(GATE or BALL) VALVES: A plumbing component that controls water circulation. As you perform the many tasks that are required to achieve and maintain water chemistry, you will need to become familiar with the positioning (either "open" or "closed") of these valves. The positioning will be explained on a case-by-case basis, where they apply, within the text of poolmanual.com. Note: There are typically no valves for aboveground pools, with the exception of some aboveground pools that may use slide valves to stop circulation when they need to clean the pump basket; if so, these slide valves are usually installed before the pump and after the pump.