Source: Title: The toilet refills by itself. The flapper is only half the story. \| Mountain Man Plumbing Content revision: 416fe5c631937549042c70064a40726653c266eb9778efbdfbf8a323408dbc4d Source modified (WordPress record): 2026-09-19T16:17:38+00:00 Extracted at: 2026-10-25T18:45:14+00:00 Freshness: Stored anonymous publication. Refreshed in the background after content changes and daily; the previous successful version remains available during refresh. See extracted-at time. Live inventory requires its tool. ## On this page - [Which part it actually turned out to be]() - [← DIY plumbing tips]() MOUNTAIN MAN PLUMBING / DIY Toilet Repair # The toilet refills by itself. The flapper is only half the story. A new flapper is everyone's first move and it's right about half the time. Listen to it before you buy anything: a short refill every few minutes and a constant run send you to opposite ends of the tank. Mountain Man Plumbing · Updated Sep 19, 2026 The toilet refills by itself. The flapper is only half the story. The house goes quiet and then the toilet fills for ten seconds on its own. Or it never stops at all, and you've stopped hearing it. Both are the same underlying thing: water is escaping the tank, and the fill valve keeps topping it back up. Almost everyone's first move is a new flapper. It's the cheapest part and the easiest to reach. It's also right about half the time, which is why so many people put one in and nothing changes. **Right now:** if it is running continuously rather than refilling in short bursts, close the stop under the tank tonight. A fill valve stuck open wastes thousands of gallons a day. An intermittent refill is a slow leak and can wait for the weekend. ### Which part it actually turned out to be Of 150 people who identified a part. Several threads involved more than one. Flapper or tank ball **52%** Fill valve or ballcock **47%** Flush valve seat or canister **37%** Water level set above the overflow **29%** That third bar is the one that catches people. The flush valve seat is the plastic rim the flapper closes against. A new flapper sealing onto a pitted, scaled thirty-year-old seat will still leak, and you will have correctly diagnosed the symptom and replaced the wrong half of the joint. The EPA's own guidance says the same thing: if the leak doesn't stop after a new flapper, [replace the seat and overflow tube assembly](). There is a way to do that without pulling the tank, and it's further down this page. ## Two different sounds, two different halves of the tank. Before you take the lid off, listen. This one distinction sends you to opposite ends of the tank. | What you hear | What's happening | Where to look | | --- | --- | --- | | Silence, then a short refill, then silence again | Water is seeping from tank to bowl until the level drops enough to trigger the fill valve | The flapper and the seat it sits on | | Continuous running that never stops | Water is pouring into the overflow tube, so the tank never reaches its shut-off level | The fill valve and the water level setting | For the continuous one, take the lid off and just look at the overflow tube — the open pipe in the middle. If water is running into it, a new flapper cannot possibly help you. Either the level is set too high or the fill valve isn't shutting off. **Set the lid down flat somewhere it can't be knocked.** It's heavy unglazed porcelain, it breaks into sharp pieces, and a replacement lid for an older toilet is often harder to find than any part inside the tank. ## The dye test, done properly. For the quiet-then-refills version, this settles it in ten minutes for free. The method comes from [the EPA's WaterSense leak protocol](): 1. Put a dye tablet or a few drops of food coloring in the **tank**, not the bowl. 2. Wait five to ten minutes and don't flush. 3. Look in the bowl. Color in the bowl means the flapper or the flush valve seal is leaking. 4. Flush straight away afterwards, or the dye can stain the tank and bowl. Then run a finger around the seat the flapper closes onto. If it feels rough, gritty or pitted rather than smooth, that's your answer, and a flapper alone won't fix it. ## What it's costing while you decide. About one in five of the people asking about this had found it on their water bill rather than by hearing it. The numbers behind that are worth seeing, and they're from the EPA rather than from us: ### Gallons wasted per day EPA WaterSense figures, drawn to scale against each other. A slow flapper seep **30** A fill valve stuck open **4,300** A slow flapper leak runs about 30 gallons a day, roughly 900 a month — annoying, not alarming. A fill valve stuck open is a different category entirely: the EPA puts it at up to 4,300 gallons a day and as much as $1,400 a month in water. That's the one to deal with tonight rather than next weekend. Worth knowing generally: the EPA says **about 20% of toilets leak**, and that a flapper should be treated as a wear item and replaced roughly every five years. ## Three free things to check before you buy a part. A quarter of people had already put a new part in before asking for help, and about a third of those said it made no difference. Some of them never needed a part at all. - **Chain slack.