If your toilet keeps running after you flush, you’re not being fussy — it’s genuinely worth sorting. A “little hiss” can turn into a constant trickle into the pan, and that’s basically your water bill quietly doing press-ups all day.
Most of the time it’s not a dramatic problem. It’s a worn seal, a float set wrong, or a button that’s not quite returning. The trick is: don’t start reefing on plastic parts inside the cistern like you’re fixing a car. Toilets are surprisingly easy to make worse.
What “running” actually is (in plain terms)
When a toilet “runs on”, one of two things is happening:
- Water is leaking from the cistern into the pan, so the cistern keeps topping itself up.
- The cistern is overfilling, and the excess is going down the overflow (often into the pan on modern toilets).
Different cause, same noise.
The usual suspects (what we see all the time)
1) The flush valve seal is tired or dirty
This is the big one.
What you’ll notice:
- Water trickling into the pan even when nobody’s touched the loo
- The cistern refilling every few minutes for no reason
Why it happens:
- Rubber seal wears out
- Limescale builds up
- A tiny bit of grit gets caught and stops it sealing
You can’t always see it, but it’s enough to keep the cistern “chasing” the level.
2) The fill valve/float isn’t shutting off properly
What you’ll notice:
- The water level sits very high in the cistern
- You hear a constant fill noise
On many UK toilets now, the overflow goes straight into the pan, so you don’t get water pouring out the back like the old days. It just quietly runs into the bowl.
3) The button or handle is sticking (or set wrong)
This is the “I have to jiggle it” situation.
What you’ll notice:
- It runs until you press the button again
- It’s worse after someone’s taken the lid off / fitted a new flush plate
Sometimes the push rods are set a touch too long, or the lid isn’t sitting right and it’s rubbing.
4) Hard water and limescale making everything a bit sticky
If you’re in a hard-water area, assume limescale is involved somewhere.
It can make seals less flexible and valves slower to shut.
5) Concealed cistern weirdness
If it’s a wall-hung toilet or a concealed cistern, the same faults apply — it’s just more awkward to access, and parts can be more model-specific.
Quick checks you can do safely (no tools, no drama)
1) Take the lid off and watch one flush
Put the lid somewhere safe (they crack easily).
Flush once and look for:
- Is the water level rising too high?
- Can you see water going into the overflow?
- Does the flush valve drop back down cleanly?
2) The food colouring test (properly useful)
Put a few drops of food colouring in the cistern. Don’t flush.
Wait 10–15 minutes.
- If colour shows up in the pan, your flush valve seal is leaking.
It’s simple, but it tells you a lot.
3) Lightly press the flush valve down
If you can gently press the flush valve down and the running stops, that usually points back to the seal not seating properly.
4) Check the water level isn’t right up to the top
If it’s very high, it’s often a fill valve/float setting issue.
The one thing to do if it’s running constantly
If it’s properly running and you can’t stop it quickly, don’t leave it for “later”.
- Turn off the isolation valve to the toilet (usually on the pipe feeding the cistern)
- No isolation valve? Turn off the stop tap
That stops the waste while you decide whether you’re adjusting something or calling someone.
What not to do (because this is how people end up calling us anyway)
- Don’t over-tighten plastic nuts inside the cistern — they split.
- Don’t force anything that feels stuck.
- Don’t buy a random “universal” part unless you’re sure it matches your valve.
- Don’t ignore it for weeks. A slow trickle adds up.
When to call a plumber
Call a plumber if:
- You’ve done the quick checks and it’s still running
- The parts look old/brittle (or you can see cracking)
- You can’t isolate the toilet easily
- It’s a concealed cistern and access is tight
- You see any sign of water outside the toilet (damp, staining, soft flooring)
Most fixes are straightforward: replace the flush valve seal, replace the fill valve, set the levels correctly, and make sure everything is seated how it should be.
Quick note on “is it expensive?”
Usually it’s not a big job — it’s just one of those things that’s annoying until it’s done. The expensive version is leaving it long enough that it causes a leak, damages flooring, or you end up replacing a whole toilet because the fittings have seized and snapped.
