The short answer
- The Department of Energy puts a typical tank water heater at 10 to 15 years and a tankless unit at around 20.
- Sediment, a used-up anode rod and unchecked thermal expansion are the three biggest reasons tanks fail early.
- Rusty hot water, rumbling, moisture at the base and a dripping relief valve are the warning signs.
- A tank in an attic or finished space should be replaced on age, not on failure. A leak there does real damage.
Typical lifespans, and how to tell your heater's age
The Department of Energy's figures are a fair starting point: 10 to 15 years for a storage tank, gas or electric, and around 20 years for a tankless unit, which has no tank to rust through. Plenty of tanks fall short of 10, and some go past 15, depending on the water, the installation and the maintenance.
To find your heater's age, look at the rating plate on the side. The manufacture date is usually coded into the serial number, often as a month and year in the first few characters. Each manufacturer uses its own pattern, and their websites explain how to read it.
Sediment: the slow insulator
Minerals and grit settle to the bottom of a tank. On a gas heater, that layer sits between the burner and the water, so the steel bottom runs hotter and the burner runs longer. The rumbling and popping you hear is water trapped under the sediment flashing to steam.
Spartanburg Water's lake-fed supply is soft, so mineral buildup here is slower than in hard-water parts of the country, but some sediment still collects. Draining a few gallons from the valve at the bottom once a year, or a full flush, keeps it from building up. On a tank that has gone years without one, a flush can stir up more than it removes, and a technician can judge whether it is worth doing.
The anode rod: the part that rusts so the tank does not
Inside every steel tank is an anode rod, a length of magnesium or aluminum that corrodes on purpose so the tank lining does not. Once the rod is used up, the water goes to work on the tank itself, and rust-through follows.
Anode rods are replaceable, and checking one every few years is the single best thing you can do to stretch a tank's life. Water softeners, very hot settings and some water chemistry use them up faster. A rotten-egg smell in the hot water is also an anode issue, and a different rod material often cures it.
Thermal expansion and water pressure
Water expands when it is heated. In an open system, that extra volume pushes back into the city main. But many homes now have a check valve or pressure-reducing valve at the meter or main, which closes the system. Then every heating cycle spikes the pressure inside the tank, flexing the steel and the fittings over and over.
An expansion tank on the cold line gives that water somewhere to go. The plumbing code also requires a pressure-reducing valve where street pressure is over 80 psi. A relief valve that drips after the heater runs, or a faucet that spits when first opened, are clues that pressure is the problem.
Signs a water heater is near the end
Any one of these on a tank that is past 10 years is a reason to start planning a replacement rather than waiting for the failure:
- Rusty or brown hot water, when the cold water runs clear
- Rumbling, popping or banging as it heats
- Moisture, rust or a small puddle around the base
- A relief valve that drips or discharges
- Hot water that runs out noticeably faster than it used to
- Rust or corrosion on the fittings at the top, or on the gas burner area
When to call a pro
Call when the tank shows water at the base, rust in the hot water or a discharging relief valve, or when it is past 10 years and sits in an attic, a closet or above a finished floor. A yearly flush and an anode check every few years add years to a tank.