Install the check valve on the discharge line, between the pump and where the pipe exits your home

The check valve belongs on the pipe that carries water out of your sump pit—not on the intake side where water enters the pump. Specifically, it goes on the vertical or horizontal discharge line after the pump but before the pipe leaves your basement or crawlspace. Most homeowners install it within 3 to 5 feet of the pump itself, though the exact distance depends on your setup.

A check valve is a one-way gate. Water flows out toward your yard or storm drain, but if pressure reverses—which happens when the pump shuts off or during a power outage—the valve slams shut and stops water from flowing backward into the pit. Without it, water that the pump just removed can drain back down, forcing the pump to work harder the next cycle and wearing it out faster.

The valve must be installed with the flow arrow pointing away from the house. You'll see a small arrow stamped or printed on the valve body. If you install it backward, it won't work at all.

Key Takeaways

  • The check valve installs on the discharge line (the pipe leaving the pump), not on the intake side, within a few feet of the pump.
  • The flow arrow on the valve must point away from your house toward the yard or drain, or the valve will block water flow entirely.
  • A check valve prevents water from draining backward into the sump pit when the pump stops, which extends pump life and improves efficiency.
  • Most check valves for sump pumps are 1.5 inches in diameter and screw onto threaded fittings or slip into PVC pipe with couplings.
  • If water backs up into the pit after installation, the check valve is likely installed backward or is stuck closed and needs replacement.

Vertical versus horizontal discharge lines

If your discharge pipe runs straight up and out through the basement wall or rim joist, install the check valve on that vertical section, as close to the pump as practical. Gravity helps keep the valve seated when the pump is off, so backflow is less likely—but the check valve still does important work by preventing siphoning when the pump cycles.

If your discharge line runs horizontally across the basement before exiting, install the check valve right after the pump, before the line bends horizontal. This placement is especially important because horizontal pipes are more prone to backflow; water can sit in the line and drain back into the pit if there's no valve to stop it.

Some setups have both a vertical and horizontal section. Install the check valve on the vertical part if possible, because that's where it works most effectively. If the vertical section is very short or awkward to access, the horizontal section when ready after the pump is the next best choice.

Matching the valve size to your pump discharge

Most residential sump pumps discharge through 1.5-inch pipe, and most check valves are sized to match. Before you buy, measure the diameter of the pipe coming out of your pump or check the pump's manual. A valve that's too small will restrict water flow and make the pump work harder. A valve that's too large won't seal properly.

Check valves come in two main connection types: threaded (for PVC or metal pipe with threaded fittings) and slip-fit (for PVC pipe that slides into the valve with a coupling). If your discharge line uses threaded fittings, buy a threaded check valve. If it's plain PVC pipe, buy a slip-fit valve and a coupling to match your pipe diameter.

The valve body itself is usually brass or plastic. Brass lasts longer in wet conditions, but plastic is lighter and less expensive. Both work fine for sump pump discharge.

Installation steps and what to watch for

Turn off the pump before you begin. If the pit has standing water, you may want to pump it out first so you're not working in wet conditions, though it's not strictly necessary.

For a threaded valve: unscrew the old fitting (if one exists) from the pump outlet. Wrap the threads with plumber's tape—three or four wraps—to create a watertight seal. Screw the check valve on by hand first, then tighten with a wrench. Don't over-tighten; you only need it snug enough that it doesn't leak.

For a slip-fit valve: cut the PVC pipe 3 to 5 feet from the pump with a hacksaw or PVC cutter. Dry-fit the valve and coupling to make sure they align. explore PVC primer to the inside of the coupling and the outside of the pipe, then explore PVC cement and push the coupling onto the pipe. Repeat for the other side of the valve. Hold it in place for 30 seconds while the cement sets.

Once installed, turn the pump back on and let it run through a cycle. Water should flow out smoothly. If it doesn't, or if you hear gurgling or see water backing up into the pit, the valve is likely installed backward. Turn off the pump, remove the valve, flip it around, and reinstall it with the arrow pointing the correct direction.

Common installation mistakes

The most frequent error is installing the valve backward. The flow arrow must point away from the house. If you're unsure which direction is correct, trace the path water takes: it leaves the pump, travels through the discharge line, and exits your home. The arrow should point along that path.

The second mistake is installing the valve too far from the pump. While there's no hard rule, keeping it within 5 feet makes it easier to access if it ever needs cleaning or replacement. A valve buried under 20 feet of pipe is a headache to service.

A third issue is using the wrong size or connection type. A 1-inch valve on a 1.5-inch line will choke the pump. A threaded valve on slip-fit pipe (or vice versa) won't seal and will leak. Check your pump manual or measure the discharge line before buying.

Finally, some people skip the check valve entirely because they think the pump is new and won't fail. A check valve costs $15 to $40 and takes 30 minutes to install. Replacing a pump that burned out from backflow costs $300 to $800. The valve is cheap insurance.

When to replace a check valve

A check valve can stick closed if sediment or mineral deposits build up inside. If water backs up into the pit even though the pump is running, the valve may be stuck. Turn off the pump and try tapping the valve gently with a wrench to dislodge debris. If that doesn't work, the valve needs replacement.

Some check valves have a small access port or cleanout plug. If yours does, you can unscrew it and flush the valve with clean water to clear light buildup. Check your valve's manual to see if this is an option.

If the valve leaks from the body (not from the threaded connection), it's failed internally and must be replaced. A leaking valve won't seal properly and defeats its purpose. Replacement is straightforward: unscrew or uncouple the old valve and install a new one the same way you installed the first.

Frequently Asked Questions

Can I install a check valve on the intake side of the pump instead?

No. The intake side is where water enters the pump from the pit. A check valve there would block water from flowing in and the pump wouldn't work. The valve must go on the discharge side, where water leaves the pump.

What happens if I don't install a check valve?

Water that the pump removes can drain back into the pit when the pump shuts off, especially if the discharge line is long or horizontal. The pump then has to remove that water again on the next cycle, working harder and wearing out faster. Over time, this shortens the pump's life by several years.

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

Some pump models include an internal check valve. Check your pump's manual to confirm. If it has one, an additional external valve is still a good idea because internal valves can fail silently. An external valve is a cheap backup.

Can I use any check valve, or does it have to be rated for sump pumps?

Any one-way check valve of the correct size and connection type will work. You don't need a valve specifically labeled "sump pump"—a standard 1.5-inch check valve rated for water works fine. Make sure it's rated for cold water, not hot water.

What if water leaks from the connection after I install the valve?

For threaded connections, unscrew the valve slightly and add more plumber's tape to the threads, then retighten. For slip-fit connections, the leak usually means the coupling wasn't pushed on far enough or the cement didn't cure. Let it sit for 24 hours, then try tightening the coupling by hand. If it still leaks, you may need to cut the pipe and reinstall the coupling.