What a check valve does and why it fails

A check valve is a one-way gate in your well pump plumbing that lets water flow up into your pressure tank but stops it from flowing backward down the well when the pump shuts off. When it fails, water drains back down the well line overnight, and your pump has to refill the entire line before it can build pressure again — you hear the pump cycle on and off even when nobody is using water, and it takes longer to get pressure at the tap.

Check valves fail because sediment, mineral buildup, or debris gets stuck in the flapper or ball inside, preventing it from sealing. Sometimes the internal spring weakens after years of opening and closing. A failed check valve is one of the most common reasons a well pump short-cycles — turns on and off in quick bursts instead of running steadily.

Replacing one is a job most homeowners can do with basic tools in an afternoon, though you will need to turn off power to the pump and drain the line. The valve itself costs between $30 and $150 depending on the size and type.

Key Takeaways

  • A check valve prevents water from flowing backward down your well when the pump stops, and a failed one causes the pump to short-cycle and lose pressure overnight.
  • You must turn off power to the pump at the breaker and drain the pressure tank before opening any fittings to avoid injury from pressurized water.
  • The check valve is usually located on the discharge line between the pump and the pressure tank, installed vertically with the flow arrow pointing upward.
  • Replacement involves unscrewing the old valve, cleaning the threads, screwing in the new one, and refilling the line while watching for leaks at the connection.
  • If the pump still short-cycles after replacement, the problem may be a failed pressure switch or a leak in the tank itself, not the check valve.

Locate the check valve on your pump setup

The check valve sits on the discharge line — the pipe that carries water from the pump up toward your pressure tank. On most installations, it is mounted vertically between the pump and the tank, with an arrow stamped or printed on the body showing the direction water should flow (always pointing upward toward the tank).

If your pump is in a basement or pump house, follow the pipe coming out of the top of the pump. The check valve is usually the first fitting you encounter, often threaded directly onto the pump outlet or connected with a union fitting. On some older systems, the check valve may be inside the pressure tank itself, but that is less common in residential wells.

Look for a cylindrical or ball-shaped brass or steel body about 2 to 4 inches long. If you are unsure, trace the line from the pump upward and look for any fitting that has a flow direction arrow or the words "check valve" stamped into it.

Turn off power and drain the system safely

Before you touch any fitting, switch off the pump at the circuit breaker. Do not rely on a switch at the pump itself — go to the main electrical panel and flip the breaker to the pump circuit to OFF. This prevents the pump from starting unexpectedly while you are working and pressurizing the line.

Next, open a faucet or hose bib on the lowest point of your plumbing system to drain water from the pressure tank and discharge line. If you have a drain valve at the base of the pressure tank, open that instead — it drains faster. Let water run until it stops flowing, which means the line is depressurized. This step is critical: opening a fitting on a pressurized line can spray water forcefully and cause injury.

Once the line is drained, you can safely unscrew the check valve. If water still trickles out when you loosen the fitting, the line was not fully drained — close the fitting, open the drain valve again, and wait longer.

Remove the old check valve and clean the connection

Use an adjustable wrench or pipe wrench to hold the fitting or union below the check valve steady while you unscrew the valve itself with another wrench. Turn counterclockwise. If the valve is stuck, explore penetrating oil (like WD-40) and wait 15 minutes before trying again — do not force it, because you can crack the brass or strip the threads.

Once the valve is off, inspect the threads on the pump outlet or fitting where it was connected. Wipe away any debris, mineral deposits, or old thread sealant with a clean rag. If the threads are corroded or damaged, you may need to replace the entire fitting, not just the valve.

Wrap the threads of the new check valve with plumber's thread seal tape (also called PTFE or Teflon tape) — wrap it clockwise around the male threads three to four times. This creates a watertight seal and makes the valve easier to remove later if needed. Do not use pipe dope or thread sealant compound on the check valve itself; tape is sufficient and cleaner.

Install the new check valve and refill the line

Screw the new check valve onto the threads by hand first, turning clockwise until it is snug. Then use a wrench to tighten it another quarter to half turn — do not over-tighten, because you can crack the valve body or strip the threads. The valve should be oriented vertically with the flow arrow pointing upward toward the tank.

Once the valve is installed, close the drain valve at the pressure tank and turn the pump back on at the breaker. Water will flow back up the discharge line and refill the pressure tank. Watch the connection where you installed the new valve for any drips or sprays. If water leaks, turn the pump off, drain the line again, and tighten the connection a bit more with the wrench.

Let the pump run until it builds pressure and shuts off on its own — you will hear the pressure switch click. This usually takes a few minutes. Once the pump shuts off, listen for any hissing or dripping at the check valve. If it is dry and quiet, the installation is complete.

Test the pump and watch for short-cycling

Over the next 24 hours, observe whether the pump still short-cycles. Turn off all water use and listen to the pump. A healthy pump should stay off for at least 30 minutes to an hour before the pressure drops enough to trigger it to run again. If the pump turns on and off every few minutes even when nobody is using water, the check valve may not have been the problem.

Short-cycling can also be caused by a failed pressure switch (the device that tells the pump when to start and stop), a leak in the pressure tank, or a waterlogged tank with no air charge. If replacing the check valve did not solve the problem, those are the next things to investigate.

If the pump now runs normally and holds pressure overnight, the check valve replacement was successful. You should not hear the pump cycling on and off at night, and you should have normal water pressure at all taps.

Frequently Asked Questions

Can I replace the check valve without draining the whole system?

No. The discharge line between the pump and tank stays pressurized even when the pump is off, because water is trapped above the check valve. Opening any fitting without draining first will release that pressure suddenly and can cause injury. Always drain the line first.

What size check valve do I need?

The check valve must match the diameter of the pipe it connects to — usually 1 inch or 1.25 inches for residential wells. Look at the pump outlet or the fitting the old valve was screwed onto and measure the diameter, or take the old valve to a plumbing supply store and ask for the same size.

How do I know if the check valve is actually bad?

The most reliable sign is short-cycling: the pump turns on and off every few minutes even when water is not being used. You may also notice that it takes longer than usual to get water pressure at a tap after the pump has been off for a while. If the pump runs normally and holds pressure, the check valve is probably fine.

Can I install the check valve horizontally instead of vertically?

No. Check valves must be installed vertically with the flow arrow pointing upward. Installing one horizontally or upside down prevents it from sealing properly and defeats its purpose. Always orient it the way the arrow indicates.

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

Turn off the pump, drain the line again, and tighten the connection with a wrench — usually another quarter turn is enough. If it still leaks, remove the valve, add more thread seal tape, and reinstall it. If leaking continues, the threads may be damaged and you may need a new fitting.