** Fluidmaster's spec is about half an inch of slack with the flapper closed. Too tight and the chain holds the flapper a hair off its seat — a leak you'd blame on worn rubber. Too loose and the chain slips under the flapper and props it open. Cut the excess off once it's right, for the same reason. - **The handle and its arm.** If the lever doesn't drop all the way back on its own, it's holding the chain up. Corroded brass arms bind; some plastic ones sag until the arm rests on the tank rim. - **Where the refill tube points.** That thin tube from the fill valve should clip to the *top* of the overflow pipe and squirt down into it. If someone has pushed it down inside the pipe below the water line, it siphons, and the toilet cycles forever. Fluidmaster call this out as a cause of a valve that won't shut off. Pulling the tube back up is free. ## Buying the right flapper. Flappers come in two sizes and they are not interchangeable. Measure the flush valve opening — the round hole at the bottom center of the tank that the flapper covers. [Korky]() give a useful eyeball version: about the size of an orange or a baseball is a 2-inch valve, about the size of a grapefruit or a softball is a 3-inch. Most toilets made before 2000 are 2-inch; 3-inch became common after that, but there's no clean cutoff, so measure rather than guess from the age of the house. **The old flapper measures bigger than the valve it fits.** Fluidmaster's rule: an existing flapper about 3 inches across means you need a **2-inch** flapper, and one about 4 inches across means a **3-inch**. People measure the rubber, read the number off it, and buy the wrong one. Two things that will send you home empty-handed: - **Canister or tower valves aren't flappers.** If the middle of your tank has a tall cylinder that lifts straight up when you flush, rather than a hinged rubber flap, you need a seal specific to that model. No universal flapper fits it. - **One-piece toilets and some proprietary valves.** Kohler one-piece toilets take a specialty flapper and the generic repair kits don't fit. American Standard Champion and a few others have their own seals. Take a photograph of your open tank with you, and get the model number while you're in there — it's usually stamped into the porcelain inside the tank, above the water line, or molded under the lid. Fluidmaster and Korky make most of the flappers and fill valves sold in the US and both will match a part to that number. ## A new flapper on a worn seat still leaks. This is the 37% bar, and it's where most people give up and call someone. You don't have to. A flush valve seat repair kit — the common one is the Fluidmaster 555C, and there are equivalents — is a peel-and-stick replacement seat that bonds on top of the damaged one. **No tank removal.** It works on plastic, ceramic and metal seats, fits 2-inch flush valves, and suits toilets made from about 1985 onward. 1. Turn off the stop under the tank, flush, and hold the flapper open to drain as much as you can. 2. Sponge the tank out. It has to be properly dry, not damp. 3. Unhook the old flapper and scrub the seat clean. If it's brass, Fluidmaster say to use steel wool or wet-and-dry paper, then rinse and wipe it. 4. Dry the seat thoroughly — a hairdryer for a minute does it. Keep the heat moving. The adhesive will not bond to a wet seat, and this is the step people rush. 5. Peel and center the new seat over the old one, press it down hard, and check the flapper can swing fully open without fouling anything. 6. Fit the new flapper, set the chain, turn the water on and test a flush. If your flush valve is the 3-inch type, or the valve body itself is cracked rather than just worn, there's no sticker fix — the whole valve comes out, and that means draining and lifting the tank off the bowl. That's a genuine step up in difficulty, because it also means new tank-to-bowl bolts and a new spud gasket. ## The fill valve, and the measurement almost nobody checks. If water is running into the overflow pipe, the fill valve or its level setting is the problem. Adjusting the level is free — most modern valves have a screw or a clip on the float. **Set the water about half an inch below the top of the overflow pipe.** Any higher and it simply pours down the overflow forever. If adjusting it doesn't hold, or the valve hisses, chatters or shuts off slowly, replace it. It's a twenty-minute job and one of the better value repairs in a house. But when you fit the new one, there are three heights that matter, and only one of them is about performance: | What | Where it goes | Why | | --- | --- | --- | | The **C.L.** mark stamped on the fill valve | At least 1 inch above the top of the overflow pipe | Plumbing code. This is the backflow limit — below it, tank water can be drawn back into your drinking water | | Top of the fill valve cap | Roughly 3 inches above the overflow pipe | Fluidmaster's shortcut: get this right and the C.L. mark lands where it should | | Top of the overflow pipe | At least 1 inch below the tank lever hole | So an overflowing tank drains down the pipe instead of out of the hole in the porcelain | Fill valves adjust for height by twisting the lower shank — counter-clockwise to raise, clockwise to lower. Don't touch the lock ring; it's holding the body and shank together under pressure. **Hand-tighten the supply nut and stop.