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Check Valve Guide

Quick answer: A sump pump check valve lets water leave the pit but stops the water in the discharge pipe from draining back when the pump shuts off. Without one, the pump re-pumps the same water and cycles more often. Install it arrow-up just above the pit, add a weep hole if required, and replace it when it leaks or slams.

What a check valve does

When the pump stops, the column of water standing in the discharge pipe wants to fall back into the pit. In a typical installation that might be several gallons. If it returns, the water level jumps, the float rises again and the pump restarts — wasted energy and extra wear. A check valve is a one-way gate: flow from the pump pushes it open; when flow stops, it closes and holds the column.

Types of check valve

Swing check. A hinged flap that swings open with flow and falls closed when it stops. Simple and common; can close with an audible thump.
Spring-loaded or silent check. A spring pushes the disc closed before the water reverses, reducing water hammer. Usually quieter.
Ball check. A ball seats against an opening; less common in sump applications.
Clear check valves. Have transparent bodies so you can see the flap or debris.

Many sump check valves have rubber unions at each end, which makes the valve and pump easier to remove for service.

Installation basics

Install the valve in the discharge pipe just above the pit, where it is easy to reach.
Follow the arrow on the body: it must point in the direction of flow, away from the pump.
Some valves are rated for vertical orientation only; follow the manufacturer's instructions.
Clamp unions properly; a loose clamp can leak or let go under pressure.
Each pump in the pit needs its own check valve before any tee joining two pumps.

The weep hole

Many pump manufacturers recommend drilling a small hole, often about 1/8 inch, in the discharge pipe between the pump and the check valve, just above the pump and below the water line. It releases trapped air so the pump can prime. Without it, an air pocket can stop the pump from moving water even though the motor runs — an air lock. A weep hole sprays a small stream back into the pit while the pump runs, which is normal. Always follow your pump's instructions.

Water hammer

When a swing check slams shut, the moving water column stops suddenly and a pressure spike travels through the pipe, causing a bang that can loosen fittings over time. Quiet, spring-assisted valves close earlier and reduce the spike. Supporting the pipe firmly and avoiding very high velocities also help. The discharge pipe calculator estimates velocity in your line.

Worked example: what the valve saves

A homeowner has 12 feet of vertical 1½-inch PVC and 20 feet of horizontal run that drains back toward the house. A 1½-inch pipe holds about 0.106 gallons per foot, so the vertical rise holds about 1.3 gallons and the sloped run perhaps another 2.1 gallons that could flow back — about 3.4 gallons. Their 18-inch pit holds about 1.1 gallons per inch, so without a working valve each shut-off would add about 3 inches back to the pit. With a 7-inch drawdown, almost half of each cycle would be re-pumping the same water, increasing starts by roughly 40–80% depending on inflow. Replacing a failed valve noticeably cuts cycling.

Illustrative note (2026-10-29): Pipe volumes and cycle impacts in this example were calculated on 2026-10-29 using simplified assumptions. Actual drain-back depends on pipe routing and slope.

Signs a check valve is failing

The pump restarts shortly after shutting off, even without new inflow.
You hear water rushing back into the pit after the pump stops.
A loud bang every time the pump stops.
Visible leaks at the valve body or unions.
The flapper is stuck open with debris in a clear-body valve.

Replacing a check valve

Replacement is a common, modest job when the valve uses rubber unions: unplug the pump, loosen the clamps, catch the water that drains, fit the new valve in the correct direction and tighten. If the valve is solvent-welded into rigid pipe, cutting and gluing is required. A licensed plumber can do it quickly and check the weep hole and pipe supports at the same time.

When a check valve is not recommended

Some pump manufacturers or installers omit check valves on very short discharges where drain-back is minimal, or where the discharge is designed to drain completely to prevent freezing. Some designs use a check valve plus a freeze-relief fitting outside. Follow your pump's instructions and local code; a plumber can advise.

Check valves on dual-pump systems

With a primary and backup pump, each needs its own check valve before the tee that joins their discharges. Otherwise, one pump will push water back through the other instead of out of the house. Combination systems often include both valves pre-installed.

Choosing a replacement valve

Match the valve to your pipe size — most residential discharges are 1½ inch, and some larger pumps use 2 inch. Do not downsize the line at the valve. If water hammer has been a problem, a spring-assisted quiet valve is usually worth the small extra cost. A clear-body valve makes inspection easy during routine tests. Unions at both ends make future pump swaps quicker, and stainless hose clamps resist corrosion in damp basements. Keep the old valve's box or a photo so the next replacement is straightforward.

Frequently asked questions

How long does a check valve last?

It varies; many are replaced with the pump. Replace any valve that leaks, sticks or slams.

Can I install a check valve horizontally?

Some are rated for horizontal use; others are not. Check the product markings.

Why is there water spraying from a small hole in my pipe?

It is probably the weep hole, which is normal while the pump runs.