CFS TWO STAGE FILTER SET — GROOVED SEDIMENT AND CARBON, 10 X 2.5 INCH A two stage system works because the two cartridges do different jobs in the right order. This set covers both positions in one service. STAGE ONE: GROOVED SEDIMENT The first cartridge is grooved rather than smooth walled. The fluted outer surface adds filtering area without tightening the media, so it holds more sand, silt and rust before flow starts to fall off. The fibers run coarse on the outside and tighter toward the core, spreading debris through the wall instead of packing it all on one face. That is the cartridge working for its living so the one behind it does not have to. STAGE TWO: CARBON The second cartridge reduces the chlorine taste and odor that make water unpleasant to drink and cook with. Running it behind the sediment filter is what makes it last. A carbon cartridge doing sediment work exhausts on grit long before its adsorption capacity is used up, which is why people who skip the prefilter end up buying carbon twice as often. WHAT IT REPLACES This set replaces AP110 in the sediment position and AP117 in the carbon position. Both are standard 10 by 2.5 inch cartridges, so they seat in AP11T, AP101T and most other slim line housings. On the sediment side the grooved cartridge also covers WHKF-GD05, WHCF-GD05, P5, FPMBG-5-975 and CFS110. AP117 is also sold as APS117 and AP017. WHEN TO CHANGE THEM Change both together, about every three to six months depending on your water. If you are on well water or your line runs heavy sediment, the first stage will call for it sooner. The usual sign is pressure dropping at the taps, and that is almost always the sediment cartridge rather than the carbon. This is a compatible replacement set. Part numbers are for reference only.