Why does my toilet cistern take ages to refill?

A modern fill valve is a small plastic assembly with a fine filter, a rubber diaphragm and a float. It is designed to shut off cleanly against mains pressure, which means the water path through it is deliberately narrow — and narrow paths block.

What you'll need

• A screwdriver and an adjustable spanner
• A cloth and a bowl
• A replacement diaphragm washer if needed

Step by step

1. Check the isolating valve first.There is usually a small screwdriver-slot valve on the supply pipe below the cistern. If the slot is not in line with the pipe, it is partly closed. Open it fully and retest.
2. Compare with the rest of the house.If every cold outlet is slow, the problem is the incoming supply or the stopcock, not the cistern. Test the kitchen cold tap.
3. Take the cistern lid off and watch it fill.A weak trickle from the fill valve means a restriction in the valve. A good flow that simply takes time in a large cistern is normal.
4. Turn the water off and drain the cistern.Close the isolating valve, flush, and hold the flush open to empty it. Have a cloth ready for the last inch.
5. Dismantle the fill valve head.Most modern valves unclip or unscrew at the top. Photograph the arrangement first. Inside is a rubber diaphragm sitting over a small nozzle.
6. Clean the filter and the nozzle.There is usually a fine gauze filter at the inlet and a pinhole nozzle under the diaphragm. Grit and limescale in either will strangle the flow. Rinse both and clear the pinhole with a soft brush, not a pin.
7. Inspect the diaphragm.A hardened, split or distorted diaphragm will not lift cleanly. They cost very little and are sold generically for common valve brands.
8. Reassemble and refill.Open the isolating valve slowly and watch the fill rate. It should fill a standard cistern in well under two minutes.
9. Consider replacing the whole valve if it is old.Complete fill valves are inexpensive. If the body is scaled and the parts are unavailable, replacing the valve is faster and more reliable than nursing it.
10. Reset the water level afterwards.Check the level sits about 20mm below the top of the internal overflow, and adjust the float if not.

Tips

• Turn the isolating valve slowly when refilling. A sudden surge dislodges scale into the newly cleaned nozzle.
• In a hard water area, cleaning the filter annually keeps a fill valve working for many years.
• Note the brand and model on the valve before ordering parts — the diaphragms are not universal.

Common mistakes

• Assuming a slow fill means low mains pressure — A partly closed isolating valve and a blocked filter produce exactly the same symptom and are far more common. Test another cold outlet before blaming the supply.
• Clearing the nozzle with a pin or a needle — Scoring the plastic seat enlarges it unevenly, so the valve then fails to shut off cleanly and the cistern overfills. Use a soft brush and descaler.

If it doesn't work

Questions people ask