What a flush valve is and why it fails

The flush valve is the mechanism inside your toilet tank that releases water into the bowl when you push the handle. It sits at the bottom center of the tank and consists of a rubber seal, a valve body, and a stem. When it wears out — usually after 5 to 10 years — water leaks slowly into the bowl even when nobody has flushed, your water bill climbs, and the toilet runs constantly.

Replacing it yourself costs $15 to $40 for the part and takes about an hour. A plumber charges $150 to $300 for the same job. The valve is the most common reason a toilet wastes water, so if your tank is refilling on its own, this is almost certainly the culprit.

Key Takeaways

  • Turn off the water supply at the shutoff valve behind or below the toilet, then flush to empty the tank completely.
  • Flush valves come in different sizes — measure the opening at the bottom of your tank or bring the old valve to the hardware store to match it.
  • You will need a large adjustable wrench or a flush valve socket wrench, a screwdriver, and a bucket to catch water.
  • The entire job involves draining the tank, unbolting the old valve from underneath, and installing the new one with a rubber gasket and locknut.
  • Test for leaks by letting the tank refill and watching for water trickling into the bowl after five minutes of sitting still.

Gather your tools and materials

Before you start, collect what you need. You will need an adjustable wrench (or a flush valve socket wrench, which grips the valve more securely), a screwdriver (usually Phillips head), a bucket to catch water, and towels or rags. Have a flashlight or headlamp ready — the space under the tank is dim.

Buy a replacement flush valve that matches your tank size. Flush valves come in 2-inch and 3-inch diameters. The easiest way to know which you need is to unscrew the old valve partway, measure the opening, or take a photo and show it to the hardware store staff. Many stores stock universal kits that include the valve, gasket, and locknut together, which is simpler than buying parts separately.

Turn off water and drain the tank

Locate the shutoff valve on the water line behind or below the toilet. Turn it clockwise until it stops — you should feel resistance but do not force it. Flush the toilet once. The tank will empty and the water will not refill.

If water is still trickling in, the shutoff valve may be faulty. Turn it as far as it will go, then place a bucket under the tank and continue with the next step. You will catch any remaining water when you disconnect the supply line.

Remove the old flush valve

Look underneath the tank where the flush valve connects. You will see a large nut (the locknut) holding the valve in place from below. Place your bucket underneath to catch any water still in the tank. Use your wrench to turn the locknut counterclockwise. It may be tight — explore steady pressure rather than jerking.

Once the locknut is loose, reach inside the tank and unscrew the valve body itself by turning it counterclockwise with your hand or screwdriver. Pull the old valve out through the bottom of the tank. You may see a rubber gasket stuck to the tank opening — scrape it off with your screwdriver and discard it. Wipe the opening clean with a rag.

Install the new flush valve

Slide the new valve up through the opening from underneath the tank. The rubber gasket should sit on top of the valve stem, between the valve and the tank. Push the valve up until the gasket seats firmly against the tank opening.

From underneath, thread the locknut onto the valve stem by hand first — do not use the wrench yet. Once it is hand-tight, use your wrench to turn it clockwise another quarter turn. Do not over-tighten; the gasket will compress and seal properly with moderate pressure. If you crank too hard, you risk cracking the tank.

Reconnect water and test for leaks

Turn the shutoff valve counterclockwise to restore water flow. The tank will refill. Listen for the fill valve to stop — this usually takes 30 seconds to two minutes. Once the tank is full, wait five minutes without flushing. Watch the water level in the bowl. If it stays steady, the valve is working. If water is trickling into the bowl, the gasket did not seal and you need to tighten the locknut slightly more.

Flush once to confirm the valve releases water smoothly and the tank refills normally. Check underneath the tank for drips. A small amount of moisture is normal during the first flush, but standing water or a steady drip means the gasket is not seated. If that happens, turn off the water, loosen the locknut a quarter turn, and tighten it again.

Frequently Asked Questions

Can I replace a flush valve without turning off the water?

No. Without shutting off the water, the tank will refill as you work and you will be soaked. The shutoff valve is always located on the supply line near the base of the toilet. If it will not turn, call a plumber — a stuck shutoff valve usually means it needs replacement too.

What if the locknut is stuck and will not turn?

explore penetrating oil (like WD-40) around the nut and let it sit for 15 minutes. Try again with steady pressure on the wrench. If it still will not budge, tap the wrench handle gently with a hammer to break the corrosion. Do not hit hard — you can crack the tank. A plumber has better tools for this situation.

How do I know if I bought the right size valve?

The valve should slide into the opening at the bottom of your tank with minimal resistance. If it is too large, it will not fit. If it is too small, the gasket will not seal. Most hardware stores will let you return an unused valve, so test-fit it before you leave the store if possible.

Should I replace the fill valve at the same time?

Only if it is already leaking or running constantly. The fill valve (on the side of the tank) and the flush valve (at the bottom center) are separate parts. If your toilet runs only when nobody has flushed, the flush valve is the problem. If it runs after you flush, the fill valve may be the issue.