A toilet that trickles into the bowl or refills constantly wastes water and can raise your bill if you have a water meter. Most faults come down to the fill valve, the float setting or a worn seal on the flush valve. This guide helps you diagnose the fault and replace the part.
Step by step
Diagnose the fault
Lift the cistern lid and watch a full flush and refill. If water overflows into the overflow pipe, the fill valve is not shutting off. If water trickles into the bowl constantly, the flush valve seal is usually worn. If the flush is weak, check the chain, cable or button mechanism.
Adjust the float first
Many problems are solved by lowering the float so the water stops below the overflow level. Most modern fill valves have an adjustment screw or clip; follow the marked water line inside the cistern.
Isolate the water and empty the cistern
Turn off the isolation valve on the supply pipe with a flat screwdriver, or the main stopcock if there is none. Flush to empty the cistern and soak up the remaining water with a sponge.
Note the make and take photos
Photograph how everything is connected and note any brand name on the parts. Take the old part to a plumbers' merchant or match it carefully, as flush valves in particular vary between cistern types.
Replace the worn part
Disconnect the supply, remove the old fill valve or flush valve following the part's instructions, and fit the new one with its washers seated correctly. Tighten plastic nuts by hand plus a small turn, as overtightening can crack them.
Restore water and test for leaks
Open the isolation valve slowly, let the cistern fill and check under the cistern and around every joint with dry tissue. Flush several times and adjust the water level.
Ready-to-use checklist
- Flat screwdriver for the isolation valve
- Adjustable spanner or water pump pliers
- Sponge and bucket
- Towels
- Replacement fill valve or flush valve seal
- Torch
- Phone for photos
Practical tips
- A few drops of food colouring in the cistern will show a leak into the bowl within 30 minutes without flushing.
- Keep the old part until the new one is working, in case you need to compare.
- If the isolation valve is stiff, do not force it; use the main stopcock instead.
Common problems
Water still trickles into the bowl after adjusting the float. What next?
The seal or washer on the flush valve is likely worn or dirty. Clean it first; if it is cracked or misshapen, replace the seal or the whole flush valve.
The joint under the cistern drips after refitting. What should I do?
Turn off the water, check that the washer is seated flat and not pinched, then retighten gently. Replace any washer that looks damaged.
The cistern refills very slowly. Why?
The fill valve or its filter may be partly blocked with debris or limescale, or the isolation valve may not be fully open. Clean or replace the valve and open the isolation valve fully.