Toilet cistern not filling or filling slowly: causes and fix

What you need

• 17 mm spanner or an adjustable spanner
• Flat-nose pliers
• Old toothbrush
• Bucket and cloth
• Phone torch
• Cleaning vinegar or descaler (Viakal)
• Universal side-entry or bottom-entry fill valve (Wirquin Jollyfill, Geberit Unifill or Roca replacement, 15 to 25 euros at Leroy Merlin, Bricodepot or a ferretería, the local hardware shop)
• Stainless steel braided flexible hose, 3/8 by 1/2 inch, 30 cm (3 to 6 euros)
• 1/2 by 3/8 isolating valve with washer, if the current one is seized (5 to 8 euros)

Step by step

1. 1

   Open the isolating valve fully

   Turn the valve on the wall anticlockwise until it stops. If water comes in, you are done. If the valve will not move, do not force it: go to step 2 after shutting off the mains for the flat.

2. 2

   Free the float

   Take the lid off the cistern. Check that the float rises and falls without touching the wall, the overflow or the chain. If it rubs, turn the fill valve slightly or shorten the flush chain. Lift and release the float: if water comes in, the problem was mechanical.

3. 3

   Turn off the water and disconnect the hose

   Close the isolating valve (or the mains). Put the bucket underneath and unscrew the hose nut at the base of the fill valve with the 17 mm spanner. Open the valve for a second over the bucket: if a strong jet comes out, the inlet is fine.

4. 4

   Clean the inlet filter

   At the valve connection you will see a fine plastic mesh, sometimes inside the nut. Pull it out with the tip of the pliers, brush it with vinegar and rinse. If it is broken, replace the whole valve. Screw the hose back on by hand plus a quarter turn with the spanner, no PTFE tape.

5. 5

   Clean or replace the diaphragm

   If it still fills slowly, take off the top cap of the valve (turn anticlockwise or press the tabs). Underneath is the rubber diaphragm: soak it in vinegar for 20 minutes, clean the seat with the brush and reassemble. If it is rigid or cracked, go to step 6.

6. 6

   Replace the fill valve

   Unscrew the large nut holding the valve from outside the cistern, lift it out from inside and fit the new one with its rubber washer inside the tank. Tighten the outer nut by hand, connect the hose and adjust the float height so the level sits 2 cm below the overflow.

7. 7

   Turn the water on and check

   Open the isolating valve slowly. A 6-litre cistern should fill in 60 to 90 seconds at normal pressure. Flush twice, check it shuts off properly and run dry paper around the nut and the hose to rule out drips.

Why the cistern is not filling: causes from most to least common

Isolating valve partly closed or seized. Someone closed it for a repair and left it half open, or limescale has blocked the spindle. It is the first check: open it fully by turning anticlockwise. If it will not turn, do not force it with pliers; the valve can snap and then you will have to shut off the mains.

Inlet filter blocked. All modern fill valves (Roca, Geberit, Wirquin) have a fine mesh at the hose connection. On the coast, with hard water and supply cuts that bring sand through, it blocks within one or two years. The symptom is clear: the cistern fills very slowly and the water comes in with a whistle.

Float caught. The float on the fill valve rubs on the cistern wall, the overflow tube or the flush chain and stays up, so the valve thinks the tank is full. It happens a lot after replacing the flush mechanism without repositioning the fill valve properly.

Fill valve diaphragm split or scaled up. Inside the cap of the fill valve is a rubber diaphragm that opens and closes the flow. If it splits, water pours in and will not shut off; if it scales up, it will not open and the cistern does not fill. After 6 to 8 years in hard water, this is normal.

Flexible hose kinked or delaminated inside. Rubber hoses with steel braid degrade from the inside and the inner rubber peels away and acts as a plug. If the hose is more than 10 years old or the steel is rusty, replace it without a second thought (3 to 6 euros).

Low pressure or a supply cut. If the taps in the house are also weak, it is not the cistern. In the urbanisations of Torrevieja, Orihuela Costa or the Marina Alta there are scheduled cuts and pressure restrictions in summer.

How to tell which one it is: diagnosis in 5 minutes

Take the lid off the cistern and lift the float by hand. Let it go: if it drops freely and you hear water coming in, the valve works and the problem was that it was caught. If no water comes in even with the float down, close the isolating valve, unscrew the hose from the valve and open the isolating valve for a second over a bucket. If a good jet comes out, the inlet is fine and the fault is in the fill valve (filter or diaphragm). If only a trickle comes out, the problem is upstream: isolating valve, hose or pressure.

One last trick: look at the dried water mark inside the tank. If the cistern has been filling halfway for weeks, you will see a limescale line well below the overflow. It means the float is set low or the valve is closing early because of limescale.

Common mistakes when fixing a cistern that will not fill

The most expensive mistake is forcing a seized isolating valve with a big spanner. The brass spindle snaps and water comes out under pressure with no way to shut it off. If the valve will not turn by hand, shut off the mains for the flat and replace it with a new 1/2 by 3/8 valve (5 to 8 euros) using a 17 mm spanner.

Another common mistake is buying a bottom-entry fill valve when your cistern is side-entry, or the other way round. Look at where the hose comes in: through the side of the tank is side-entry; from underneath is bottom-entry. The universals from Wirquin and Geberit come in both versions and the box says which.

The third is raising the float to maximum so it fills faster. It does not fill faster, it only fills fuller, and if it goes above the overflow it starts leaking into the pan. The correct level is 2 cm below the tube. And never put chlorine tablets in the tank: they destroy the valve diaphragm within months.

When to call a professional

Call a plumber if the isolating valve will not close or leaks when you move it, if the cistern is concealed and the work has to be done through the push-button opening, if the water connection is old iron pipe (very common in 1960s and 1970s buildings in the centre of Valencia or Alicante) or if, after replacing the valve, it still does not fill and you suspect a building pressure problem.

With our plumber service, replacing a cistern mechanism costs 60 to 110 euros, with the mechanism included in most cases, and replacing the isolating valve or the hose is added at the price of the part. We arrive in 20 to 45 minutes in Valencia, Alicante, Torrevieja and Benidorm, and in 30 to 60 minutes in Dénia, Jávea, Calpe and Altea. Fixed price by WhatsApp with a photo and 6 months written guarantee. See the rest of our rates in how much a plumber charges.

Limescale and sand: tips for the coast

The water in Alicante and Valencia is hard and, after every supply cut, it brings grit and rust from the building's pipes. That mixture is what blocks the fill valve filter. If you live in an urbanisation with frequent cuts, clean the filter with vinegar every spring: it takes 10 minutes and saves you the valve.

In holiday homes that sit empty for months, the water standing in the hose leaves limescale on the diaphragm. Close the isolating valve when you leave and, when you return, run a tap before opening it. If your cistern is also leaking into the pan, check the guide toilet cistern keeps running, and if it is a Roca, repairing a Roca cistern mechanism.