Hydronix HF3 series Slim Rib cap Filter housings have pressure Relief (PR) buttons and are ideal for Typical applications such as pre- and post-filtration for ro systems, under sink filters, counter top systems, ice machines, food service and other applications. Hydronix HF2 series housings are available in a variety of colors and are made to house 2.5" diameter cartridges. Most 9 7/8" and 10" cartridges fit easily and caps are available in 1/4", 3/8" and 1/2" female NPT ports.