A dual flush toilet leaks silently when the central flush valve seal stops seating properly, so water trickles from the cistern into the pan all day. Put a few drops of food colouring in the cistern, wait twenty minutes without flushing, and colour in the pan confirms the leak. Replacing the seal usually stops it.
Your water bill has crept up, the meter moves when nothing is on, and you have checked every tap in the house. The toilet looks fine: no hiss, no overflow, and the cistern refills and stops like it should. That is exactly how a dual flush leak behaves, and it is one of the most common reasons a metered household in Cornwall pays for water it never used.
Why dual flush toilets leak without making a noise
An older toilet with a handle and a siphon is almost leak-proof by design, because the siphon only moves water when the handle lifts it. Push button dual flush cisterns work differently. A flush valve tower sits in the middle of the cistern, with a rubber seal on its base resting on the outlet. Pressing a button lifts the tower, water drops into the pan, and the tower falls back to close the outlet.
That seal is the weak point. It only takes a little limescale, a grain of grit, or a few years of hardening rubber for it to sit slightly proud of the outlet. Water then seeps past it continuously. The fill valve tops the cistern up so slowly that you never hear it, and the trickle runs down the back of the pan below the waterline where you cannot see it either.
Three faults account for most silent dual flush leaks:
- A perished or scaled flush valve seal. The rubber washer on the base of the tower no longer seats cleanly. This is the cause in the majority of cases and is the one the dye test picks up.
- A button cable set too tight. If the buttons sit slightly pressed, or the cable was adjusted short after a repair, the valve never fully drops back. The seal cannot close because it is being held a fraction open.
- A float set too high. On most push button cisterns the flush valve tower doubles as the overflow. If the water level rises above the top of the tower, water pours straight down it into the pan. This one is sometimes audible as a faint tinkle.
Cornwall’s soft water slows the limescale, but it does not stop the rubber ageing, or stop grit from a supply pipe repair or a new meter ending up on the seal.
How much water a silent leak wastes
Because the leak never stops, the volume adds up in a way a dripping tap never does. A slow trickle past a flush valve seal often amounts to several hundred litres a day, and a badly seated valve can pass far more. Over a quarter that is tens of cubic metres, which on a metered supply is real money.
The leak is also on your side of the meter, so South West Water measures it as consumption. If your bill has risen and you cannot explain it, our guide on reasons a water bill climbs with no obvious cause covers the wider checks. A silently leaking toilet sits near the top of that list because it is so easy to miss.
For a holiday let or second home the sums are worse, because a seal that failed in October can run until someone mentions the bill in March.
The dye test, step by step
You need food colouring, or a dye tablet from a plumbers’ merchant. The test takes twenty minutes and costs pennies.
- Lift the cistern lid. On most push button toilets you unscrew the chrome collar around the buttons first, then lift the lid straight up. Do not force anything.
- Wipe the pan dry above the waterline. This lets you spot fresh water running down the back of the bowl later.
- Add the colouring to the cistern water. Ten to fifteen drops is plenty. Do not flush.
- Wait twenty to thirty minutes. Leave the toilet alone and keep the household away from it.
- Look in the pan. Any tint in the pan water, or a coloured streak down the back of the bowl, means water is passing the flush valve seal. A clear pan means the seal is holding and the problem is elsewhere.
Flush a couple of times afterwards so the colouring does not stain the pan. If you have more than one toilet, test them all. Two leaking cisterns in one house is not unusual when they were fitted at the same time.
The tissue test as a back-up
No colouring to hand? Dry the back of the pan and lay a square of toilet paper against it for an hour. A wet or torn square tells you the same thing, though it is less sensitive than dye.
Fixing it
Most dual flush leaks are a straightforward job, and many people are comfortable doing the first two fixes themselves.