** Overtightening cracks the plastic valve shank or the tank itself, and you find out at two in the morning. And don't put plumber's putty, thread sealant or pipe dope on that connection — American Standard say outright that it voids the warranty. The rubber washer is the seal. ## The drop-in bleach tablets are eating your tank parts. If there's a blue or a bleach tablet sitting in your tank, that is very likely why you're reading this. In-tank cleaners containing chlorine sit in contact with every rubber and plastic part for months and destroy them from the outside in. This is not a plumber's opinion, it's in the paperwork that came with the toilet: - **Kohler:** "Never use in-tank toilet cleaners. This will void the warranty." Their homeowner's guide is blunter — products containing chlorine "can seriously damage fittings in the tank," and that damage "can cause leakage and property damage." - **American Standard:** the warranty "DOES NOT COVER any damages caused by the use of in-tank cleaners. These products can seriously corrode the fittings in the tank." Take it out and clean the bowl from the top instead. If your water is heavily chlorinated to begin with, chlorine-resistant flappers exist and are worth the extra dollar or two. ## Where to hand it over. - The shutoff under the toilet is seized, corroded, or won't fully close — deal with that before anything else. - Water is appearing on the floor at the base, or on the ceiling below. That isn't a running toilet, it's a leak. - The tank is cracked, or you find water between the tank and the bowl. - You've replaced the flapper and the seat and it still seeps. - The flush valve needs replacing and you'd rather not lift a tank off a bowl. - It's a pressure-assist toilet with a sealed vessel inside the tank. Different system, different parts, leave it alone. [Running toilet repair ↗]() · [All toilet repairs ↗]() · [Water around the base ↗]() · [Find your main shutoff ↗]() Where the numbers come from: 291 homeowner questions about running and phantom-flushing toilets posted to public plumbing forums, scoped to threads about a toilet that won't stop filling rather than clogs or base leaks, and sorted by what each person described. Because they count what people wrote down, each figure is a floor. Water-waste figures, the dye test protocol, the 20% leak rate and the five-year flapper guidance are from the US EPA WaterSense program: the Free of Water Leaks technical sheet, WaterSense at Work Section 3.1, and the Know Your Flow leak detection guide. Flapper sizing is from Korky's sizing guide and Fluidmaster's flush valve guidance; water level, C.L. mark, valve height, chain slack and refill tube placement are from Fluidmaster's 400A and 400C installation instructions; the seat repair method is from Fluidmaster's Flusher Fixer instruction sheet. The in-tank cleaner warnings are quoted from Kohler's one-year limited warranty and homeowner's guide and from American Standard's toilet warranty. Every procedure and figure here was checked against those sources. Related repair guides [Toilet repair & replacement ↗]() YOU CAN HAND IT OVER FROM HERE. ## Need a second set of eyes? Tell us what happened and what you’ve tried. - [Text a photo ↗]() - [Book a visit ↗]() ## Another useful thing to know. [All DIY tips ↗]() ![Water comes up out of the washing machine standpipe. Before you snake it, find out if the drain can breathe.]() DIY TIP / 01 Laundry Plumbing ### [Water comes up out of the washing machine standpipe. Before you snake it, find out if the drain can breathe.]() A washing machine doesn't drain, it pumps, and a top-loader pushes up to 17 gallons a minute into a pipe that has to take every bit of it. Ten minutes with a five-gallon bucket tells you whether the line is actually blocked or just can't get air, and those two need opposite repairs. Read the tip ↗ ![The pipes bang when the water shuts off. Check the pressure before you buy an arrestor.]() DIY TIP / 02 Water Pressure / Pressure Regulator ### [The pipes bang when the water shuts off. Check the pressure before you buy an arrestor.]() Banging, humming, whistling and ticking are four different faults with four different repairs. A fifteen-dollar gauge on a hose bib tells you whether an arrestor will fix the banging or do nothing at all, and it takes two minutes. Read the tip ↗ The tub won't hold water. Almost everything that's wrong is behind two screws in the overflow plate. DIY TIP / 03 Shower / Tub Plumbing ### [The tub won't hold water. Almost everything that's wrong is behind two screws in the overflow plate.]() Six different mechanisms all get called a tub stopper, and the repair is different for each one. Most of the time you never open a wall. And the hard part usually isn't the fix, it's getting the old one out. 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