Toilet Keeps Running? 5 Causes and How to Fix It Yourself
Most running toilets can be fixed in under 30 minutes with a part from any hardware store. This guide covers all 5 causes, exact DIY steps, and the London costs if you'd rather call a plumber.
Water waste: A running toilet wastes 200-400 litres per day -- adding 150-300 pounds to your annual water bill in London
How It Works
How a Toilet Cistern Works
Fill valve opens
Water enters the cistern through the fill valve (also called ballcock or float valve). The float sits low while the cistern is empty, keeping the valve open.
Float rises and shuts the valve
As water rises, the float rises with it. Once the cistern is full, the float reaches its cutoff point and closes the fill valve -- stopping the flow.
Flush valve opens on flush
When you press the flush handle, the flush valve (flapper or siphon) lifts, releasing water rapidly from the cistern into the bowl to clear waste.
Either valve failing = constant running
If the fill valve won't shut off, water keeps entering. If the flush valve won't seal, water drains silently into the bowl. Both result in the toilet running constantly.
Cistern diagram (top view, lid removed)
Water enters left (fill valve). Water exits bottom (flush valve). Overflow tube is the safety escape route if fill valve fails.
Diagnosis Guide
5 Reasons Your Toilet Keeps Running
Listed from most to least common. Green border = DIY fix. Grey border = plumber recommended.
Worn flapper (flush valve seal)
DIY fixSymptom: Toilet runs continuously after every flush. Water trickles into the bowl without stopping.
Cause: The rubber flapper degrades over time -- heat, water hardness and cleaning chemicals accelerate the process. When it no longer seals properly, water slowly drains from the cistern into the bowl.
Fix: Turn off the isolating valve, flush the cistern empty, unhook the old flapper and clip on a new universal one (or a brand-matched replacement). 15 minutes, no tools needed.
Ballcock / fill valve not shutting off
DIY fixSymptom: You can hear water running into the cistern constantly. The cistern may feel perpetually warm from incoming water.
Cause: The float arm or ball float isn't rising high enough to shut the fill valve once the cistern is full. Can be caused by a waterlogged float, a bent arm, or a worn valve seat.
Fix: Try bending the float arm gently downward (this lowers the cutoff point). If the float is waterlogged, unscrew and replace it (a few pounds). If the valve seat is worn, replace the whole fill valve (10-25 pounds).
Overflow tube set too low
DIY fixSymptom: Water constantly trickles out of the overflow pipe (usually exits through the outside wall). Cistern fills but never stops.
Cause: The water level is set too high and constantly spills over the overflow tube into the bowl or outside. The float cutoff point is too high relative to the tube height.
Fix: Adjust the float arm downward to lower the water level so it sits 25mm below the top of the overflow tube. Bend the arm (older brass ballcocks) or turn the adjuster screw (modern valves).
Faulty flush valve (not sealing after flush)
Symptom: Water drains slowly from the cistern into the bowl between flushes. The toilet may need multiple flushes to clear.
Cause: The flush valve seat or mechanism is worn, warped or corroded. Common in older siphon flush systems and in dual-flush drop valves where the valve body has degraded.
Fix: Siphon flush: replace the siphon diaphragm washer (5-10 pounds, 30-60 minutes -- requires draining the cistern fully). Drop-valve flush: replace the whole flush valve unit (10-30 pounds).
Cracked cistern body
Symptom: Visible water pooling around the base of the cistern or on top of the toilet. A hairline crack may only be visible when the cistern is full.
Cause: Physical impact, frost damage, or age-related stress fractures in the ceramic or plastic cistern body. Water escapes continuously through the crack.
Fix: Cistern replacement is the only reliable fix -- crack sealants are a temporary measure at best. The cistern must be drained, disconnected and replaced with a matching or compatible unit.
Step-by-Step Guide
How to Replace a Toilet Flapper
Applies to standard single-flush toilets with a rubber flapper (most UK toilets pre-2005 and many newer ones). Time required: 15-20 minutes.
