Why do I have to flush the toilet twice?

A flush works by dumping a volume of water fast enough to start a siphon in the pan's trap. Anything that reduces the volume or slows the delivery breaks that, and the toilet then empties lazily instead of pulling everything through. That gives you the diagnostic route: check the volume, then the speed of delivery, then the pan.

What you'll need

• A bucket
• Descaler or citric acid solution
• A small mirror and a torch
• A piece of wire or a thin cable tie for clearing rim jets

Step by step

1. Take the lid off and check the water level.Look for a moulded fill line on the cistern or on the flush valve. Water should sit at that line, typically about twenty millimetres below the top of the internal overflow. A level even a couple of centimetres low takes a noticeable amount out of the flush.
2. Raise the level if it is low.Adjust the clip or screw on the fill valve's float stem to let the water rise higher, then flush and refill and check where it settles. Do not raise it above the fill line — it will simply run into the overflow.
3. Check which button you are actually pressing.On a dual flush, the smaller of the two buttons is the reduced flush and is deliberately not enough for solids. This sounds trivial and is a genuinely common answer, especially on unmarked chrome buttons where both look the same.
4. Check the dual-flush set-point.Inside, most dual-flush valves have a small adjustable float or clip that sets how much of the cistern the reduced flush uses. If it has slipped down, the half flush drops to almost nothing. Slide it to increase the volume and test.
5. Check the flush valve is opening fully.Take the lid off, hold the button or lever down and watch. The flap should lift clear and stay up for the full flush. A cable that has stretched, a rod out of adjustment or a button not travelling fully means the valve opens partly and closes early.
6. Look under the rim for scale.In a hard water area the small jets around the underside of the rim scale up and the water no longer sweeps round the bowl — it dribbles down one side. Use a mirror and a torch to look. This is very common and is often the whole answer.
7. Descale the rim jets.Turn off the supply, flush to empty the cistern and lower the pan water, then apply a descaler or citric acid gel up under the rim and leave it as long as the product allows. Then work a piece of wire or a cable tie gently into each jet to clear it. Repeat if the scale is heavy — this is not a one-application job on a badly scaled pan.
8. Rule out a partial blockage with a bucket test.Fill a bucket with several litres and tip it into the pan quickly, from about waist height. A healthy pan takes it and siphons cleanly. If the level rises and drains slowly, you have a partial blockage in the pan trap or the drain, and no amount of cistern adjustment will fix it.
9. Consider the pan itself.Some water-saving pans genuinely need a full flush for solids and were sold on the assumption people would use the dual flush appropriately. Older pans with a damaged or scaled trap flush poorly permanently. If everything above checks out and the bucket test is clean but the pan still needs two flushes, the pan is the limitation.

Common mistakes

• Raising the cistern water level above the fill line to get a stronger flush — Water above the line runs straight into the internal overflow and into the pan, so it wastes water continuously and adds nothing to the flush.
• Putting a bottle or a brick in the cistern to save water on a toilet that already flushes weakly — It reduces the flush volume further and turns one flush into two, which uses more water overall.
• Assuming a weak flush means a blockage — In most cases the pan is fine and the cistern is delivering less water than it should. The bucket test separates the two in thirty seconds.

If it doesn't work