Why Is My Toilet Constantly Running Water? Causes and Fixes

The three parts that cause almost every running toilet

Lift the cistern lid and you will find a fill valve (sometimes called a ballcock), a flush valve or syphon, and an overflow. Nearly every constantly running toilet comes down to one of these not sealing or shutting off properly.

The fill valve tops the cistern up after a flush and should close when the float reaches the set level. If the float is set too high or the valve is worn, water keeps flowing and drains away through the internal overflow, which is the steady trickle you hear. The flush valve sits at the bottom and is meant to hold the water back with a rubber seal or diaphragm. When that perishes or gets a bit of limescale under it, water leaks into the pan, the level drops, and the fill valve kicks in again every few minutes.

• Hissing or a constant trickle down the back of the pan: usually the fill valve or float set too high
• Toilet refills by itself every 10 to 30 minutes without being flushed: usually a leaking flush valve seal or syphon diaphragm
• Water dripping from an outside pipe near the toilet: an older external overflow doing its job, so the fill valve is the culprit

Why it matters more on the south coast than you might think

Southern Water and Portsmouth Water supply most of our area, and the Solent has some of the hardest water in the country. Limescale builds up on valve seats and diaphragms far faster here than in soft water regions, which is why a toilet that was fine for eight years can suddenly start running. If your kettle furs up quickly, your cistern parts are suffering the same way.

A faulty flush valve can lose anywhere from 200 to over 400 litres a day. On a metered supply that can add well over a hundred pounds a year, and if you are on a water meter with a sewerage charge the cost roughly doubles. It also keeps the cistern cold and damp, which encourages condensation and mould on the back wall in bathrooms without decent ventilation.

Checks you can safely do yourself

Before calling anyone, a couple of five minute checks will tell you which part is at fault. Put a few drops of food colouring in the cistern, do not flush, and wait 20 minutes. If colour appears in the pan, the flush valve seal is leaking. If the water level in the cistern sits right at the top of the overflow tube, the float needs adjusting down or the fill valve is not closing.

Adjusting the float is usually a screw or a sliding clip on modern fill valves and takes a minute. Replacing a flush valve seal or a syphon diaphragm is a straightforward job on many cisterns, with parts costing between 5 and 20 pounds from a plumbers merchant. Isolate the water first using the small isolation valve on the supply pipe, and take the old part with you so you get the right match, as there are dozens of variants.

• Turn the isolation valve a quarter turn with a flat screwdriver so the slot sits across the pipe
• Flush to empty the cistern before removing anything
• Do not overtighten plastic nuts, as cracked cistern fittings are a far more expensive problem than the leak you started with

When it is worth calling a plumber

Some cisterns are not worth the fight. Close coupled toilets with a bottom entry fill valve, concealed cisterns behind a tiled or boxed in wall, and older Ideal Standard or Twyford units with obsolete parts often need the cistern lifting off or the flush valve replaced as a complete unit. If the isolation valve is seized, which is common on fittings that have not been turned in years, you will need the mains stopcock and the confidence to work fast.

A straightforward valve replacement typically takes us under an hour, and the cost depends on the parts needed and whether the cistern has to come off. Where a cistern is cracked, the flush mechanism is discontinued, or the pan is fixed to the floor with old cement rather than screws, it can be more economical to fit a new WC than to keep patching it. We will always tell you honestly which option makes sense rather than assuming you want the bigger job.