Big Blue Filter Housing: What It Holds and Why It Matters A Big Blue filter housing is the sturdy shell that holds your cartridges and sits on the main water line. The housing does the holding while the cartridge inside does the cleaning, so the two need to match. These two 4.5 x 20 inch cartridges are a good example. One is a solid gray-white block, and the other has a ribbed white body with blue end caps. Both are sized to slide into a standard Big Blue housing without adapters. Because the housing is large, it moves more water at once than a slim under-sink unit, so showers and appliances keep their pressure. Check the cartridges every few months and replace them when flow slows. Visit : https://store.afwfilters.com/big-blue-filter-kits/