Three kinds of filters are used in the swimming pool, hot tub and spa industry. Diatomaceous earth (DE), sand filters, and cartridge filters. DE and sand filters are used in cool-water pools and are not suitable for hot tubs and spas. Hot tubs and spas use cartridge filters made of non-woven, spun-bonded, polyester fabric. Avoid filters made from paper or cotton-paper blends because you can’t clean them. They must be replaced when dirty. Polyester cartridge filters are made from two or six-ounce fabric. There are two filtration methods available—suction and pressure. In the suction method, water from the tub is sucked through the filter to the pump, heater, and jets. Sometimes the filter sits right inside the hot tub or spa. Suction filtration requires about four times as much filter area as pressure filtration. If you don’t have a large filter, you have to clean it often. On some systems, a dirty suction filter can cause such flow restriction that there may be damage to the pump or heater. This filtration method is cheaper than pressure filtration because it does not require an expensive housing to hold the filter cartridge. This method is used on most spas. Pressure filtration is more expensive, but safer for the equipment and easier to maintain. The filter sits in a pressure housing located near either the tub or pump. A good six-ounce polyester fabric in a well spaced and sized filter affords good element life, excellent filtration, and little possibility of damage to pump or heater at reduced water flow. Great Northern® recommends pressure filtration. However, suction filtration is acceptable on budget sized and priced tubs. Manufacturers determine filter size by examining two factors—bather load (number of people, how dirty they are, the frequency of use) and circulating pump volume. If you had a very large pump, perhaps you‘d need a 100 square foot or larger filter. (This is large and expensive!!) If an average family used this large filter, they’d need to clean it only once a year. A more reasonably sized 25 square foot filter requires cleaning every six to eight weeks.