Dye test for a leaking toilet

The short answer

Dye in the cistern, half an hour of nobody touching it, then look in the pan. That is the whole test, and the result is binary in the useful direction.

The steps themselves are on the dye test procedure in full, with the calculator that converts what you see into a daily volume.

How this works in practice

The test works because of where the water sits. The cistern holds standing water above the pan, and the flush valve is the only thing between them. If the valve seals, coloured water stays in the cistern. If it does not, gravity moves it.

That is also why it detects the fault nothing else does. A passing flush valve makes no noise, leaves no damp, and refills so slowly that the fill valve never runs long enough to be heard. The toilet looks and sounds normal while continuously losing water.

• Do it when the toilet will genuinely be left alone. One flush restarts the test, and a household toilet is rarely undisturbed for half an hour by accident.
• Colour the water properly. The dye reaching the pan is already diluted, so a faint cistern gives an ambiguous answer.
• Test every toilet, not just the suspected one. Around eight per cent of toilets are leaking at any time, so a second one is a realistic possibility rather than bad luck.
• Look at the overflow outside while the dye is in. It costs nothing extra and it distinguishes the two faults in one test.

What changes the answer

Four things, and the first two produce a false negative, which is the failure that matters.

• Not waiting long enough. A weeping valve at the bottom of the range moves only a few litres an hour, so half an hour can produce a tint faint enough to miss. Overnight settles it.
• Somebody flushing. The test is over the moment the cistern empties, and a half-remembered flush is the most common reason a dye test is inconclusive.
• The cistern mechanism. A siphon cannot pass water silently the way a drop valve can, so a negative result on a siphon cistern is more informative than a negative on a drop valve. which mechanism your cistern uses covers why.
• Which fault you have. The dye test finds a passing flush valve. A passing fill valve shows at the overflow instead, and a leak elsewhere in the property does not involve the toilet at all.

A positive result is conclusive: colour in the pan cannot get there any other way. A negative result is narrower than it feels, because it rules out one fault on one toilet.

Assumptions and sources

Nothing on this page rests on a constant, and the procedure needs no citation. It either shows colour in the pan or it does not, and the mechanism behind it is that water flows downhill through a seal that is not sealing.

The one figure that appears here, the share of toilets leaking at any time, is measured but reached second hand, and it is cited on the calculator page with the intermediary named rather than presented as though the original research had been read directly.

What to do next

A positive result and a negative result lead to different places.

• Positive, colour in the pan: you have found a passing flush valve. Classify what you can see, weeping or trickling or running, and convert it into a daily volume and a cost.
• Positive, colour at the overflow: the fill valve is passing. That is the higher-volume fault, because an overflow runs to waste continuously rather than into the pan.
• Negative on every toilet: the toilets are not the cause. Move the test to the meter, which measures the whole property rather than one appliance.
• Ambiguous: repeat overnight with more dye. An ambiguous dye test almost always means not enough dye or not enough time.

Whichever it is, the meter is what turns a category into a number. confirm the rate of loss at the meter measures the whole property over an interval you choose.

Common questions