How a toilet fill valve works

The float falls, the valve opens; the float rises, the valve shuts. That is all a fill valve knows how to do, and a toilet that hisses, runs or refills by itself in the night is usually that one motion going wrong, or the flapper next to it letting the water out.

What the fill valve does

Korky, which makes them, describes the job plainly. The fill valve, also called a ballcock, refills fresh water in the tank after each flush. As water leaves the tank, the float drops and opens the valve to let more in. Once the tank refills to the right level, the float rises and shuts the valve off. The float takes more than one form. Portland’s water bureau describes the style in use for decades as a ball on a float arm, and the newer style as a float that rides up and down the valve. Fluidmaster’s valve uses a float cup on the valve body, and Korky’s uses a float inside the valve.

Two jobs: the tank and the bowl

The bowl needs refilling too, and once the flapper has closed the tank no longer feeds it. So the fill valve does a second job through a small hose. Korky: the fill valve also refills the toilet bowl after a flush through the refill tube, which connects the top of the fill valve to the overflow tube, so the bowl refills while the tank refills.

The overflow tube is the top of the flush valve, and its open rim stands above the water line. Korky gives it two jobs. It directs that refill water down to the bowl, and it prevents a flood: if a failing fill valve keeps running, the rising water spills into the tube and down to the bowl instead of over the rim of the tank.

Where the water line belongs

The fill valve decides how full the tank gets, and every source we opened that sets the water line puts it below the top of the overflow pipe. How far below is where they part: Fluidmaster and Korky each give one figure for their own valves, and Portland’s water bureau gives a range. So we leave the number to the instructions for your valve.

Set the valve or its float too high and, as Korky puts it, the valve will not shut off. Denver Water’s description of that fault: the float arm is not set properly, which causes water to constantly flow down the overflow tube. The valve keeps running, the bowl keeps receiving, and the meter keeps turning.

The critical level: why the valve stands tall

A fill valve should carry a mark, C.L., for critical level. Fluidmaster’s sheets say the mark must sit at least 1 inch above the top of the overflow pipe, and its 400A sheet, dated 2014, calls that a requirement of what it names the Universal Plumbing Code, without giving an edition. The building inspection office in Bloomington, Minnesota, prints the reason on its information sheet: the critical element in the installation is a minimum 1 inch air gap, which prevents the water in the tank from being siphoned back into the house drinking water.

Tank water is not drinking water, and the air gap is what keeps the supply line from siphoning it backward. Bloomington’s handout lists the rules behind it: every flush tank must have an approved anti-siphon ballcock, it must deliver water through an air gap or a vacuum breaker, and its critical level must be 1 inch above the top of the overflow. Where a valve has no C.L. mark, the bottom of its vacuum breaker is taken as the critical level. That handout is dated 2006 and does not say which code, or which edition, its rules come from. The plumbing code your state or city has adopted, in the edition it adopted, is the one that applies; every source we opened that gives a figure gives the same 1 inch.

Two mistakes get made at the overflow pipe, and the sources warn against both:

• Cutting the overflow tube short. Bloomington’s handout says not to cut the overflow tube off, because the flush will be incomplete. Fluidmaster’s sheets give the other boundary: the top of the overflow pipe should sit at least 1 inch below the tank lever hole. The two Fluidmaster sheets that mention cutting the pipe to that line, the 507 flush valve sheet and the 400ARHRLS fill valve sheet, both place the cut in installing a flush valve; the fill valve sheets set the C.L. height by adjusting the fill valve itself.
• Pushing the refill tube down into the overflow pipe. Fluidmaster’s flush valve sheet says not to put the refill tube into the overflow pipe: it will siphon water down the pipe, and the valve will turn on and off trying to keep the tank full. Korky says the same, that a refill tube pushed into the overflow tube can create a siphon. The flow from the refill tube belongs just above the overflow pipe, and the clip holds it there.

How a fill valve fails

Korky puts the life of a fill valve at typically 5 to 7 years, depending on the water, and gives the reason: some water sources carry sediment that can build up inside the valve and cause poor performance. Fluidmaster’s 400ARHRLS sheet says much the same about a valve that will not turn off: dirt may have got into the valve when the water was turned on, and the sheet’s remedy is to flush the debris out.

