To fix a running toilet, lift the cistern lid and watch the water. If it trickles into the bowl, the flush valve or flapper seal is worn and needs replacing. If the water rises past the overflow tube, the fill valve is faulty or set too high. Both parts are inexpensive cistern components and most are a straightforward swap.
Key takeaways
- A running toilet usually means a worn flush valve seal or a faulty fill valve.
- A simple dye test in the cistern tells you which of the two parts is the culprit.
- A constantly running toilet can waste thousands of litres a week, so it is worth fixing fast.
- Universal cistern kits suit most toilets, but a concealed or wall-hung cistern is a plumber's job.
How a toilet cistern works
Fixing a running toilet is easy once you know the two parts that do the work. The fill valve, mounted on one side of the cistern, refills the tank with clean water after a flush and shuts off at a set level. The flush valve sits in the centre and holds water back until you press the button, then releases it into the bowl.
A running toilet is one of these two failing. Either the flush valve is leaking water into the bowl so the cistern never fills, or the fill valve is not shutting off so water keeps pouring in and draining away through the overflow. Working out which one is the whole job.
Diagnose the cause with a dye test
Lift the cistern lid and look inside, then run this quick test to pinpoint the fault.
- Check the water level. If it sits above the top of the central overflow tube, the fill valve is the problem: it is not shutting off, or it is set too high.
- If the level is correct but the toilet still runs, suspect the flush valve. Put a few drops of food colouring in the cistern and wait 15 minutes without flushing.
- If colour appears in the bowl, the flush valve seal or flapper is leaking and needs replacing.
- Listen for a faint hiss or trickle. A hiss usually points to the fill valve, a trickle into the bowl points to the flush valve.
Fix the flush valve or the fill valve
Turn off the water at the isolation valve behind or beside the toilet, then flush to empty the cistern before you start.
For a worn flush valve, the simplest fix on many toilets is a new flapper or seal washer, which lifts straight out and clicks back in. If the whole valve is scaled or cracked, fit a complete replacement flush valve, which means undoing the cistern from the bowl on close-coupled suites.
For a faulty fill valve, first try adjusting it down so the water shuts off below the overflow tube. If it still will not seal, swap it for a new fill valve: disconnect the supply hose, undo the lock nut underneath, lift the old valve out, and fit the new one. Universal cistern kits from any hardware store fit the majority of standard Australian toilets.
When to call a licensed plumber
Most running-toilet repairs are well within DIY range, but some setups are not. Get a licensed Adelaide plumber involved if any of the following applies.
- The toilet has a concealed cistern in the wall or is a wall-hung suite, where access and parts are specialised.
- Water is leaking from between the cistern and the bowl, or from the base of the toilet onto the floor.
- You have replaced the valves and the toilet still runs, which can mean a hairline crack in the cistern.
- The isolation valve is seized or leaking, so you cannot safely turn the water off.
- The toilet is old and replacement parts for that model are no longer available.
Why a running toilet is worth fixing quickly
A running toilet is easy to ignore because nothing floods, but it quietly wastes water. A toilet that runs continuously can lose thousands of litres a week, and on Adelaide's water and sewerage charges that adds up on the quarterly bill.
Hard water makes it worse. Adelaide's mineral content builds scale on the flush valve seal and the fill valve, which is why these parts fail faster here than in soft-water regions. If you replace cistern parts often, that is normal for the local water, and fitting good-quality valves rather than the cheapest option will buy you longer between repairs.