Check Valve

A check valve (also called a non-return valve or one-way valve) is a valve that opens to allow flow in one direction and closes automatically when flow reverses — requiring no manual operation. Check valves are used throughout plumbing systems wherever backflow would cause damage, contamination, or system failure: sump pump discharge lines, water heater expansion tank connections, irrigation systems, and backflow prevention assemblies.

How check valves work

The two most common check valve mechanisms are:

• Swing check: a hinged disc (clapper) swings open as forward flow pushes it, and gravity (or backpressure) swings it closed when flow stops or reverses. Common in larger-diameter lines.
• Spring check (lift check): a spring-loaded disc or ball is pushed open by forward flow pressure. When pressure drops or reverses, the spring snaps the disc closed. More reliable for vertical installations and systems with low flow velocity.
• Ball check: a ball sits in a seat and is displaced by forward flow; reverse flow pushes the ball back into the seat. Common in sewage and sump pump applications.

Common applications

• Sump pump discharge: prevents pumped water from flowing back into the sump pit when the pump shuts off
• Water heater expansion tank: a check valve in the cold water supply line to the water heater creates a closed system that requires an expansion tank to absorb thermal expansion
• Irrigation backflow prevention: prevents irrigation water from siphoning back into the potable supply
• Boiler systems: prevents hot water from flowing backward through cold supply connections

Maintenance

Check valves in residential plumbing rarely need maintenance, but they do fail — typically by getting stuck open (due to debris fouling the seat) or stuck closed (corrosion or a stuck spring). A failed check valve on a sump pump line allows discharged water to immediately refill the pit, causing the pump to run continuously. Cost: $10–$80 depending on size and type.