What Does a Sump Pump Check Valve Do?

A sump pump check valve is a one-way valve on the discharge pipe that lets water flow out of the pit and stops it from falling back in when the pump shuts off. Liberty Pumps' installation manual for its 230, 240, and 450 series sump pumps says plainly that "a check valve is required to prevent the backflow of liquid after each pumping cycle." Without one, the water you just pumped up the pipe drains right back into the pit, and the pump has to move it again.

Manufacturers also call for a tiny hole in the pipe below the check valve, and they do not agree on its size. Zoeller's instructions for its 1073 and 1075 sump pumps say to drill a 3/16 inch hole in the discharge pipe level with the top of the pump, while Liberty Pumps says the hole should be no more than 1/8 inch and drilled low on the pipe, just above the threaded connection to the pump. This post explains what the valve does, where it goes, why that hole matters, what the loud thud means, and how to tell when a valve is failing.

How Does a Sump Pump Check Valve Work?

Inside the valve is a flapper or disc that swings open when the pump pushes water up the pipe. When the pump stops, the column of water in the pipe starts to fall back. That backward push, helped by gravity or a spring, closes the flapper and holds the water in place.

Boshart, a check valve maker, describes what happens without a valve: the water pumped vertically "will all come back into the basin once the pump turns off." The pump then turns on again sooner than it should, which Boshart says "will reduce the pump's life, as well as waste energy cost." North Dakota State University Extension makes the same point, saying both pump types it covers should have a check valve on the discharge pipe "so water doesn't flow back into the sump when the pump shuts off."

If you are comparing pump styles, our breakdown of pedestal vs. submersible sump pumps for PA homes covers how each is set up.

Do I Need a Check Valve on My Sump Pump?

For a standard basement sump pump, yes. Liberty lists the check valve as required, and NDSU Extension says both pump types should have one. In Liberty's troubleshooting table, "check valve not installed, stuck open, or leaking" is listed as a cause when the pump cycles too frequently.

Short cycling is the main cost of skipping it. Every time water falls back, the pit refills faster, the pump starts more often, and the motor and switch wear sooner. On a wet spring night in Westmoreland or Allegheny County, that can add up to a lot of extra starts.

A few systems come with a valve built in. Zoeller's instructions for its 508 series battery backup pumps note the check valve is incorporated in the discharge of the backup pump. Read your own pump's manual before adding a second valve to a line that already has one.

Where Does the Check Valve Go on a Sump Pump?

The valve sits on the discharge pipe above the pump, usually on the vertical section coming out of the pit. Boshart's advice is to install it "as close to the floor as you can," and says anything under 2 feet above the floor is ideal. Keeping it low limits how much water sits between the pump and the valve.

Two other pieces often go with it:

• A union, so the pump can come out without cutting pipe. Zoeller's 1073 and 1075 instructions call for "rigid 1-1/2 in. discharge pipe, union, check valve according to code."

• A shutoff valve after the check valve. Liberty says a gate valve should follow the check valve "to allow periodic cleaning of the check valve or removal of the pump."

Which way does the arrow point?

Every check valve has an arrow on the body showing the direction of flow. It must point away from the pump, in the direction the water travels out of the house. Boshart warns that if the valve is not installed in the right direction, it will not open. Liberty's troubleshooting table lists "check valve stuck closed or installed backward" as a problem to look for when water is not moving.

Zoeller says its 1-1/2 inch Quiet Check Valve can be installed vertically or horizontally in discharge lines. Other valves may be limited to one position, so follow the instructions on the one you buy.

Why Is There a Small Hole Between the Pump and the Check Valve?

That hole, often called a weep hole, vent hole or relief hole, prevents air lock. An air lock happens when air gets trapped in the pump housing. The pump motor spins, but it cannot push water because it is pushing against a pocket of air under a closed check valve.

The fix is a small hole that lets the trapped air escape. Zoeller's instructions for its 400 series pumps say that if a check valve is installed, a vent hole of about 3/16 inch "must be drilled in the discharge pipe below the check valve and pit cover to purge the unit of trapped air." Zoeller adds that the hole must stay below the basin cover and be cleaned periodically.

