If your toilet won’t stop running or you notice water constantly trickling into the bowl, your fill valve may be “letting by.” This means water is leaking past the valve even when it should be shut off. It’s a common problem that can waste hundreds of litres of water every day and raise your utility bills. In this guide, we’ll explain what causes a fill valve to let by, how to fix it, and which replacement valves are the most reliable.
What Does “Letting By” Mean?
A fill valve is supposed to close tightly once the tank reaches the correct water level. When it’s letting by, water seeps past the valve’s internal seal and continues flowing into the cistern or down the overflow pipe. The result is a toilet that keeps topping itself up or trickles water into the bowl nonstop.
Common Causes of a Fill Valve Letting By
- Worn or hardened seal: The rubber washer inside the valve can wear out, preventing a watertight closure.
- Debris or limescale buildup: Dirt, grit, or mineral deposits stop the seal from seating properly.
- Misadjusted float: If the float is set too high, the valve won’t close before water spills into the overflow.
- Cracked or damaged valve body: Physical wear or age can compromise the housing and internal components.
- High water pressure: Excessive mains pressure may force water past the valve even when closed.
How to Fix a Fill Valve That’s Letting By
- Turn off the water supply: Shut off the isolation valve behind or below the toilet.
- Check the float setting: Adjust the screw or clip so the water stops about 1 inch (25mm) below the overflow pipe.
- Clean the valve: Remove the cap and seal, rinse away debris, and soak parts in vinegar if limescale is present.
- Replace the seal: Many manufacturers sell inexpensive seal kits if the rubber washer is worn.
- Replace the valve: If cleaning or seals don’t work, it’s time for a new fill valve.
When to Replace Instead of Repair
Repairing is worth a try, but if your fill valve is older than 5–7 years, replacing it is often quicker and more reliable. Modern valves are inexpensive, easy to install, and designed to last.
Recommended Replacement Fill Valves
Fluidmaster 400UK Universal Fill Valve
- Adjustable height for different cisterns.
- Quiet fill technology reduces noise.
- Compatible with most UK toilets.
Verdict: The best all-round replacement for reliability and ease of installation.
Siamp 99B Universal Inlet Valve
- Universal fit with adjustable float.
- Durable design with brass threads.
- Popular in many UK installations.
Verdict: A durable option if you want long-term performance.
Macdee Dual Flush Repair Kit
- Includes both a fill valve and dual flush outlet valve.
- Great for older cisterns with multiple worn components.
- Easy to install in modern UK toilets.
Verdict: Best choice if your toilet also needs a new flush valve alongside the fill valve.
Comparison Table
| Model | Best For | Key Features | Price |
|---|---|---|---|
| Fluidmaster 400UK | Everyday replacement | Quiet fill, universal fit, adjustable height | Check Price |
| Siamp 99B | Durable performance | Universal fit, brass threads | Check Price |
| Macdee Repair Kit | Full repair | Fill + flush valve included | Check Price |
FAQ
- Can I just clean my fill valve instead of replacing it?
Yes. If debris or limescale is the issue, cleaning may restore function temporarily. But if the seal is worn, replacement is best. - How do I know if my valve is letting by?
If you hear running water, see water in the bowl when the toilet isn’t flushed, or notice your tank constantly topping up, the valve is leaking through. - Do I need a plumber to replace a fill valve?
No, most homeowners can do it with basic tools in under 30 minutes. - Will a new valve save water?
Absolutely. A leaking fill valve can waste hundreds of litres per day — replacing it pays for itself quickly in saved water bills.
Conclusion & Call to Action
If your fill valve is letting by, the problem is usually a worn seal, debris buildup, or a misadjusted float. While cleaning and seal replacement can work as short-term fixes, replacing the valve entirely is often the most reliable solution. Modern valves like the Fluidmaster 400UK or Siamp 99B are easy to install, affordable, and designed to last for years.
Stop wasting water and money — fix or replace your leaking fill valve today and enjoy a quiet, efficient, leak-free toilet.