Plumbing & water
Toilet keeps running? Here’s the fix
A toilet that will not stop running has exactly two suspects: the fill valve that refills the cistern, or the flush valve seal that holds the water back. A splash of food colouring identifies the guilty party within the hour.
By the Tickover team · Published 29 June 2026 · 4 min read
Inside every cistern, two parts do all the work. The fill valve opens after a flush and closes when a float tells it the cistern is full. The flush valve is the gate at the bottom: a siphon in older UK toilets, a drop valve with a rubber seal in most push-button models. A running toilet means one of them has stopped doing its one job.
Diagnose it by sound and sight
A constant hiss, with the water level creeping above its marked line and trickling into the central overflow tube, convicts the fill valve: it is not shutting off, so the excess pours quietly into the pan. A silent cistern that refills itself every so often (ghost flushing, in the trade) points to the flush valve seal, which is letting water seep from cistern to pan and triggering top-ups.
Which valve is it?
The food-dye test
Name the culprit in an hour
Lift the cistern lid carefully
Push-button lids are captive: unscrew the button collar or the button itself first. Never yank the lid.
Check the water level
Water sitting above the marked line, or dribbling into the overflow tube, means the fill valve is guilty and you can skip the dye.
Add food colouring to the cistern
A generous squirt of anything dark. Cheap dye, expensive knowledge.
Wait an hour without flushing
Longer is better. Overnight is conclusive.
Read the verdict
Coloured water in the pan means the flush valve seal is passing water. A clean pan and a stable level means the leak is intermittent, so test again another day.
Fixing each fault
Fill valve: modern valves have an adjustment screw or clip to lower the float, which often cures it. If the valve still hisses past a lowered float, swap the whole unit: isolate the supply at the little valve on the inlet pipe, flush to empty, and the new valve fits with hand-tight connections.
Flush valve seal: isolate, flush to empty the cistern, then twist the drop valve out (most release with a quarter turn) and replace the rubber seal at its base. While you are in there, scrape any limescale off the seat it lands on. Older siphon toilets cannot leak this way, so a running siphon toilet is nearly always the fill valve instead.
Concealed and wall-hung cisterns are the exception to all this cheerful DIY: access panels, cramped fittings and manufacturer-specific parts make them a plumber’s job, and no shame in it.
If a plumber does it
Flush or fill valve replaced
Daytime call-out, first hour
Typical UK ranges, mid-2026, as a guide only. Your home, your area and the job itself all move the number — always compare real quotes.
Why it will not wait
On a water meter, a running toilet is the most expensive fault in the house, quietly passing a few hundred litres a day around the clock. Water companies call them leaky loos and take them seriously; your bill will do the same. Unmetered homes waste the same water, only invisibly. Either way it is the same instinct as a dripping tap: small, constant, fixable.
Not sure what you’re dealing with?
Tell Self-Diagnose what the cistern is doing, hissing, ghost flushing or refusing to fill, and it will pinpoint the part, the urgency, and a vetted plumber if the cistern is the concealed kind.
If your toilet has the opposite problem, nothing going anywhere, read how to unblock a toilet without making it worse. And for the whole system around it, the home plumbing guide is the map.
Quick answers
Why does my toilet keep running after flushing?
Either the fill valve is not shutting off, so surplus water escapes down the overflow into the pan, or the flush valve seal at the bottom of the cistern is worn and letting water seep through. A stuck or misaligned flush button causes the same symptom. The food-dye test separates them in about an hour.
How much water does a running toilet waste?
A steady leak from cistern to pan can pass a few hundred litres a day, which on a water meter is real money every quarter. Water companies call these leaky loos, and some offer help tracing them because the waste is so large. Fixing the valve usually pays for itself within a billing cycle or two.
Can I replace a toilet fill valve myself?
On a standard exposed cistern, yes: isolate the supply, flush to empty, unscrew the old valve from the inlet and fit the new one hand-tight. Allow an hour the first time. Concealed or wall-hung cisterns are harder to reach and use brand-specific parts, so hand those to a plumber, typically £80–£160.
Why is my push-button toilet running?
Push-button cisterns use a drop valve, and its rubber seal hardens, warps or collects limescale until it no longer seats cleanly. The button mechanism can also stick or sit misaligned, holding the valve slightly open. New seals are cheap, and descaling the valve seat while the cistern is empty doubles your odds of a lasting fix.
This guide is general information for UK homes, not advice about your specific property. Anything involving gas must be done by a Gas Safe registered engineer, and notifiable electrical work by a qualified electrician. If you smell gas, call the National Gas Emergency line on 0800 111 999. If you’re ever unsure, stop and get a professional in — that’s exactly what Tickover is for.
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