Price it before
the wrench.

Almost always one of two parts inside the tank. Either the flapper at the bottom is no longer sealing, so water seeps into the bowl and the tank keeps topping itself up, or the fill valve is set too high and water is spilling into the overflow tube. A few drops of food coloring in the tank tell you which: wait fifteen minutes without flushing, and color in the bowl means the flapper. Both are inexpensive parts, and a running toilet is a reason to replace a part, not the toilet.

Where the water comes from decides what it is. Water from the pipe on the side of the heater is the temperature and pressure relief valve, either relieving pressure as it is designed to or failing. Water at the drain valve near the bottom is often that valve. Water at the fittings on top is a connection. Those are all repairs. Water seeping from the tank body itself means the tank has corroded through, and a tank cannot be patched. In every case, turn off the power or gas and close the cold-water valve on top of the heater.

We would rather you did not. It often fails to clear a real blockage, and then it sits in the pipe, hard on older metal drains and dangerous to whoever opens the trap next, including you. Pull the stopper and clear the hair first, then try a plunger. If the drain is still slow, it needs a snake, not more chemicals.

Usually where the water line comes into the house: on the basement wall facing the street, in a crawlspace, in the garage near the water heater, or in a utility closet. In warmer climates it is sometimes outside near a hose bib. There is also a valve at the meter, but that one belongs to the water utility. Find yours before you need it, and turn it clockwise to close it.

The U.S. Department of Energy puts a storage tank water heater at 10 to 15 years and a tankless unit at more than 20. Hard water shortens a tank's life, and flushing the sediment and replacing the anode rod lengthens it. Age alone is not a reason to replace one that is working, but it is the honest starting point when a repair is being weighed against a new heater.

On an electric heater, check the breaker, then the red reset button behind the upper access panel, which trips when the water overheats. On a gas heater, check whether the pilot is lit. If either one fixes it but the problem keeps coming back, something is causing it and that is worth a visit.

When more than one fixture backs up at once, or a drain keeps clogging after it has been cleared, the blockage is in the main line and a camera is how you find out why: roots, a sag that holds water, or a broken section. If anyone recommends replacing a sewer line, ask to see the footage first. It is your pipe, and the video is the evidence.