Why Your Toilet Keeps Running and How to Fix It

The flapper test

Take the tank lid off and watch the flapper, the rubber seal at the bottom of the tank that lifts when you flush and drops back to seal it.

1. Flush once and watch the flapper drop back into place.
2. Look at the seal itself for warping, cracking, or mineral buildup around the edge where it meets the valve seat.
3. Add a few drops of food colouring to the tank water and wait fifteen minutes without flushing.
4. Check the bowl. If the colour has crept into it, water is bypassing the flapper seal even though the tank looked fine.

Replacing a worn flapper

A flapper is one of the cheapest parts in the house to replace, and the swap does not need the water shut off for more than a minute.

1. Shut off the water supply at the valve behind the toilet and flush to empty most of the tank.
2. Unhook the flapper from the two pegs or the ring at the base of the overflow tube, and disconnect the chain from the flush lever.
3. Take the old flapper to the hardware store, or note the toilet brand, to match the replacement exactly, since flapper sizes are not fully universal.
4. Clip the new flapper onto the pegs, connect the chain with a small amount of slack, and turn the water back on.
5. Flush and watch the flapper seat fully before considering the job done.

The fill valve

If the flapper looks fine and the food colouring test comes back clean, the sound is more likely coming from the fill valve, the taller mechanism on the left side of the tank that refills after a flush. A worn fill valve can run constantly at a low hiss, or cycle on and off with nobody near the toilet. Fill valves are a single unit that unscrews from the bottom of the tank once the water is shut off and the supply line disconnected, and a new one is a straightforward swap onto the same fitting, typically finished by clipping the refill tube into the overflow tube and adjusting the float to the correct height.

Checking the water level

Sometimes both parts are fine and the problem is simply that the water level is set too high, so it constantly trickles into the overflow tube in the centre of the tank.

1. Look for the waterline inside the tank relative to the top of the overflow tube.
2. The correct level sits about 2.5 centimetres below the top of that tube, and is often marked on the inside of the tank wall.
3. Adjust the float, either the screw on top of the fill valve or the clip on a float arm, down slightly if the water sits above that mark.
4. Flush and recheck, adjusting again in small increments rather than all at once.

When it is not any of these

If the tank is not overfilling, the flapper seals properly in the food colouring test, and you can still hear running, check the supply line into the tank for a slow drip you cannot see easily from above, and check whether the toilet itself is cracked somewhere below the waterline, which is rare but does happen on older units. That is the point where it is worth a second pair of eyes rather than swapping parts at random.