CHECK VALVE PURPOSE - In general a pool check valve in installed in your circulation system to keep water from flowing backwards in the system when the pump is turned off. For common uses of a check valve see "When To Install a Check Valve in Your Pool Plumbing".
CHECK VALVE TYPES - There are two common types of Check valves on the market today: the Spring Check Valve and the Spring Flapper Check Valve. The Spring Check Valve has an internal panel that moves against a light spring to open and shut the valve. The Spring Flapper Check Valve has a flap on one end of the valve that pivots up against a light spring when water is flowing from the pump, then releases down to shut off any opposite flow when the pump is off.
SPRING CHECK VALVES - This picture shows the center panel inside a typical Spring Check Valve. I have used a stick to push the center of the panel to allow flow through the valve. This Check Valve does not open as much as the Flapper valve so would appear to impede flow more than the Flapper Valve.
CHECK VALVE SIZES - These Check Valve come in different port sizes to fit the common system piping including 1 1/2, 2" and 2 1/2". Some of the valve are designed to fit two sizes. For example, a 1 1/2 pipe would fit inside the port, and a 2" pipe would fit outside the port. Also these valve come with different spring strengths. Most have a 1/2 # spring for light resistance. Some have 2# resistance for situations with greater head or backflow pressure like solar panels mounted on an 8' high roof.
UNION VS NON-UNION - Valve come with and without unions. Unions allow you to remove the valve for cleaning without having to cut the valve out of the plumbing. Some valves can be accessed and repaired through the top of the valve. For these types, the valve body does not need to be removed and is typically installed into the plumbing without unions - commonly referred to as a "slip" connection - pipe "slips" into the valve and is permanently glued.
CLEARING DEBRIS - Debris will occasionally get caught in the valve keeping it from closing completely. For instructions on how to clear debris from a typical Spring Flapper Valve, see "How To Clear Debris From a Pool Spring Flapper Valve." For instructions on how to clear debris from a typical Spring Check Valve see "How To Clear Debris From a Pool Spring Valve."
REPAIRING A CHECK VALVE - If the spring mechanism in a Check Valve is weak or broken, you will have to replace it. Unfortunately, not all Check Valve can be repaired. The Spring Check Valves discussed above cannot be opened for repair.so they must be replaced. On a Spring Flapper style Check Valve, the Flapper mechanism can be accessed and replaced through the top of the valve. See "How To Repair a Pool Spring Flapper Check Valve". for further instructions.