Quick Answer:
If the tank still refills slowly after a new fill valve, the problem is usually upstream: the valve height or installation may be off, the fill-valve inlet screen may be clogged, or the shutoff/stop valve or supply line is restricting flow. Check installation height and the inlet screen first, then test the shutoff and supply before buying another fill valve.
Why This Happens
- Fill valves need to sit at the correct height and orientation for the float and refill path to work. If installed too low or crooked, flow can be reduced.
- Most fill valves have a small mesh or plastic inlet screen where the supply hose connects. Sediment or mineral deposits can restrict flow even with a new valve.
- Shutoff valves, old angle stops, and supply lines can collect debris or corrode internally. A partially closed or clogged stop often limits flow despite a new valve at the tank.
- Household water pressure or devices added to the supply (for example, smart shutoff units) can also reduce flow.
Step-by-Step What to Do
Turn off the water and get ready
- Shut the toilet shutoff valve fully clockwise.
- Flush to drain the tank. Keep a towel and a small bucket or cup handy to catch remaining water when you disconnect fittings.
Confirm fill valve height and proper installation
- Check the manufacturer’s recommended height relative to the overflow tube. The fill valve top should usually be several inches above the overflow—follow the instructions that came with the valve.
- Make sure the valve is straight and the mounting nut is snug (hand-tight plus a quarter turn with a wrench). A twisted or loose valve can restrict flow.
- Verify float or adjustment cup moves freely and isn’t hitting the tank wall or overflow tube.
Inspect the inlet screen
- Place a bucket under the supply line. Unscrew the supply hose from the fill valve.
- Look inside the valve inlet for a small mesh screen or filter. Gently clean it with a toothbrush or rinse it. Replace the fill valve gasket if damaged.
- Reconnect and test. If flow improves, the screen was likely the limiter.
Check the stop valve and supply line
- Fully open the shutoff valve. If it feels stiff or loose, it may be failing.
- To test flow from the stop: disconnect the supply hose from the shutoff (have a bucket ready) and open the shutoff briefly to see how fast water flows directly from the pipe. Slow flow here means the stop or the house piping is the bottleneck.
- If the shutoff is clogged, replacing the angle stop or the flexible supply hose will often fix the issue.
Compare hot and cold behavior
- If you notice different behavior by side, check which supply is slow. If it’s only the hot side, troubleshoot the water heater or hot-side piping. For that case see Slow refill only on hot water side as a related topic to explore further.
Watch for added devices or pressure issues
- If you recently installed any devices on the supply (for example, a smart shutoff), they can limit flow or add resistance. Check or temporarily bypass those devices. For this situation, see Slow refill after installing smart shutoff for additional checks.
- If household water pressure is low everywhere, the toilet will be slow even with a clear valve; a plumber or your water utility can help confirm pressure levels.
Reassemble and test
- Reconnect the supply hose to the fill valve, open the shutoff slowly, and watch the fill rate. Adjust float height if necessary to reach correct fill level.
- If flow is still poor after these checks, the supply upstream (stop valve, pipe, or municipal pressure) is the likely cause.
What Not to Do
- Don’t keep replacing fill valves without checking the supply—restriction upstream is common; if uncertain, a plumber is appropriate.
- Don’t overtighten plastic fittings—overtightening can crack parts and cause leaks.
- Don’t use harsh chemicals in the tank to try to clear internal screens or valves; those can damage parts and create other problems.
When to Call a Professional
- The stop valve is seized, leaks, or you can’t get acceptable flow when the shutoff is fully open.
- You find low pressure in multiple fixtures—this can be a house or municipal supply issue that a plumber should assess.
- You’re uncomfortable disconnecting supply fittings or the supply line is corroded and needs replacement.
Safety Notes
- Always shut off the local shutoff before disconnecting plumbing. Keep towels and a bucket ready to catch water.
- Use basic hand tools only. If a fitting won’t move by hand and you must use extra leverage, stop and call a plumber to avoid breaking pipes or fittings.
- Turn the water back on slowly after work to watch for leaks and avoid water hammer.
Common Homeowner Questions
- Why check the inlet screen? It traps sediment; cleaning it is quick and often restores full flow.
- Could the new valve be defective? Yes, but a supply restriction is more common—test the shutoff and supply before assuming the valve is bad.
- Is it okay to replace the angle stop myself? Yes if you’re comfortable shutting off the main water and using a wrench; otherwise call a plumber to avoid leaks or damage.