Check valve for a pool: operation, selection, placement, and testing (complete guide)

It's a component that costs just a few euros, yet it is responsible for a surprising number of unpleasant surprises: a filter that empties into the pond every time the power goes out, dirty water sent back to the fish, a flooded air pump, or a surface pump losing its prime. A check valve allows water or air to flow in only one direction and closes as soon as the flow reverses: when chosen and installed correctly, it protects your system; when chosen incorrectly, it can also reduce your pump's flow rate. Here's everything you need to know.

How a check valve works

All check valves work on the same principle: a valve element, disc, flap, or ball is pushed off its seat by the flow when the water moves in the correct direction. As soon as the flow stops or reverses, the valve element returns to press against its seat, under the action of a spring, its own weight, or the pressure of the water flowing back down, and blocks the passage. No electricity, no adjustment: it is a purely mechanical component.

The downside: to open the valve, the flow must overcome the resistance of the valve element. This is called pressure loss. Pond pumps, however, are designed to move large volumes at low pressure. A check valve that is too small or has a spring that is too stiff can significantly reduce the flow rate of a pond pump, whereas this would go unnoticed on a borehole pump.

The different types of check valves

When is a check valve useful?

Filter placed higher than the pond, or pressure filter

When the pump stops, the water contained in the pipe and filter flows back down into the pond through the pump. With a pressure filter, this backflow can send some of the dirt trapped in the foam back toward the fish. A check valve on the pump discharge blocks this return flow and keeps the circuit full, which also makes restarting easier.

Surface pump on the suction side

A pump installed out of the water, higher than the pond level, loses its prime if its suction pipe empties. The foot valve, installed at the submerged end of the pipe, keeps the water column in place. Its strainer also protects the pump from leaves and small fish.

Air pump placed below the water level

If the compressor is below pond level, for example in an underground equipment room, water can rise through the air pipes when the compressor is stopped and flood the device. The first solution is to place the air pump higher than the water level. If this is impossible, an air check valve on each outlet is essential.

Several pumps on the same manifold

When two pumps discharge into the same pipe, a check valve behind each one prevents water pushed by one from flowing back out through the other when it is stopped.

Where to place the check valve

1. On the discharge side, near the pump: this is where it retains the greatest volume of full pipe and protects the pump.
2. Accessible: a check valve gets clogged. Plan for detachable fittings on both sides, or place it out of the water if possible.
3. In the correct direction: the arrow molded onto the body indicates the flow direction, from the pump to the filter or waterfall. If installed backwards, it blocks everything.
4. In the correct position: a swing check valve only works when installed horizontally, with the hinge on top, or on a vertical pipe with upward flow. A spring check valve can be installed in any position.
5. Never on the suction side of a submersible pump: the pump must draw freely; otherwise, it works too hard and overheats.

Choosing the right diameter

Choose the check valve based on the diameter of the discharge pipe, not the pump outlet diameter if it is smaller. A reduction followed by a check valve adds two head losses. For larger installations, prefer a swing check valve, which restricts flow less, and check the pump curve to ensure that the delivery head remains compatible.

How to test a check valve

A faulty valve is indicated by a filter that drains, a surface pump that loses its prime, or water in the air hose. To test it:

1. Disassemble it and rinse it with clean water.
2. Blow in the direction of the arrow: air should pass without excessive effort.
3. Blow in the opposite direction: nothing should pass through.
4. Water test: hold it vertically, with the arrow pointing upward, and fill it from the top. The water should remain in the valve without dripping.
5. Inspect the inside: filamentous algae, snail shells, gravel, or limescale deposits prevent the seal from seating properly. A weakened spring or hardened seal requires replacement.

Check your valves every spring restart and after each winter shutdown: limescale and frost are their two enemies.

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Frequently asked questions

— Does a check valve reduce flow?

A little, especially if it is spring-loaded or too small. Choose one that matches the pipe diameter and prefer a swing check valve for high flow rates.

— Which way should a check valve be installed?

The arrow engraved on the body must point in the direction of flow, from the pump toward the filter or waterfall.

— Can a valve be installed on the suction side of a submersible pump?

No. Only surface pumps use a foot valve on the suction side.

— How can I tell if my check valve is leaking?

Blow through it in both directions: air should pass only in the direction of the arrow. Filled with water, with the arrow pointing upward, it should not drip.

In summary

• The check valve allows flow in only one direction.
• It is installed on the discharge side, near the pump, with the arrow pointing in the direction of flow.
• Spring-loaded: all positions; swing check: lower pressure loss.
• Foot valve for a surface pump, air check valve for an air pump below the water level.
• Test it by blowing through it in both directions, and clean it every spring.

Last updated: September 28, 2026.