Why Your Toilet Won't Stop Running
A running toilet usually means the fill valve is either stuck open or failing to seal properly. The flapper is the other common culprit, but if you've already swapped that and the water keeps trickling through, the fill valve is what you need to tackle next. I've replaced more of these than I care to count, and the process is genuinely straightforward if you just know which way to turn things. Start by shutting off the water supply. That little knob on the wall or floor behind the toilet turns clockwise until it stops. Flush the toilet to drain most of the water out of the tank. You'll still have some residue at the bottom, which is fine. Sponge it out if you want a dry workspace, though it isn't strictly necessary. Disconnect the supply line from the bottom of the fill valve. It's a plastic nut, hand-tight usually, sometimes you might need pliers but grip it gently because these nuts crack easier than you'd think. If the nut is seized from years of mineral buildup, apply a little penetrating oil and wait five minutes before trying again. Force is the fastest way to strip the threads on your toilet's brass bushing.
Once the supply line is free, there's typically a large plastic locknut underneath the tank holding the fill valve in place. Loosen that by turning counterclockwise. Some valves use a clip system instead of a nut, particularly the cheaper models. Look for a rectangular tab you pull upward to release. Remove the old valve and clean the inside of the tank where it sat. Mineral deposits and sludge accumulate there over time, and if you skip this step the new valve won't seal properly against the bottom of the tank. A coarse cloth and some vinegar work fine. I once installed a perfectly good Fluidmaster valve and had it leaking within an hour because I didn't scrub the gasket surface. Took thirty seconds I should have spent the first time around. Insert the new fill valve through the hole from the top. Thread the locknut back on from underneath and tighten it by hand first, then give it a quarter turn with pliers. Do not overtighten. These plastic components don't need much torque and cracking a nut is a real problem that leaves you fishing for a replacement toilet bushing.
Reconnect the supply line. Turn the water back on slowly and watch for leaks at both connections. While it's filling, adjust the valve height if your model allows it. Most modern fill valves have a screw on top or a clip you slide up and down the stem. The water level needs to sit about half an inch below the overflow tube opening. Too high and water runs constantly into the overflow. Too low and you get a weak flush. Attach the refill tube from the fill valve to the overflow tube. This is the small flexible hose that sends a bit of water into the bowl after each flush. Without it, the bowl won't refill properly and you'll notice standing water that looks wrong the next morning.
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What Most People Miss
The water pressure in your home matters more than you might expect. If your supply pressure is below 20 psi, many fill valves struggle to shut off cleanly. They'll click but leave a small stream passing through. In that case, you might need a valve designed for low-pressure systems, or consider installing a pressure regulator if the rest of your house has similar issues. Standard valves assume 40 to 80 psi range. Another thing that catches people out: the type of fill valve you buy matters depending on your tank's design. Older toilets often have a 7/8 inch hole for the fill valve inlet. Newer ones are increasingly moving toward 1 inch. Measure your existing hole before purchasing, or grab a universal conversion kit that works with both sizes. The cheapest fill valves on the shelf are usually the ones that fail within two years. Spend the extra dollar on a brand like Fluidmaster or Korky, and you'll likely outlast the toilet itself. If your fill valve is one of the older style float-ball models with an arm and spherical float, just replace the entire assembly. Those designs are obsolete and repair parts are increasingly hard to find. The whole valve costs about fifteen dollars and installs in under ten minutes.
When Replacement Isn't Enough
Sometimes the valve seats are corroded inside the toilet's inlet thread. If you install a new valve and water still seeps around the base even after tightening, the bushing inside the toilet may be worn. This is common in homes built before 1990 with original plumbing fixtures. The workaround is to wrap the valve's threaded portion with a couple of layers of PTFE tape before threading the locknut back on, or replace the entire toilet if the damage is severe. There's no point patching a tank that's leaking from its foundation. Also worth noting: replacing the fill valve won't fix a running toilet caused by a misaligned or degraded flapper. If you haven't checked the flapper yet, do that first. It's a five-minute job and costs about three dollars. The fill valve is the second thing you should investigate, not the first.
