Quick Answer:
If only the upstairs toilet keeps refilling, the likely fix is to lower the fill level so the water line sits below the overflow tube and confirm the fill valve is holding. Turn off the water, make the adjustment, and test. If the water still reaches the overflow or the adjustment won’t stay, call a plumber.
Why This Happens
When only one toilet refills at odd moments, the usual causes are a fill valve that no longer holds or a float set too high. A mispositioned refill tube or a slow flapper leak can also cause the tank to slowly lose water, which triggers the fill cycle. If the problem started after work nearby, see Phantom flush after remodel for related causes to check.
Step-by-Step What to Do
Step 1 — Turn off the water and observe
- Close the toilet shutoff valve (usually on the wall behind the bowl).
- Flush to empty the tank so you can see the mechanism clearly.
- Look for obvious issues: a broken float arm, a refill tube pushed into the overflow tube, or a cracked fill valve.
Step 2 — Lower the float
Lower fill level so water line sits below overflow tube. How you adjust depends on the fill valve type:
- Float cup style: squeeze the clip on the float cup or slide the float down to lower the shut-off point.
- Float ball on an arm: turn the adjusting screw or bend the arm slightly downward (do this gently).
- Newer integrated valves: follow the small adjustment screw or dial on the valve body to drop the water level.
Step 3 — Reconnect and test
- Turn the water back on slowly and let the tank fill.
- Check that the final water line is a little below the top of the overflow tube (about 1/4 to 1/2 inch lower is typical).
- Watch for several minutes to ensure the valve holds and the toilet stops filling.
Step 4 — If it still refills
- Check the refill tube: it should end above the overflow tube, not routed inside it.
- Inspect the flapper and chain for slow leaks or misalignment that let water escape slowly.
- If a small adjustment won’t hold, the internal seal of the fill valve may be failing and the valve will likely need replacement.
What Not to Do
- Don’t let water sit above the overflow tube — it will spill into the drain and can cause hidden leaks. Don’t allow water above overflow tube; plumber if adjustment won’t hold.
- Don’t jam parts to force a stop; forcing components can break the valve or the float assembly.
- Don’t ignore continuous running or slow refills — that wastes water and can lead to damage to ceilings and floors below an upstairs bathroom.
When to Call a Professional
Call a plumber if:
- The water level won’t stay below the overflow after adjusting the float.
- You find cracks in the tank or parts that are broken and you’re unsure how to replace them.
- There are signs of water damage below the bathroom (stains, sagging ceiling, damp insulation).
Safety Notes
- Shut off the supply before working on the toilet to avoid flooding.
- If water has leaked into the ceiling below, stop using the toilet and address the leak quickly to prevent structural or mold damage.
- Use gentle force when adjusting plastic components—too much force can cause snapping and larger repairs.
Common Homeowner Questions
- Why does the upstairs toilet refill but the downstairs one is fine? Different valves and floats age differently; the upstairs unit likely has a failing fill valve or a misadjusted float.
- How low should the water line be? The water should sit a little below the overflow tube—about 1/4 to 1/2 inch lower is normal.
- Could a new device nearby cause this? Yes—changes to plumbing or added equipment can affect pressure or how valves sit; see Random refills after installing filter for similar scenarios.