Filter Water Direct carries a full line of Sterling Water Filters Replacement Cartridges to fit both ten inch and twenty inch big blue housings. Most of these installations had one ea five mic sediment and one ea GAC (Granular Activated Charcoal) filter but some had three housings and the third was usually polyphosphate housing. If you open the third housing and it looks as if it had small granules inside, then odds are it was a polyphosphate. This is designed to slowly dissolve into your water and encapsulate hard water particles making it easier to maintain your equipment. These filters should be changed every six months, or at the same time the Sterling Water Conditioner is cleaned. It’s also a good idea to take a good look at the inside of the cartridge housings and clean them with chlorine bleach if necessary. The large “O” Rings sometimes wear out and stretch which will create a leak. It’s a good idea to always have a few extra spares on hand. When it’s time to change your filters, be sure and by-pass the filter set if you’re lucky enough to have a by-pass. Also your housing might have a red pressure relief button on top. After you by-pass the filter set, press the red button to relieve the pressure inside the housing. If you don’t have a by-pass, then you will need to turn off your water supply and open a few faucets to relieve the water pressure. Use the specially designed wrench to twist the Sterling Water Filters Replacement Cartridges off.