Toilet Cistern Running Continuously: How to Replace the Flush Valve and Fill Mechanism
A continuously running toilet cistern can silently waste over 200 liters of potable water every single day, adding €30 to €60 to your monthly water bill. In 95% of cases, the fault is caused by a hardened silicone flapper seal or an improperly adjusted inlet float valve. Here is how to fix it in 15 minutes.
Step 1: Identify Which Component Is Failing
Remove the ceramic cistern lid (unscrew the dual-flush push button first) and observe the water level inside:
Case A (Inlet Float Valve Fault): Water level rises above the center overflow tube and spills continuously down the middle. The inlet float is set too high or its internal diaphragm is blocked with limescale.
Case B (Flush Mechanism Seal Fault): Water level remains below the overflow rim, but a steady trickle continues running into the toilet bowl. The bottom silicone base seal on the flush valve is warped, slimy, or encrusted with calcium deposits.
How to Fix the Flush Valve Base Seal
1. Turn off the small water isolation valve behind the toilet and flush to empty the tank.
2. Twist the central flush mechanism counterclockwise a quarter-turn and lift it out of the cistern bayonet base.
3. Remove the circular translucent silicone washer from the bottom of the mechanism.
4. If covered in black slime or limescale, wash it or replace it with a new universal €2-€4 silicone seal, clip the mechanism back in, and test.
How to Adjust or Replace the Inlet Float Valve
Adjust the plastic screw on the float arm to lower the shut-off water level by 2 to 3 cm below the overflow tube. If water fails to shut off completely when lifting the float manually, replace the complete side or bottom-entry inlet valve (€12-€20).
FAQ
Why do cistern rubber seals degrade so quickly?
Chlorine in tap water and aggressive bleach cistern blocks harden and warp silicone and rubber washers over 3 to 5 years, preventing an airtight seal.
How much does a plumber charge to replace a toilet cistern mechanism?
A complete overhaul including a new dual-flush valve, silent compact inlet float, and labor typically costs between €65 and €110.