Turn off the isolating valve
The valve is behind or beside the toilet on the supply pipe. Turn clockwise until it stops. If there is no isolating valve, turn off the mains stopcock.
Flush to empty the cistern
Hold the flush handle until all water has drained from the cistern. Mop up any remaining water with a sponge if needed.
Remove the cistern lid
Lift the lid straight up and set it aside carefully -- ceramic lids are heavy and fragile.
Unhook the flapper from the flush valve ears
The flapper has two side clips (ears) that hook onto pegs on either side of the flush valve seat. Squeeze and lift to unclip.
Disconnect the chain from the flush handle lever
Unclip or unhook the chain from the lever arm inside the cistern.
Match or source a replacement flapper
Take the old flapper to B&Q, Screwfix or a plumbers merchant for an exact match. Note the brand if visible (Armitage Shanks, Ideal Standard, RAK, Duravit). A universal flapper works for most standard toilets.
Clip new flapper onto valve ears and reconnect chain
Hook both ears onto the valve pegs. Connect the chain to the lever arm with 10-20mm of slack -- too tight and the flapper won't seal; too loose and it won't lift fully on flush.
Turn water back on and test
Open the isolating valve slowly. Let the cistern fill, then flush and observe. Adjust chain length if the flush is weak or the toilet continues to run.
No tools needed
A standard flapper replacement requires zero tools. The only purchase needed is the replacement flapper (3-10 pounds from B&Q, Screwfix, or any plumbers merchant). Universal flappers fit most UK toilets.
London Note
Dual-Flush Toilets: Different Fix
Most London properties built or refurbished after 2000 have dual-flush toilets. These use a ceramic cartridge mechanism rather than a rubber flapper -- the fix is different.
Leaking dual-flush: usually cartridge seal failure
The ceramic disc or rubber seal inside the cartridge wears over time. Water seeps past the seal and trickles continuously into the bowl.
DIY cartridge replacement: 15-40 pounds for a kit
You can buy a replacement cartridge kit matched to your toilet brand. The job is more involved than a flapper swap -- the cistern must be fully drained and the button assembly removed first.
Full mechanism replacement is often better value
A complete cistern mechanism (20-50 pounds) often makes more sense than just the cartridge, especially on older dual-flush systems. Fitted by a plumber: 80-150 pounds.
Not sure which type you have? If your toilet has two buttons on top (or a split button), it is a dual-flush. If it has a lever handle, it likely uses a siphon or flapper mechanism.
Repair Costs
Running Toilet Repair Costs -- London 2026
| Fix | DIY Cost | Plumber Cost |
|---|---|---|
| Flapper replacement | £3-10 | £60-120 |
| Float valve adjustment | £Free | £60-80 |
| Fill valve replacement | £10-25 | £80-150 |
| Full cistern mechanism | £20-50 | £80-150 |
| Cistern replacement | £30-150 (part) | £100-250 fitted |
| Toilet replacement | £80-300 (toilet) | £150-300 fitted |
All London plumber prices include VAT and labour. Fixed quote before work starts. No call-out fee on jobs over 2 hours.
Diagnosis Tip
How to Check If Your Toilet Is Running Silently
The food dye test -- takes 10 minutes
Put a few drops of food colouring into the cistern (not the bowl)
Wait 10 minutes without flushing
Look at the water in the bowl -- if the colour has spread there, the flapper is leaking (silent running)
If the bowl water is still clear, the flapper is fine -- the issue is likely the fill valve or overflow level
Silent running is the most wasteful type
A silent leak through a worn flapper can waste 200-400 litres per day in your London home without any audible sound. On a metered supply, that adds 150-300 pounds to your annual bill. Many households don't notice for months.
Toilet Running Questions
Frequently Asked Questions
Toilet Still Running After Trying These Fixes?
London plumbers available same day. If the DIY steps above haven't resolved it, a plumber can diagnose and fix most running toilet faults in a single visit -- usually in under an hour.
No call-out fee on jobs over 2 hours · Same-day London plumbers · Fixed quote before work starts