Why Does My Toilet Keep Running? A Cardiff Plumber Explains

Start by working out where the water is going

There are only two possibilities. Either water is leaking from the cistern down into the pan, or the cistern is overfilling and spilling into the overflow. Telling them apart takes one test.

Lift the cistern lid and look at the water level. There should be a fill line marked on the inside wall, or on the plastic body of the fill valve, usually around 20mm below the top of the overflow pipe. If the water is sitting at or above the overflow, your fill valve is the culprit. If the level is correct but you can still hear running water, the flush valve seal is leaking.

• Dye test: put a few drops of food colouring in the cistern, do not flush, and wait 20 minutes. Colour appearing in the pan confirms a leaking flush valve.
• Listen at the pan: a faint hiss or visible ripple at the back of the bowl points the same way.
• Water climbing steadily above the fill line means the float or fill valve is not shutting off.

The three parts that cause almost every case

Flush valve seal. In most modern dual flush toilets the flush valve is a tall plastic tower sitting in the middle of the cistern with a rubber washer at its base. Limescale, and Cardiff sits in a moderately hard water area, builds up on that washer until it no longer seats properly. This is by far the most common cause we see, particularly on cisterns fitted eight years ago or more.

Fill valve. Older siphon-style toilets use a brass ball float on an arm; newer ones use a vertical float on a plastic stem. Grit from the mains or a perished diaphragm stops it closing fully, so the cistern creeps up and drains away through the overflow.

Flush button or handle. A dual flush button that has been pushed hard, or a button rod set too long, can hold the valve fractionally open. Press the button once and watch the valve drop fully back down. If it hesitates, that is your answer.

What you can sort yourself

A fill valve set too high is a genuinely simple fix. On a float arm, bend it gently downwards or turn the adjustment screw; on a modern stem valve, squeeze the clip and slide the float down a centimetre. Flush, let it refill, and check the level sits below the overflow.

You can also lift out the flush valve, most twist off with a quarter turn after isolating and emptying the cistern, and inspect the seal. If it is coated in scale, soaking it in white vinegar for an hour and rinsing it sometimes buys you another year. If the rubber is hard, cracked or distorted, it needs replacing rather than cleaning.

Before you touch anything, find the isolation valve on the supply pipe going into the cistern and turn the slot a quarter turn with a flathead screwdriver. If there is no isolation valve, or it is seized, stop there. Seized valves that shear are how a small job becomes a flooded floor.

When it is worth calling someone

Call a plumber if the cistern is concealed behind tiling or a furniture unit, if the toilet is close coupled and the cistern would need lifting off the pan, or if you have replaced the seal and it is still running. Repeated failures often mean the flush valve body itself is worn or the wrong replacement part was fitted, and there are dozens of near identical but incompatible valves on the market.

As a rough guide, a straightforward flush valve or fill valve replacement in the Cardiff area typically falls somewhere in the region of 90 to 160 pounds including the part, depending on the make of the toilet and how accessible the cistern is. Concealed cisterns and discontinued models cost more because of the time involved in identifying and sourcing the right component. Any plumber worth using will tell you on the phone what it depends on rather than quoting a flat figure blind.