Short definition
A flush valve is the outlet assembly at the bottom of a toilet tank: the seat the flapper rests on, the integrated overflow tube, and the spud-nut connection to the bowl. When a flapper replacement doesn’t fix a running toilet, the flush valve seat is usually the next suspect — often eroded by chloraminated WA water.
What it is
Three components in one assembly:
- Seat. The flat ring the flapper presses against to seal the tank. When eroded or pitted, no flapper will seal.
- Overflow tube. Vertical tube running up from the seat. Prevents the tank from overfilling (water spills into the tube and goes to the bowl) and routes refill water from the fill valve into the bowl during the fill cycle.
- Mounting hardware. Spud nut threaded shank at the tank bottom, plus a tank-to-bowl gasket (spud washer) sealing between the tank and the bowl casting.
Standard seat sizes: 2 inch (older / standard toilets), 3 inch (modern HET / 1.28 gpf), and 4 inch (large-trapway designs). The flapper or canister flush valve must match the seat size.
Why it matters to a homeowner
The classic diagnostic flow: dye-test confirms a flapper leak, you replace the flapper, and the toilet still leaks. The flush-valve seat is eroded — pitted by years of chlorine or chloramine attack. Two cures:
- Fluidmaster 555C resurfacing kit — a thin stick-on patch that creates a fresh sealing surface. $5 to $10. Works for moderate erosion.
- Replace the whole flush valve. $15 to $40 part. More work — you remove the tank, swap the assembly, reinstall — but solves any seat condition. Pro labor $150 to $300.
When a plumber says “I’ll need to replace the flush valve,” they’re describing this assembly.
Common failure modes
- Eroded / pitted seat. Flapper can’t seal. Resurface or replace.
- Cracked overflow tube. Tank water bypasses the bowl during refill; flush is weak. Replace assembly.
- Cracked tank around the spud. Replace tank or whole toilet.
- Worn tank-to-bowl gasket. Leak at the tank-bowl junction. See tank-gasket.
Common variants
- Flush valve (this entry, tank-side) vs. flushometer (commercial direct-supply). Different mechanism, similar function — the flushometer uses water-line pressure directly instead of a tank reservoir.
- 2-inch vs. 3-inch flush valve seat. Flapper or canister must match.
- Flush valve vs. fill valve (refill valve). Flush valve is the outlet; fill valve is the inlet.