period for the Aquasana AQ 4000 water filter system is 500 gallons or every 6 months.
Replacing the Filter Cartridges - remove the threaded caps by turning in the opposite direction of the arrows. Remove both filter cartridges and dispose of them. Install the new filter cartridges with the "A" cartridge being placed in the "A" housing with the O Ring end first. Install the "B" cartridge into the "B" housing with the O Ring end last. Reinstall the treaded caps by turning in the direction of the arrows and tighten by hand only. After the cartridge installation is complete, turn on the cold water at the faucet and pull the diverter valve pin to activate the water flow through the system. Check for leaks around the threaded caps. The initial flow of water may be slightly discolored and cloudy due to carbon fines and air bubbles. This is normal. Once the water flows clear in about 2-3 minutes, the Aquasana water filter system is ready for use.