The toilet keeps running: which part has failed
A cistern that keeps refilling nearly always has one of two culprits: the fill valve, which doesn't shut off when the tank is full, or the flush valve, which lets water leak into the bowl. With the lid off, the isolating valve (llave de escuadra) to hand and ten minutes, you'll know which, and whether it needs adjusting or replacing.
Step 1 of 4
What exactly is your cistern doing?
Listen and watch the bowl for a moment. A healthy cistern fills after you flush and then goes quiet. If you hear a constant hiss, if the tank tops itself up every so often without anyone using it, or you see a thin stream of water running down the inside of the bowl, water is escaping somewhere. Also check whether it's a traditional cistern, with the ceramic tank in view, or a concealed one built into the wall where you only see the flush plate. The steps are different.
Why a toilet cistern keeps refilling
A cistern tank has two parts that work against each other. The fill valve lets water in and the float closes it when the level reaches the top. The flush mechanism releases water into the bowl when you press and then has to seal with a rubber washer. If the cistern keeps refilling, one of the two has failed: either the fill valve doesn't shut off and water overflows into the middle tube, or the flush valve leaks and the tank empties slowly, so the fill valve opens again and again.
How to tell which one in ten minutes
Take the lid off and look at the level. If the water reaches the top of the overflow, it's the fill valve. If it's below that but water still runs into the bowl, close the isolating valve (llave de escuadra) and mark the level with a pencil. If it drops with the water off, it's the flush valve. The colouring trick confirms it without waiting: a few drops in the tank and, if they show up in the bowl without flushing, the seal isn't sealing.
What you can fix yourself
Lower the float with its screw or clip until the water sits a finger or two below the overflow. Free it if it's catching on the side or on the mechanism. Clean the limescale off the flush seal and the rim it sits on with vinegar. Fully open an isolating valve that was left half closed. That solves a good share of cases without paying anyone.
When parts need replacing
When the float won't shut off even if you lift it by hand, when the seal is hard or split, or when the same fault comes back after a few weeks. In hard water areas like Valencia, limescale eats away at the seals and adjustments don't last long. The isolating valve and the flexible hose deserve a separate look: if they drip or won't close, they're replaced together. What the market charges to replace the mechanism or the whole cistern is in how much it costs to replace a toilet cistern.
What we do
Tell us on WhatsApp or through the form what you found with these steps, with a photo of the open tank and another of the valve. We give you a fixed price in writing, usually in under two hours, and an independent professional who works with us comes to your home. If something else turns up when it's taken apart, it's quoted separately and you decide.
Frequently asked questions
Does a running toilet waste a lot of water?
Yes, and it's the fault you notice least until the bill arrives. A trickle running day and night makes no noise and wets nothing, but it never stops. If you can't fix it today, close the isolating valve between uses and only open it to fill the tank when you need to.
Can I replace the cistern mechanism myself?
If the isolating valve closes properly and the tank is a standard one, it's a job plenty of people do at home. The risk is in what's around it: a seized valve that snaps, seals that aren't replaced and end up dripping underneath, or a mechanism that doesn't fit the flush button. If you're unsure about any of the three, better not to start.
Why does water overflow into the middle tube?
Because the fill valve doesn't shut off in time. Either the float is set too high, something is catching it, or the valve's seal has limescale and no longer closes. Lift the float by hand: if the water stops, lowering it with its screw is enough. If water keeps coming in even when you lift it, the fill valve needs replacing.
What do I do if the isolating valve won't close?
Don't force it with pliers, because old ones break inside and then water comes out under pressure. Close the flat's main stopcock if you need to cut the water, and make a note to have that valve replaced. It's the one that saves you the day a flexible hose bursts, so it's worth keeping it in good shape.