A running toilet can waste 200+ litres of water a day. That's money literally going down the drain. Here's how to figure out what's wrong.
How a Toilet Works (Quick Version)
The cycle:
- You flush → water leaves the tank
- Float drops → fill valve opens
- Tank fills → float rises
- Float reaches set level → fill valve closes
- Tank sits ready for next flush
A running toilet means: Water is continuously flowing - either into the bowl, into the overflow, or both.
Cause 1: Faulty Flapper / Flush Valve Seal
The most common cause.
What happens: The rubber flapper (or drop valve in modern toilets) doesn't seal properly. Water constantly leaks from tank to bowl.
How to check:
- Remove cistern lid
- Add food colouring to the tank water
- Wait 15 minutes (don't flush)
- Check the bowl - if it's coloured, water's leaking past the flapper
Why it fails:
- Rubber perishes over time
- Mineral buildup prevents seal
- Flapper warped
- Something caught under the seal
The fix: Clean or replace the flapper. See our guide on fixing running toilets.
Cause 2: Float Set Too High
What happens: The float (ball or cup type) is set too high. Water fills past the overflow tube and runs constantly.
How to check:
- Remove cistern lid
- Look at the water level vs the overflow tube
- If water is running into the overflow, the float is too high
Why it happens:
- Float adjusted incorrectly
- Float mechanism has drifted
- Replacement parts set wrong
The fix: Adjust the float lower. Ball floats: bend the arm down. Cup floats: adjust the screw or clip on the fill valve.
Cause 3: Fill Valve Won't Shut Off
What happens: Even with the float at the right level, the fill valve doesn't close properly. Water keeps running.
How to check:
- Lift the float manually to its highest position
- If water keeps running, the fill valve isn't shutting off
Why it fails:
- Worn washer/diaphragm inside valve
- Debris blocking the valve
- Valve mechanism worn out
The fix: Sometimes cleaning helps. Often needs replacement (they're cheap - £15-25).
Cause 4: Stuck or Misaligned Float
What happens: The float is stuck, tangled, or rubbing against something, so it can't rise properly.
How to check:
- Watch the float as the tank fills
- Does it rise smoothly and freely?
- Is it catching on anything?
Common issues:
- Float arm bent
- Float ball has water inside (it's sinking)
- Cup float stuck on guide rod
The fix: Free the obstruction, straighten the arm, or replace damaged parts.
Cause 5: Cracked Overflow Tube
What happens: The overflow tube itself is cracked or damaged, so water drains through even below the normal level.
How to check: Look at the overflow tube carefully. Any visible cracks? Is water escaping from it below water level?
The fix: Replace the overflow tube or whole flush valve assembly.
Cause 6: Flush Valve Not Seating
What happens: After flushing, the flush valve doesn't drop back down properly.
How to check: Flush and watch the mechanism. Does the valve/flapper return to a closed position?
Common issues:
- Chain too short (holding flapper open)
- Valve mechanism stuck
- Debris preventing closure
The fix: Adjust chain length, clean mechanism, or replace.
Quick Diagnosis Guide
| Symptom | Likely Cause |
|---|---|
| Water runs into overflow tube | Float too high or fill valve fault |
| Water leaks to bowl silently | Flapper/flush valve seal |
| Running after flush, then stops | Slight seal issue |
| Never stops running | Fill valve or float problem |
| Tank doesn't fill properly | Fill valve fault or low water pressure |
The Parts (Modern Toilet)
Fill valve: Lets water into the tank, controlled by float
Float: Rises with water level, tells fill valve when to stop
Flush valve/flapper: Releases water into bowl when you flush
Overflow tube: Safety outlet if water rises too high
Handle/button mechanism: Lifts the flapper or operates the flush valve
Should You DIY?
DIY friendly:
- Adjusting float level
- Replacing flapper
- Cleaning parts
- Replacing fill valve (moderate DIY)
Consider calling a plumber:
- If you've tried obvious fixes
- If there's a leak somewhere you can't find
- If the toilet is old and weird
- If you're not comfortable
Temporary Fix
If you need to stop the running until you can fix it properly:
- Turn off the water supply (valve behind toilet)
- The running will stop
- Flush won't refill until you turn water back on
Not a solution, but buys you time.
Water Waste
A constantly running toilet can waste:
- Slow leak: 20-30 litres/day
- Moderate: 100-200 litres/day
- Bad: 400+ litres/day
On a water meter: That's noticeable on your bill. Even without a meter, it's environmental waste worth fixing.
Can't figure it out? I fix toilets across Exeter - usually in under 30 minutes. Call 01392 964094 or get a quote.
Sam Hembury
Sam is the founder of Hembury Contracting, providing professional handyman services across Exeter and Devon. With years of experience in property maintenance, he shares practical tips to help homeowners tackle common tasks.
