The flapper valve
The most common cause of a continually running toilet is a worn flapper valve. This is the rubber seal at the bottom of the tank which can wear down and dissolve over time. To check if this is the problem:
- Lift the valve and run your fingers around it. If you can feel grit and limescale it will need cleaning. If it feels distorted or damaged it will need replacing.
- Look at the seat below the flapper valve. If it is scored and worn then the entire flush valve system will need replacing.
The chain
If the chain connecting the arm lever to the rubber flapper valve is too short then the valve will always be slightly open, letting water leak through. If this is the problem, you'll need to replace the chain with a longer one.
Float ball
Check the float ball. If it is positioned too high it will allow water to continually spill into the overflow pipe. To fix the problem:
- Reposition it by gently bending the arm.
- Make sure it is not touching the side of the tank or bent too low.
- If the float ball is damaged and letting in water, it will need replacing.
- If the float ball is positioned correctly but the water level seems too high then the problem is with the ballcock.
- Check the ballcock washer as this may be worn. If so, lift it out and replace – or if necessary replace the entire ballcock mechanism.