How to Stop a Running Toilet and Prevent Overflow

Flushing toilet bowl landscape
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A constantly running toilet is a common problem in many homes, not to mention a major annoyance and a significant waster of fresh water. Fortunately, addressing this issue is simple and will help you reduce your water usage. Start with a simple lesson on toilet mechanics.

How Your Toilet Works

A basic understanding of the mechanisms in your toilet and how they work will help you make the necessary repairs.

  • When you press the handle, a chain lifts a flap (called a flapper) that allows the water in your tank to flow into the bowl. As the tank empties, the flapper drops and closes the opening to begin the refill cycle.
  • A large plastic float drops as the water empties out of the tank. The float is connected to a float valve that opens when the float is down and closes when the tank is full and the float is up.
  • An overflow tube in the center of the tank drains excess water into the bowl if the level in the tank gets too high. This tube is also the channel through which the float valve sends water to the bowl during the refill cycle.

A constantly running toilet, or a toilet that tops up its tank by itself, can stem from a variety of issues, including a faulty flapper, a high water level, or a water-logged float. If none of these seem to be the problem, you probably have a broken valve. The best solution is simply to replace it.

Checking for a Faulty Flapper or Chain

Your toilet’s flapper is a plastic or rubber cap that keeps water in your tank. Over time, your flapper can become brittle and create a faulty seal. If your tank doesn’t refill or hold water, it’s likely due to a sub-par flapper. Here’s how to troubleshoot your flapper:

  1. Check the flapper’s consistency. In many cases, flappers become hard and stop creating an adequate seal. Feel your flapper to ensure it’s still soft and able to hold back water.
  2. Check the chain. Make sure the chain connected to your flapper isn’t catching on anything. If you have an overly long chain (with lots of extra hanging down), trim off some excess to prevent tangles. If the chain is rusty or isn't designed for this use, replace it with a new flapper chain.
  3. Check for jams. Flappers can become pinched in their hinges. Look for a side of the flapper that’s jammed and preventing a full seal.
  4. Check for alignment. Flappers can become dislodged and cause leaks. Make sure your flapper is seated directly over the drain.

Adjusting the Water Level

Your toilet’s overflow tube ensures that the tank doesn’t overfill and flood your bathroom. But if the fill valve's float is set too high, small amounts of water will leak into the overflow tube and into the bowl, resulting in the fill valve turning on automatically and topping up the tank periodically.

  1. Lower the float setting on the fill valve. Some valves have a metal rod and a small clip that you squeeze to slide the clip (and float) up and down on the rod; move the clip down to lower the water level. If you have an old fill valve with a long rod and a tank ball, carefully bend the rod in the middle so the ball goes a little deeper into the tank.
  2. Flush the toilet and let it refill and stop by itself.
  3. Check the water level. It should be about 1/2 to 1 inch below the top of the overflow tube.
  4. Adjust the float more, if necessary, and flush again, until the refill stops at the proper level.

Checking for a Water-Logged Float

Water-logged floats can also cause constant running. If water is trapped inside your float, it will sit lower in the water and leave your float valve partially open. Check your float by unscrewing it (if applicable) and shaking it. Replace the float if you hear water inside. 

Replacing a Fill Valve

Your fill valve controls the flow of water into your toilet’s tank. A broken valve may fail to shut off or may shut off inconsistently, leading to non-stop or unintended refill cycles. Submerged float valves are the most common sign of a faulty fill valve. Because fill valves are inexpensive and easy to replace, it's better to replace an old fill valve than to attempt a repair.

  1. Turn off the water supply by closing the shutoff valve on the pipe leading to the toilet. Remove the tank lid. Flush the toilet to empty the tank.
  2. Place a plastic bin or bucket below the water supply hose. Disconnect the supply hose from the bottom of the fill valve.
  3. Remove the old fill valve by unscrewing the locking nut on the bottom of the water supply shank and pulling the entire valve assembly out of the tank. Residual water in the tank will drain into the bin or bucket.
  4. Adjust the height of the new valve to fit the height of the tank, following the manufacturer's directions. Insert the threaded end of the valve into the hole in the tank, and secure it with the locking nut, tightening by hand. Use pliers to turn the nut a bit further to create a watertight seal, but be careful not to over-tighten.
  5. Attach the supply hose to the fill valve and tighten it snugly (not too tight) with pliers. Clip the valve's refill hose onto the top of the overflow tube so the hose points down into the tube. Avoid placing the tube below your water line. This will lead to constant refilling. Most valves come with a clip or angle adapter that ensures the tube is at the right angle.
  1. Turn on the water at the shutoff valve, and let the tank refill. Fine-tune the water level, as needed, so the water stops about 1 inch below the top of the overflow tube, or as specified by the manufacturer. Replace the toilet tank’s lid.