Why a toilet runs, and how to make it stop

The three parts that matter

Everything a toilet does happens in the tank, and the tank contains fewer parts than most people expect. Lifting the lid is the whole diagnostic method, so it helps to know what you are looking at.

Flapper

The rubber seal at the bottom of the tank, connected to the handle by a chain. Flushing lifts it, releasing the tank water into the bowl, and it drops back to reseal. It is the part that wears out most often, and a worn one leaks tank water into the bowl continuously.

Fill valve

The vertical assembly on the left side, connected to the supply line. It refills the tank after a flush and shuts off when the float tells it the tank is full. When it fails, it either never quite shuts off or cycles on and off by itself.

Float

The part that senses the water level, either a ball on an arm or a cup that rides up the fill valve shaft. Its height setting decides where the water stops.

Overflow tube

The open vertical pipe in the middle of the tank. It exists so an overfilling tank drains harmlessly into the bowl instead of onto the floor. Water trickling into it is a symptom worth recognizing, because it means the fill level is set too high or the valve is not shutting off.

The one-minute diagnosis

Take the lid off, set it somewhere safe since the ceramic cracks easily, and look and listen.

1. Look at the overflow tube

   Water spilling into the top of it means the tank is overfilling. That is a fill valve or float height problem, and the flapper is innocent.

2. If the tube is dry, watch the water surface

   A water level that slowly drops, followed by the fill valve kicking in to top it up, means water is escaping past the flapper into the bowl. This on and off refilling is often called ghost flushing.

3. Confirm the flapper with food coloring

   Put a few drops in the tank, do not flush, and wait fifteen minutes. Color appearing in the bowl proves the flapper is passing water. The same test works even when the leak is silent.

4. Press down gently on the flapper with a stick

   If the running sound stops while you hold it down, the seal is the problem. If it continues, look at the chain and the fill valve instead.

5. Check the chain slack

   A chain with no slack holds the flapper slightly open, and one with far too much can tangle under it. About half an inch of slack with the flapper seated is right.

Fixing a flapper leak

Flappers are rubber sitting permanently in water, and they simply age. They warp, stiffen, or grow a mineral film that stops them seating cleanly. In-tank cleaning tablets, particularly chlorine ones, accelerate all of this considerably and are a common reason a flapper fails years early.

Replacement is a genuinely simple job: shut the supply valve behind the toilet, flush to empty the tank, unhook the old flapper from the overflow tube pegs and the chain, and fit the new one. The one detail that matters is matching the part, since flappers come in two-inch and three-inch sizes and several mounting styles. Taking the old one to the store answers every question at once.

While the tank is empty, run a finger around the seat the flapper rests on. A rough or pitted seat will defeat a brand new flapper, and mineral deposits on it clean off with a scouring pad.

Fill valve and water level problems

The correct water level is about an inch below the top of the overflow tube, and most tanks have a molded line marking it. Water above that line, or trickling into the tube, means the shut-off point is wrong or the valve is not shutting off at all.

In order of likelihood:

• The float is set too high. Cup floats adjust with a screw or a pinch clip on the shaft, ball floats by a screw at the valve or by carefully bending the arm down.
• The valve is worn out. A fill valve that hisses continuously, shuts off only sometimes, or cycles at random has failed internally. They are inexpensive and replaced rather than repaired.
• Debris is caught in the valve. Work on the supply line or a water main disturbance can send grit into the valve seat. Some models can be flushed clean by removing the cap and running the supply briefly.

What running actually costs

A toilet running at full flow moves several gallons a minute, and even a modest flapper leak loses tens of gallons a day. The EPA puts a constantly running toilet at around 200 gallons a day, which on a metered connection is a visible line item within a single billing cycle.

It also hides other problems. A seeping toilet shows up on the water meter leak test exactly the way a supply leak does, so ruling the toilets out first is standard practice before anyone goes looking for something harder to find.

When it stops being a toilet problem

Worth a professional visit rather than another part swap:

• A new flapper and a new fill valve have not stopped the running
• The toilet runs on and off and the dye test stays clean, which points at pressure or a leak elsewhere
• Water appears around the base of the toilet when it is flushed, which is a wax ring rather than a tank fault
• The shut-off valve behind the toilet will not close or weeps when used, which is worth fixing before it is needed in an emergency
• The same toilet needs parts repeatedly, where high water pressure is often the underlying cause