A check valve stops water from flowing backward into your sump pit

A check valve is a one-way gate in your discharge line that lets water flow out of your home but prevents it from draining back into the pit when the pump shuts off. Without one, water sitting in the pipe above ground can seep back down, forcing your pump to run more often and wearing it out faster. Installing one is a straightforward job that takes about 30 minutes and requires only basic tools—no special plumbing license needed.

The valve mounts on the discharge pipe between the pump and where the line exits your foundation. You can install it yourself if you have a submersible pump with a threaded discharge fitting, though some pedestal pumps and certain configurations may need a plumber's help. The cost of the valve itself is usually $15 to $40, depending on the pipe size and valve type.

Key Takeaways

  • A check valve prevents water from flowing backward into your sump pit after the pump stops, reducing wear and unnecessary cycling.
  • The valve installs on the discharge pipe between the pump outlet and the point where the line leaves your foundation.
  • You will need the correct pipe size (typically 1.25 or 1.5 inches), a check valve rated for that size, two pipe wrenches, and thread-seal tape.
  • The entire installation takes about 30 minutes and involves unscrewing the old fitting, wrapping the new valve threads, and screwing it back in place.
  • If your pump has a union fitting or your discharge line is glued PVC, you should have a plumber handle the installation.

Gather the right valve and tools before you start

First, measure or note your discharge pipe size. Most sump pumps use 1.25-inch or 1.5-inch pipe—check the pump manual or measure the diameter of the existing discharge line. Buy a check valve in that same size; common types are swing-check valves (the most affordable and reliable for sump pumps) or spring-check valves. Swing-check valves have a hinged flapper inside that opens when water flows out and closes when pressure reverses.

Gather these tools: two adjustable pipe wrenches (one to hold the fitting, one to turn the valve), thread-seal tape (also called plumber's tape or PTFE tape), a bucket or towel to catch water, and a flashlight if your sump pit is dim. If your pump is currently running, turn it off at the breaker and let any water in the discharge line drain into the pit—this takes a few minutes.

Locate the discharge fitting and prepare the area

Follow the discharge pipe from the top of your pump upward. The pipe will be threaded into a fitting on the pump outlet. This is where you will install the check valve. Place your bucket or towel underneath because some water will drip when you disconnect the old fitting.

If the discharge line is already connected to other fittings or elbows above the pump, you have two options: install the check valve directly on the pump outlet (the simplest route), or install it further up the line if there is a union fitting or threaded connection you can access. For a first-time installation, the pump outlet is the easiest spot. If the discharge line is glued PVC pipe with no threaded connections, stop here and call a plumber—cutting and re-threading PVC requires special tools and skills.

Disconnect the old fitting and prepare the threads

Use one wrench to hold the pump outlet fitting steady while you use the second wrench to turn the discharge pipe counterclockwise. Work slowly; the connection may be tight. Once the pipe is loose, unscrew it by hand and set it aside. Water will drip—that is normal.

Wipe the threads on the pump outlet clean with a dry cloth or paper towel. Wrap the threads with thread-seal tape by holding the tape taut and wrapping it clockwise around the threads three to four times. This tape prevents leaks at the connection. Smooth the tape down as you wrap so it adheres to the threads.

Install the check valve on the pump outlet

Screw the check valve onto the pump outlet by hand first, turning it clockwise until it is snug. Then use your two wrenches to tighten it further: hold the pump outlet steady with one wrench while you turn the valve body with the other. Tighten until the valve is firm but do not over-tighten, which can crack the fitting or strip the threads. A quarter-turn past hand-tight is usually enough.

Check the valve body for an arrow or label showing the direction of flow. The arrow should point away from the pump, toward the exit of your home. If your valve is installed backward, water will not flow out at all. Most swing-check valves have a clear arrow stamped on the body; if yours does not, consult the valve packaging or manual.

Reconnect the discharge line and test the pump

Wrap the threads of the discharge pipe (the part you removed earlier) with thread-seal tape the same way you did for the pump outlet. Screw the pipe back onto the check valve by hand, then use your two wrenches to tighten it. Again, firm is better than cranked—over-tightening can crack plastic fittings.

Turn the pump back on at the breaker. Listen for water flowing out of your discharge line to the outside or into your drain. The pump should run briefly and then stop. Turn it back off and wait a few minutes, then turn it on again. If water flows out both times and you see no leaks at the check valve connection, the installation is complete. If water drips from the connection, turn off the pump and tighten the fitting a bit more with your wrenches.

Common mistakes and when to call a plumber

The most frequent error is installing the valve backward—water will not discharge at all, and the pump will run continuously until you flip it around. If your pump runs but no water comes out, turn it off when ready and check the arrow on the valve body.

Leaking at the connection usually means the threads were not sealed properly or the fitting was not tightened enough. Turn off the pump, use your wrenches to tighten the leaking connection a quarter-turn, and test again. If it still leaks after a second tightening, turn off the pump, disconnect the fitting, add more thread-seal tape, and reconnect.

Call a plumber if your discharge line is glued PVC with no threaded connections, if your pump has a union fitting that you are not comfortable disconnecting, if you cannot stop a leak after two tightening attempts, or if the pump runs continuously even with the valve installed correctly and facing the right direction. A plumber can also install a check valve on a pedestal pump or in a tight space where you cannot fit two wrenches.

Frequently Asked Questions

Do I need a check valve if my sump pump already has one built in?

Some pumps have an internal check valve, but many do not. Check your pump manual or ask the manufacturer. Even if your pump has one, adding an external check valve on the discharge line provides extra protection and is a common practice. It does not hurt to have both.

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 it does not prevent backflow automatically. A check valve is the only type that works one-way without any action on your part. Using the wrong valve type will not solve the backflow problem.

What size check valve do I need?

Match the size to your discharge pipe. Most sump pumps use 1.25-inch or 1.5-inch pipe. Measure the outside diameter of your existing discharge line or check the pump manual. Buy a check valve in that exact size—a mismatched size will not thread on correctly.

Will a check valve reduce the flow of water out of my pump?

A quality swing-check valve creates minimal resistance and does not noticeably reduce flow. If your pump suddenly stops discharging water after installing a check valve, the valve is likely installed backward or is stuck closed. Turn off the pump and check the flow direction arrow on the valve body.

How often do I need to replace a check valve?

A swing-check valve typically lasts as long as your pump—often 5 to 10 years or more. If your pump runs more often than usual or you notice water backing up into the pit, the valve may be stuck or failing and should be replaced. Replacement is the same process as installation.