Toilet constantly running: the 2 possible causes & how to fix

Toilet constantly running? There are only two possible causes: a faulty ball valve or a faulty flush valve. A faulty ball valve may also result in the toilet overflowing. Either way, a fault with either of these parts will result in the toilet constantly filling. This article will help you understand what’s causing the problem and how it can be fixed.

Why is my toilet constantly running?

The answer is very simple: water is entering or leaving the cistern when it shouldn’t. The cause is either a problem with the valve that controls the flow of water into the cistern (the ball valve or float valve), or a problem with the flush valve – the valve which allows water to flow into the pan. The same goes if you have a concealed toilet cistern constantly filling.

Common ball valve problems

  • Washers and valve seatings become worn and no longer form a watertight seal.
  • Floats can become waterlogged or detached from the float arm.
  • The valve can become jammed in an open position.

While it is possible to replace washers and valve seatings, it may be more convenient and cost-effective to simply replace the whole ball valve.

If you have a siphon instead of a flush valve, the problem almost certainly lies with the ball valve. That’s because, in a toilet with a siphon, the water would have to rise above the siphon’s spillover point in order to exit the cistern and flow into the pan. This isn’t possible because the water will meet the overflow before it can leave via the siphon.

So, if the problem isn’t caused by water leaving via the siphon when it shouldn’t, it’s caused by water entering the cistern – and that points to the ball valve. This typically results in the toilet overflowing, and you’ll probably see water dripping or running out of the overflow pipe that sticks out of the wall somewhere close to the toilet or the bathroom.

What’s the difference between a siphon and a flush valve?

In a siphon, a lever raises a body of water above a spillover point at the top, and the water is siphoned out of the cistern into the pan. In a flush valve, there is no siphon effect – the valve is opened and water simply exits the cistern into the bowl via gravity.

  • Toilets with a siphon usually have a lever.
  • Toilets with a flush valve are usually operated via a button.

Toilet constantly filling – how do I know if it’s the ball valve or the flush valve?

The difficulty understanding which part is faulty is due to the fact that many modern toilets have an internal overflow, meaning that a faulty ball valve or a leaking flush valve will both cause water in the cistern to run into the bowl.

Water constantly running in the toilet bowl may not always be obvious, but there are easy ways to confirm it:

  • Put some water-based food dye into the toilet cistern. Any water running into the pan should easily show up against the white porcelain.
  • Do not flush the toilet for at least an hour. Fold up a few sheets of toilet paper and press it to the back of the pan. If it gets wet, you have a leak.

The most effective way to determine the faulty part is to shut off the water supply to the toilet cistern. You can do this via the isolation valve on the supply pipe, or by turning off the mains or the supply from the cold water storage cistern. 

  • If after 30-60 minutes, the leak stops, you have confirmed that the ball valve/float valve is the problem.
  • If the water continues to leak into the pan after this time, the flush valve is the problem.

Faulty flush valve

If the flush valve is the issue, the fault is almost certainly due to the lack of a watertight seal via the main washer. Replacing the washer or even simply refitting it upside-down should solve the problem.

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