Check Valve

A check valve (also called a non-return valve) is a simple device that allows water or other fluids to flow in one direction and blocks flow in the opposite direction. Inside, a moving part—such as a flap, spring-loaded disc, or ball—opens with forward flow and closes if flow tries to reverse. This helps prevent backflow, protects pumps and heaters, and keeps plumbing systems working as designed.

How a Check Valve Works

Most check valves operate automatically using pressure and flow. When fluid moves the correct way, it pushes the internal mechanism open. If pressure drops or flow reverses, the mechanism closes against a seat, stopping backward movement. Two common concepts you’ll hear:

Cracking pressure: The minimum pressure needed to start opening the valve. Lower cracking pressure is helpful for small pumps and low-flow lines.
Backpressure: Pressure pushing the wrong way. A check valve must seal under backpressure to stop reverse flow.

Body material and connection type vary by use, but all check valves include an arrow on the body showing the direction of intended flow—follow it during installation.

Common Home Uses and Applications

Sump pumps: Installed on the discharge line above the pump to keep the discharged water from draining back into the pit when the pump shuts off. Reduces short-cycling.
Well systems: A foot valve (a check valve with a strainer) at the end of the suction line prevents the line from losing prime. Additional in-line checks may be used near the pressure tank.
Irrigation: Prevents zones from draining backward and helps maintain pressure in drip systems. Some sprinkler heads include built-in checks to stop low-head drainage.
Water heaters and recirculation loops: Prevents hot water from migrating into the cold line or flowing backward through the recirculation pump when it’s off.
Hydronic heating (boilers/baseboards): Stops unwanted gravity circulation and keeps zones from interfering with each other.
Condensate pumps and appliances: Keeps condensate or appliance discharge from running back into the unit.

Note: A single check valve is not a substitute for a code-required backflow preventer where cross-connection hazards exist (for example, lawn irrigation tied to potable water). Follow local codes.

Types of Check Valves

Swing check: A hinged flap swings open with forward flow and closes by gravity or backpressure. Common in horizontal lines. Simple design, low pressure drop. Not ideal for vertical upflow unless rated for it.
Spring (in-line) check: A spring-loaded disc opens with forward pressure and snaps closed when flow stops, reducing water hammer. Works well in vertical or horizontal lines. Often called “silent” check valves.
Ball check: A ball moves off a seat under forward flow and reseats to block reverse flow. Tolerates some debris; used in sewage ejector and sump lines.
Diaphragm check: A flexible diaphragm admits forward flow and seals in reverse. Good for low-flow applications and where quiet operation is desired.
Foot valve: A check valve with an inlet strainer, installed at the end of a suction line (wells, cisterns) to prevent loss of prime and keep debris out.
Double-check/backflow assemblies: Multi-valve devices designed to protect potable water; different from basic single check valves and often require testing by a certified technician.

Materials and Connections

Materials: PVC/CPVC (cold/hot potable water), brass (domestic water, heating), stainless steel (corrosive or high-pressure), ABS (some drainage/sump uses). Match material to fluid, temperature, and pressure.
Connections: Threaded (NPT), solvent-weld (slip for PVC/CPVC), sweat/solder (copper), press-fit, or push-to-connect. Use unions or couplings to allow future service.

Selecting the Right Check Valve

Size: Match the pipe size; avoid “necking down” unless the manufacturer specifies minimal pressure loss.
Orientation: Choose a type rated for your installation position (horizontal vs vertical). Spring checks typically suit more orientations than swing checks.
Cracking pressure: For small pumps (sump, recirculation), pick a low cracking pressure to avoid extra load and short-cycling.
Flow characteristics: If noise or water hammer is a concern, choose a spring or “silent” check with a soft seat.
Debris handling: For dirty water (sump/sewage), use a ball check or a clear-body check rated for solids; consider a union-style body for easy cleaning.
Temperature/pressure rating: Verify the valve’s rating matches the system. CPVC or brass for hot water; PVC for cold water.

Installation Tips

Follow the flow arrow on the valve body.
Place sump pump check valves close to the pump discharge to minimize the volume of water that can drain back.
Support the piping; don’t let the valve carry the line’s weight.
For threaded connections, use the correct thread sealant or PTFE tape; don’t overtighten.
On sump pumps, drill an anti-airlock weep hole if the pump manufacturer specifies it (commonly a small hole in the discharge pipe below the valve).
Use unions or shielded couplings so you can remove the valve for service.
Purge air after installation and check for leaks.

Maintenance

Periodic checks: Annually test critical valves by observing system behavior (e.g., sump pump cycle time) and listening for chatter or slamming.
Cleaning: For valves in dirty water, remove and rinse the internal parts. Clear-body models make inspection easier.
Replacement: If the valve chatters, leaks back, or sticks, replacement is often faster than repair, especially for inexpensive types.

Common Mistakes to Avoid

Installing backward: Always confirm the arrow matches the intended flow.
Wrong type for orientation: A swing check mounted vertically may not close reliably unless rated for vertical use.
Ignoring cracking pressure: A stiff spring can reduce flow or strain a small pump.
Wrong material: PVC on hot water, or non-rated plastics near boilers, can deform or fail.
No service access: Burying a check valve in a wall or yard without unions makes maintenance difficult.
Relying on a single check for potable protection: Use a code-approved backflow assembly where required.

Related Terms

Backflow preventer: A device (often with two checks and a relief valve) that protects drinking water. Different purpose and higher protection than a basic check.
Vacuum breaker/AVB: Prevents back-siphonage by admitting air; often used on hose bibbs and irrigation.
Pressure relief valve (PRV): Opens to release excess pressure; not directional like a check valve.
Water hammer: Shock waves from sudden flow changes; spring or silent checks can reduce it.

Practical Examples

Sump pump discharge: Install a rubber-coupled check valve on the vertical discharge. Result: water doesn’t fall back into the pit, reducing pump starts.
Well suction line: A foot valve at the bottom of the drop pipe keeps the system primed and screens out debris.
Hot water recirculation: A small spring check on the return line stops hot water from flowing backward and heating cold lines.
Boiler zone protection: A flow-check valve on each loop prevents heat from migrating when the circulator is off.
Irrigation main: A check valve at the pump discharge holds pressure and prevents zones from draining into the pump when it stops.

By choosing the right type, matching materials to your system, and installing with serviceability in mind, a check valve can quietly protect equipment, reduce nuisance cycling, and keep your home’s plumbing and mechanical systems running smoothly.