A dripping faucet or a running toilet.
Put a few drops of food coloring into the toilet tank and do not flush. If color appears in the bowl within fifteen minutes, the flapper is leaking and water is running continuously from the tank into the bowl and away. That is the single most common cause of a water bill that has jumped for no visible reason, and the part costs almost nothing.
A dripping faucet is annoying. A running toilet is expensive.
Why the toilet matters more
A faucet drip loses a steady, small amount. A leaking flapper lets the tank drain into the bowl continuously, and the fill valve keeps topping it up, all day and all night. It is often silent, because the water goes down the bowl rather than splashing, and the only symptoms are a bill and occasionally a faint hiss or the fill valve running briefly every few minutes with nobody near it.
That last one, a toilet that refills itself periodically, is called ghost flushing and it is proof of a leak.
Fixing the toilet
The flapper. The rubber seal at the bottom of the tank. They harden, distort, and stop sealing, usually after a few years, and faster where a tank cleaning product has been used. Replacing one is a five minute job with the water off at the valve behind the toilet, no tools, and the old one is the pattern you take to buy the new one.
The chain. Too short and it holds the flapper slightly open. Too long and it gets trapped underneath. There should be a little slack.
The fill valve. If the water level is above the overflow tube, it is running continuously down the overflow. Adjust the float so the level sits below the top of the tube. If it will not hold, replace the valve, which is a slightly bigger but still straightforward job.
The flush valve seat, if a new flapper still leaks. Feel for pitting or mineral build up on the seat and clean it.
Fixing a dripping faucet
Nearly always the cartridge or the washer inside, and it is a repair rather than a replacement in most cases.
- Close the isolation valve under the sink.
- Open the faucet to release the pressure.
- Plug the drain, because the small parts inside are exactly the size that goes down it.
- Take the old cartridge to buy the new one. There are many and they are not interchangeable.
A faucet that drips from the spout is usually the cartridge or washer. A faucet that leaks from the base of the spout is usually an O ring. A faucet that leaks from the handle is usually the packing or a seal at the stem.
The other things that quietly run
- An outdoor spigot dripping where nobody goes.
- A water softener stuck in regeneration.
- A pressure relief valve on the water heater discharging, which is worth investigating properly since it can mean over pressure rather than a failed valve. What a leaking heater looks like.
- An irrigation system with a stuck valve or a broken line, which is on your side of the meter and can be substantial.
- A humidifier or an ice maker plumbed into the supply.
Confirming it with the meter
The definitive test, and it takes fifteen minutes.
Close every faucet and appliance, do not flush anything, and read the meter. Wait fifteen minutes without using water and read it again. If it has moved, you have a leak on your side, and the toilet is the first suspect.
If it has moved and you cannot find it, that is a different and more serious problem. Water going somewhere you cannot see.
Why it is worth doing now
Beyond the bill: a running toilet or a dripping faucet is a fixture telling you a part has failed, and the same parts fail in ways that are not so gentle. The supply hose behind that toilet is under pressure every minute of every day, and when one lets go it does not drip. Which is one of the more expensive things that can happen to a house.
While you have the tank lid off, look at the supply hose and the shut-off valve behind the toilet. If the valve will not turn, that is worth knowing before an emergency rather than during one. What to do when a valve will not turn.
The short version
Food coloring in the tank, no flush, check the bowl in fifteen minutes. A leaking flapper is the usual answer and it is a cheap part and a five minute job. Confirm anything else with the meter test. While you are there, check the supply hose and the shut-off valve behind the toilet.