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 4 7/8" cartridges fit easily and caps are available in 1/4", 3/8" and 1/2" female NPT ports.