The symptoms Korky lists are mostly noises. A constant hiss means water is continuously escaping into the bowl and the valve is staying on to replace it. Korky lists three common causes: a worn flapper or flush valve seal leaking into the bowl, a fill valve that is not shutting off fully, and a water level set too high. Squealing or whistling comes from a restricted supply line, high water pressure, mineral or sediment buildup, a partly stuck float or diaphragm, or worn rubber parts inside the valve. A low vibrating hum, which Korky calls resonance, comes from a loose or weakened diaphragm or seal inside the valve, from water pressure making the diaphragm flutter, or from a supply line vibrating with sudden pressure changes. A bang when the valve shuts is water hammer, which Korky ties to high water pressure or a failing valve that closes too quickly, and if a new fill valve does not cure it, Korky says to call a plumber.

One cause is self-inflicted. Fluidmaster’s sheet says, in capitals, not to use in-tank drop-in bowl cleaners containing bleach or chlorine, because they damage tank components and may cause flooding and property damage.

Fill valve or flapper?

The two parts work as a pair, and a fault in either one shows the same way: a tank that refills when nobody has flushed. IBHS says to replace the flapper or the fill valve assembly if you notice intermittent or constant tank refilling when the toilet is not in use. Often the fill valve is only answering for the flapper. Korky explains it: when the flapper or flush valve seal leaks, the water level drops and the fill valve turns on to replace what was lost, and Fluidmaster’s 400A sheet reads a valve that turns on and off by itself the same way, as a tank losing water. The fill valve’s own fault is the other one, set out above: a valve set too high will not shut off, as Korky puts it, and Denver Water describes the water running constantly down the overflow tube.

EPA’s WaterSense program gives the test for a toilet leak: a few drops of food coloring in the tank, and color that shows up in the bowl means a leak. The sources we read do not agree on how long to wait, so follow the leaflet or kit you are using. Denver Water adds that the darker the color, the larger the leak, and EPA says to flush once the test is done so the dye does not stain the tank. EPA also says the flapper, the rubber part that seals the tank from the bowl, should be checked periodically and replaced at least every five years. The questions in order, one at a time, are in the Vent, Valve and Volt running toilet troubleshooter.

It is worth an afternoon. Denver Water, citing EPA, says an average leaking toilet can waste about 200 gallons of water every day.

Replacing one

Korky calls replacing a fill valve normal maintenance, and the instruction sheet that comes with the new valve is the one to follow; we will not repeat it here. The points below are in both Korky’s and Fluidmaster’s instructions unless we name a source, and they are worth knowing before you start:

• Turn off the water supply to the toilet, flush, and sponge out what is left before disconnecting anything. If the shutoff valve will not close all the way, stop here: IBHS says to operate that valve to make sure it will shut the water off and to replace it if needed, and that job is on the plumber’s list below.
• Set the valve height before anything is tightened. Fluidmaster: the C.L. mark clears the top of the overflow pipe by at least 1 inch. Korky: the water level mark stamped on its valves goes below the top of the overflow tube, by the distance its instructions give.
• Hand tighten only. Both makers say it for the mounting nut and for the supply connection, and Korky adds no sealants or tape on the supply connection. Fluidmaster gives the reason: an overtightened nut can damage the fill valve or the nut itself, or crack the tank, and the result is flooding.
• Turn the water back on and watch the tank fill. Fluidmaster says to check for leaks, and its remedy for a leak at the bottom of the tank is to turn the nut just enough to stop it. If that small turn does not stop it, stop there and shut the water off: Fluidmaster warns that overtightening can crack the tank, and Portland’s water bureau says to call a plumber if you have any concerns about a repair.
• If a new valve will not shut off, Fluidmaster’s 400A and 400ARHRLS sheets point to debris and give a way to flush it out with the cap removed. That procedure is for those valves only; follow it from the sheet. For a Korky valve that will not shut off, Korky’s first checks are the valve height and a refill tube pushed into the overflow tube.
• Fluidmaster says to inspect the supply line and replace it if it is worn or more than 5 years old, because it is under pressure.

What is yours, and what is a plumber’s

Yours: the dye test, lowering the water line, re-clipping a refill tube, and replacing a fill valve or flapper by the maker’s instructions. IBHS suggests looking at the flushing mechanism and the supply line every 6 months, and operating the shutoff valve to make sure the water supply will actually shut off.

A plumber’s: a shutoff valve that will not close or will not stop dripping, a supply connection that still leaks after the small turn Fluidmaster allows, a cracked tank, and a toilet that backs up. If the tank or bowl ever begins to overflow, IBHS’s instruction is the one to remember: turn off the water at the supply valve.

Related

• Almanac: March, Fix a Leak Week
• How a P-trap works, the water seal under a sink or tub
• Glossary: fill valve, overflow tube, critical level, flapper, backflow preventer