What is a toilet fill valve?

Lift the tank lid and it is the tall part on the left, connected in two directions. Underneath, a threaded shank passes through the tank bottom and meets the braided supply line coming off the shutoff behind the bowl. Up top, a thin flexible refill tube clips onto the rim of the overflow tube in the middle of the tank. Water should stand about 1 inch below the top of that overflow tube, and most tanks carry a molded line or a stamped mark showing exactly where. Adjustment is either a screw on the cap or a spring clip on the float rod, depending on the brand.

Two things happen at once after every flush. Most of the water refills the tank, and a small stream runs up the refill tube into the overflow to reseal the water in the bowl, which is what keeps sewer gas out of the bathroom. When the internal seal wears, it stops closing cleanly against incoming pressure. You get a faint hiss between flushes, a refill that drags past 3 minutes, a steady trickle into the overflow tube around the clock, or a tank that tops itself up every 20 minutes with nobody home. The EPA puts a leaking toilet at roughly 200 gallons a day, which shows up as $15–60 a month on a combined water and sewer bill. Ten drops of food coloring sort out which part is guilty: if the bowl stays clear for 15 minutes but the hissing continues, the fill valve is the problem, not the flapper.

Run that dye test twice a year and listen at the bathroom door on a quiet night (how often to check a toilet for leaks). Plan on a new fill valve every 5 to 7 years, or every 4 to 5 on well water and hard water where grit scours the seal (fill valve replacement timing). The swap takes 20 to 30 minutes: close the stop, flush, sponge out the last inch, unhook the supply, back off the shank nut, seat the new valve, and snug it hand tight plus a quarter turn. Keep it on the same round as the flapper and the supply line in the toilet maintenance checklist, and remember the fixture outlives its own guts by decades (how long a toilet lasts).