Liberty takes a smaller approach. Its manual recommends a hole "no more than 1/8" diameter and drilled low on the pipe, just above the threaded connection to the pump discharge." Zoeller's instructions for its 508 series backup systems also call for a 1/8 inch air relief hole and say the weep hole must be visible.

A few practical notes:

• A little spray from the hole while the pump runs is normal. Liberty and Zoeller both say so.

• Keep it inside the pit, below the cover, so the spray lands back in the basin.

• Follow your pump's manual for the size and height, since the two makers differ.

• Check it now and then. A hole clogged with mineral or debris cannot do its job.

Why Does My Check Valve Make a Loud Thud When the Pump Stops?

That bang when the pump shuts off is usually the flapper slamming shut as the water column drops. Boshart calls this the loud "clunk" often heard from regular swing check valves and identifies it as water hammer. In a finished basement, or a bedroom above the pit, it can be loud enough to wake you.

The common fix is a quiet or silent check valve. Boshart explains that these have "a spring incorporated into the flapper which forces the valve closed slowly," which it says will eliminate the clunk. Zoeller, for one, sells a 1-1/2 inch Quiet Check Valve. A few other causes of noise to rule out:

• Loose pipe that knocks against the wall or pit edge.

• A worn flapper that chatters instead of sealing.

• A pump that cycles often, which makes every thud more noticeable.

What Are the Signs a Check Valve Is Failing?

A check valve that sticks open, leaks or cracks shows up in how the pump behaves. Watch for:

• The pump turning on and off in quick bursts, even when little water is coming in. Liberty lists a check valve that is stuck open or leaking as a cause of frequent cycling.

• Water rushing back into the pit right after the pump stops.

• The pump running at times when no water should be coming in.

• The pump running but not moving water, which Liberty links to a valve stuck closed or installed backward.

• Visible cracks, a leaking body, or water seeping from the rubber couplings.

Frequent cycling has other causes too, such as a float switch set wrong. Our troubleshooting guide for a sump pump that runs constantly walks through them, and our list of warning signs your sump pump needs replacement can help you decide whether the valve or the pump is the problem.

Replacing the check valve on a sump pump

Many replacement sump check valves connect with rubber couplings and hose clamps. The general order is to unplug the pump, drain what you can from the pipe, loosen the clamps, remove the old valve, and fit the new one with the arrow pointing away from the pump. Plug the pump back in, pour water into the pit, and watch at least one full cycle for leaks. If the valve is glued in place, or the pipe near it is cracked, the job becomes a pipe repair.

FAQ

Can a sump pump work without a check valve?

It will still pump water, but much of that water drains back into the pit after each cycle. The pump then runs more often, and Liberty Pumps treats the valve as required on its sump pumps.

How big should the relief hole be?

It depends on your pump's manual. Zoeller's residential instructions call for 3/16 inch, level with the top of the pump, while Liberty says no more than 1/8 inch, just above the pump discharge connection.

Why does water spray out of a hole in my sump pump pipe?

That is the air relief hole doing its job. Liberty and Zoeller both say a small spray while the pump runs is normal, as long as it stays inside the pit.

Are quiet check valves worth it?

If the thud bothers you, a spring-loaded quiet check valve is the usual fix. Boshart says the spring closes the flapper slowly and eliminates the clunk of a standard swing valve.

Does a check valve affect how I choose a pump?

Pump size, pipe size and valve size should match. Our guide on how to choose the best sump pump for your basement covers the other features to weigh.

How high above the pump should the check valve be?

Boshart recommends placing it as close to the floor as you can, ideally less than 2 feet above it. Always keep the relief hole between the pump and the valve.

A Better Choice, Inc. Can Check Your Sump Setup

A Better Choice, Inc. has helped homeowners across the Greater Pittsburgh area since 1996 and fixes wet basements in Westmoreland, Allegheny, Butler and neighboring counties. If your pump short cycles, bangs, or cannot keep up, schedule a free inspection and the team will look at how your basement waterproofing system is moving water out of the house.

Visit A Better Choice, Inc. or call 1-800-343-2357 today. Contact us for more information.