Why Does My Toilet Keep Running After I Flush It?

It starts as a faint hiss you only notice at night, or a thin ripple running down the back of the pan. Nothing is overflowing, the toilet still flushes, so it gets left. Meanwhile a continuously running toilet can lose somewhere between 200 and 400 litres a day — on a metered supply in Kingston, that is a noticeable amount of money running quietly into the sewer.

Nearly every case comes down to one of two components. Working out which one takes about a minute.

Take the cistern lid off (lift straight up; on push-button toilets unscrew the button collar first).

• Water is trickling over the top of the overflow pipe or out through the overflow slot: the fill valve is not shutting off. The cistern is overfilling.
• Water level sits correctly below the overflow, but water is still running into the pan: the flush valve seal is leaking. Water is escaping through the bottom of the cistern.

Not sure? Put a few drops of food colouring in the cistern and don't flush for 20 minutes. Colour appearing in the pan confirms a flush valve seal fault.

The fill valve (the inlet valve) is the part that refills the cistern and shuts off when the water reaches the set level. It fails in a few predictable ways.

Limescale on the diaphragm. The water across Kingston and South West London is hard. Scale builds on the small rubber diaphragm inside the valve and stops it seating fully, so it never quite closes and you get a permanent hiss.

Grit from the mains. After works on the street or an isolation valve being operated, a fragment of debris lodges in the valve seat. Symptoms are identical to scale.

The float is set too high. On modern valves the float is a plastic cup on the valve body, adjusted by a screw or clip. If it's set above the overflow level, the cistern will always dribble into the overflow no matter how well the valve seals. A common one after somebody has "had a go" at it.

The float has taken on water. On older ball-float arms, a punctured ball sinks, the valve never gets the signal to close, and the cistern runs continuously.

The flush valve sits in the base of the cistern. On a modern dual-flush unit it's a tall plastic tower with a large rubber seal washer at the bottom; on older systems it's a siphon.

Perished seal washer. The rubber doughnut at the base of the flush tower hardens and distorts with age and limescale, so it no longer sits flush. This is the single most common cause of a quiet, continuous run into the pan — and the most commonly missed, because nothing looks wrong from above.

Dual-flush buttons sticking. Push-button mechanisms jam slightly open when the cable or rod binds, or when the button collar has been screwed down too tight. The flush valve then never fully reseats.

Debris on the seat. Grit or a fragment of a disintegrating cistern block holds the seal a fraction open. Cistern blocks that hang inside the tank are a frequent cause of flush valve failure — we'd suggest avoiding them entirely.

Failed siphon diaphragm on older lever-flush toilets shows up differently: the flush becomes weak and needs several pulls, rather than running continuously.

Individual seals, diaphragms and float adjustments are all serviceable, and on a reasonably modern mechanism that's the sensible fix. We'd lean towards replacing the whole valve when the unit is more than about ten years old, when the plastic has gone brittle and cracks as it's dismantled, or when the toilet is a concealed-cistern or wall-hung unit where a second visit is disruptive. In those cases fitting new, quality parts is the cheaper route over five years.

One genuine complication: on concealed cisterns behind a wall or in a furniture unit, access is through a small inspection panel behind the flush plate, and some budget frames make servicing awkward. That's worth having someone experienced look at rather than dismantling it on a Sunday afternoon.

On a metered supply, a continuously running toilet typically adds somewhere in the region of £200–£300 a year to a household bill — more if both the fill valve and the flush seal are passing. It also keeps the pan permanently wet-streaked with limescale, and in a flat above others, a persistent overflow discharge outside is a maintenance issue in its own right.

Compared with a short fixed-price repair visit, the maths is not close.

Maxfix repairs running toilets across Kingston upon Thames, Surbiton, New Malden and Richmond, carrying common fill valves, flush valves and seal kits on the van so most jobs are done in one visit. See running toilet repair, toilet cistern repairs and toilet repairs, or call 020 3488 0912. We provide honest, fixed-price plumbing services in Kingston.