- Clean or replace the seal. Turn off the small isolating valve on the pipe feeding the cistern, flush to empty it, then twist the flush valve tower to unlock it and lift it out. The seal on the base pulls off. Wipe the seat clean, fit a new seal of the same size, refit the tower, and refill. Take the old seal to the merchant to match it.
- Slacken the button cable. If the buttons look slightly depressed at rest, loosen the cable adjuster a turn or two so there is a little free play before they lift the valve.
- Lower the float. Adjust the fill valve so the water settles a couple of centimetres below the top of the overflow tower.
- Replace the whole flush valve. If a new seal still weeps, the plastic valve seat is probably scratched or warped. A plumber can swap the complete unit in well under an hour on most cisterns.
Repeat the dye test after the repair.
Why it shows up on the water meter
A cistern leak is a constant, low-level flow, which is what an overnight meter check is designed to catch. If you have already done the overnight meter test and seen a small steady movement, a toilet is the first thing to rule out. The dye test tells you in twenty minutes whether you have found the answer.
Fix the toilet first, then repeat the meter check with the cistern isolated at its valve. If the meter stops moving, you are done. If it keeps creeping, the leak is somewhere else.
| What you notice | Most likely cause | What to do |
|---|---|---|
| Dye appears in the pan, no sound | Flush valve seal not seating | Clean or replace the seal, retest |
| Faint constant trickle you can hear | Float too high, water down the overflow tower | Lower the float on the fill valve |
| Buttons sit slightly pressed in | Button cable adjusted too short | Slacken the cable adjuster |
| Pan clear but meter still moving | Leak elsewhere in the house or supply pipe | Isolate the toilets and repeat the meter test |
When the toilet is not the problem
A clear dye test in every toilet, and a meter that still turns, means water is escaping somewhere you cannot see. In Cornish houses that is often a supply pipe under a long drive, a heating pipe in a solid floor, or a joint behind a bathroom wall in an older cottage. Our guide to a meter that moves with everything switched off walks through the isolation checks that narrow it down.
When the home checks run out, a water leak detection survey takes over. Thermal imaging, acoustic listening and tracer gas find the point of escape without lifting floors on a hunch, and you get a report that says where the leak is and what caused it. We provide leak detection across Cornwall, from Saltash to Penzance, and a silent toilet is one of the first things we ask about when you call. It is worth twenty minutes of your time before it is worth a survey.
Keeping it from coming back
- Do the dye test twice a year. It is the cheapest leak check there is.
- Test after plumbing work. Grit dislodged by a new meter or a repaired supply pipe ends up on the flush valve seat.
- Read the meter when a holiday let is empty. Two readings a week apart with nobody there should match.
Frequently asked questions
Why is my dual flush toilet running but I cannot hear it?
The flush valve seal in the centre of the cistern is letting water seep past into the pan. The flow is slow enough that the fill valve tops up silently, and the water runs down the back of the bowl below the waterline. A food colouring test in the cistern will confirm it within twenty to thirty minutes.
How long should I leave food colouring in the cistern?
Twenty to thirty minutes is usually enough. Do not flush during that time and keep other people away from the toilet. If the pan water takes on any tint, or a coloured line appears down the back of the bowl, the flush valve seal is passing water. Flush afterwards so the colouring does not stain.
Can a leaking toilet make my water meter move?
Yes. A cistern leak is a constant flow on your side of the meter, so South West Water records it as consumption. It is one of the commonest reasons a meter creeps overnight with everything switched off. Isolate the toilet at its valve and repeat the meter check to see whether it was the cause.
Is it cheaper to replace the seal or the whole flush valve?
Start with the seal. It costs very little, fits without tools on most cisterns, and solves the majority of silent leaks. If the leak returns after a new seal, the plastic valve seat is probably worn or scratched, and replacing the complete flush valve is the lasting fix. A plumber can normally do that in under an hour.
Toilet fixed, meter still moving?
If the dye test came back clear and the meter keeps creeping, your leak is hidden. We locate it without lifting floors on a guess, and give you a report your insurer will accept.
