What It Is
A check valve is a one-way valve that allows water to move freely in the intended direction but automatically closes to block reverse flow. It has no external handle or control - it responds entirely to the direction and pressure of the water moving through it. Check valves are installed wherever backflow would defeat the purpose of the system or cause damage - most commonly on sump pump discharge lines, well pump systems, and certain water heating applications.
What It Does
The most important function of a check valve is preventing backflow. On a sump pump, for example, the pump pushes water up and out through a discharge pipe. Without a check valve, when the pump shuts off, all the water in the vertical discharge pipe falls back down into the sump pit. The pump then has to pump the same water out again on the next cycle, running more frequently than necessary. A check valve holds the water in the pipe, so the pump only moves new water.
On well pump systems, a check valve at the pump or tank prevents pressurized water in the storage tank from flowing backward through the pump when the pump is off. This both protects the pump and maintains system pressure between pump cycles.
Where It Is Located
Common check valve locations include: on the sump pump discharge line, typically installed a short distance above the pump outlet (placement varies by installation); on the output side of a well pump or at the well head; on boiler feed water lines; and on the inlet to some water heaters where backflow could cause pressure imbalances. On sump pump installations, you can usually see the check valve as a fitting with an arrow indicating flow direction on the discharge pipe coming out of the pit.
How It Works
Most residential check valves use a spring-loaded disc or a swing flap. When water pushes in the correct direction, the disc or flap opens against spring tension and water passes through. When flow stops or reverses, the spring closes the disc against a seat, sealing the valve. The seating must be clean and undamaged for the valve to seal fully - debris caught in the seat will cause a check valve to leak in reverse.
Common Problems or Signs of Failure
- Sump pump cycling very frequently in short intervals (water falling back and retriggering pump)
- Water hammer noise on the sump discharge line when pump shuts off
- Pump running but pit water level not dropping (check valve stuck closed, blocking discharge)
- Visible water dripping back down the discharge pipe after pump stops
- Well system losing pressure quickly between pump cycles
Can a Homeowner Check It?
- Listen to the sump pump after it shuts off - you should not hear water rushing back down through the discharge pipe
- Watch the pit water level: if it drops when the pump runs but immediately refills from the discharge side, the check valve has failed
- Look at the check valve body for visible cracks or leaks at the joints
- Confirm the arrow on the check valve body points in the direction of intended flow (away from the pump)
When to Call a Plumber
A failed check valve on a sump pump leads to excessive cycling that shortens the pump's life and can cause basement flooding if the pump burns out. Replacing a check valve is a straightforward repair, but it does require cutting the discharge pipe and splicing in the new valve. If your pump is cycling every few minutes or you hear water backflowing after shutdown, it is worth addressing before the pump fails.
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