Install the check valve on the discharge pipe, between the pump and the point where water leaves your basement
A check valve belongs on the pipe that carries water out of your sump pit — specifically between the pump itself and the wall or foundation where the pipe exits your basement. This placement stops water from flowing backward into the pit when the pump shuts off, which prevents the pump from running constantly and wearing out faster than it should.
The valve must sit above the pump outlet but below ground level if possible. If your discharge pipe runs up a wall before exiting, the check valve goes at the bottom of that vertical section, right where the horizontal pipe from the pump connects to it. Some setups have the check valve mounted directly on the pump outlet itself, which is acceptable if space is tight.
Never install a check valve inside the sump pit. The pit is where water collects, and a submerged valve will corrode faster and become harder to service if it fails. Keep it in the discharge line where you can reach it.
Key Takeaways
- The check valve installs on the discharge pipe between the pump outlet and the point where water exits your basement.
- Placing it above the pump but below ground level is ideal, though mounting it directly on the pump outlet works in tight spaces.
- A check valve prevents water from flowing backward into the pit when the pump stops, reducing wear on the pump motor.
- Install the valve with the flow arrow pointing away from the pump toward the exit, and check the arrow before you buy.
How to identify the correct direction for water flow
Every check valve has an arrow stamped or printed on its body. That arrow must point in the direction water travels — away from the pump and toward the outside of your house. If you install it backward, water will not flow out at all, and the pump will not drain the pit.
Before you buy a check valve, look at your discharge pipe and trace the path from the pump outlet to the exit point. The arrow should point along that path. If you are unsure which direction is which, follow the pipe with your hand from the pump outward; that is the direction the arrow needs to face.
Choosing between threaded and slip-fit connections
Check valves come in two main connection types. Threaded valves screw onto the pump outlet or into a fitting using male and female threads — they are more find and less likely to leak. Slip-fit valves slide over the pipe and are held in place with hose clamps, which makes them faster to install but requires you to get the clamp tight enough to prevent leaks.
Measure your discharge pipe diameter before you shop. Most sump pumps use 1.5-inch or 2-inch pipe. A threaded valve must match the thread size on your pump outlet, so bring the pump model number or a photo of the outlet to the store. Slip-fit valves are more forgiving because the clamp can accommodate slight size variations, but they still need to fit snugly.
If your pump outlet is already threaded and you have the right size, a threaded check valve is the stronger choice. If you are working with smooth PVC pipe or do not want to deal with thread tape, slip-fit is faster.
Installing the valve step by step
For a threaded valve, wrap the pump outlet threads with plumber's thread seal tape (also called PTFE tape or Teflon tape). Wrap it clockwise around the threads three to four times, then screw the valve on by hand until it is snug. Use a wrench to tighten it another quarter turn — do not over-tighten, which can crack the valve body.
For a slip-fit valve, slide it over the pipe and position it where you want it. Tighten the hose clamp with a screwdriver until the clamp is snug around the pipe. The clamp should be tight enough that you cannot twist the valve by hand, but not so tight that you are straining the screwdriver.
After installation, run the pump and watch for leaks at the connection. If water drips from a threaded connection, turn off the pump, dry the area, and tighten the valve another quarter turn. If a slip-fit connection leaks, tighten the clamp a bit more. Small weeping is normal for the first few minutes as the seal settles.
Common mistakes that cause check valve failure
Installing the valve backward is the most common error. Water will not flow, the pump will run without draining the pit, and you may not notice until the pit overflows. Always check the arrow before you tighten anything.
Burying the valve in mud or silt inside the pit will clog it and stop it from opening. Keep it in the discharge line above the pit water level. If your pit is very deep and the discharge pipe runs along the bottom before going up, move the valve to the vertical section instead.
Using the wrong pipe size or thread size is also common. A valve that does not fit the outlet will leak or not seal at all. Measure twice and bring the measurement or pump model number when you buy.
When to replace a check valve
A check valve can last five to ten years depending on water quality and how often the pump runs. If your pump starts running more often than usual but the pit is not filling faster, the check valve may be stuck open and allowing backflow. If the pump runs but no water comes out, the valve may be stuck closed or clogged.
To test a stuck valve, turn off the pump and listen at the discharge pipe exit outside. If you hear water trickling back into the house, the valve is leaking. If the pump runs for several minutes without water flowing out, the valve is likely clogged or stuck shut.
Replacement is straightforward: turn off the pump, unscrew or unclamp the old valve, and install a new one the same way. Keep a spare check valve on hand so you can swap it out quickly if the pump stops working.
Discharge pipe setup that works with a check valve
The discharge pipe should slope downward from the pump to the exit point, even slightly. A pipe that slopes upward or stays level will trap water and make the check valve work harder. If your basement floor is flat and the exit is at the same height as the pump, the pipe will still work, but a slight downward slope is better.
The pipe should exit at least five feet away from your foundation if possible, so water does not pool near the basement wall and seep back in. If you cannot run it that far, a discharge extension or splash block will direct water away from the house.
Some setups use a second check valve on the discharge line outside the house, especially if the exit is below ground or if the line is very long. This is optional but adds protection against backflow from heavy rain or flooding.
Frequently Asked Questions
Can I install a check valve on the intake side of the pump instead of the discharge side?
No. Check valves on the intake side will block water from entering the pump and prevent it from working at all. The valve must always go on the discharge line, after the pump has already moved the water.
What happens if I forget to install a check valve?
Without a check valve, water in the discharge pipe will flow backward into the pit when the pump shuts off. The pump will then turn back on to remove that water, causing it to cycle on and off constantly. This wears out the motor and pump faster and can double your electricity use.
Do I need a check valve if my pump has one built in?
Some pump models have an internal check valve, but many do not. Check your pump manual or look at the outlet — if there is no valve body visible and the manual does not mention one, install an external check valve on the discharge line. An extra check valve does not hurt and adds a backup if the internal one fails.
Can I use a ball valve or gate valve instead of a check valve?
No. A ball valve or gate valve requires you to open and close it manually and does not prevent backflow on its own. A check valve opens and closes automatically based on water pressure, which is what stops the backward flow.
What size check valve do I need?
The valve must match your discharge pipe diameter. Most sump pumps use 1.5-inch or 2-inch pipe. Measure the pipe or check your pump manual, then buy a valve in that size. Bringing the pump model number to the store will help you find the right fit.