The realistic numbers
A standard tank water heater lasts somewhere between eight and twelve years. Some reach fifteen. Some are done at six. The range is wide because the water going through it matters more than the badge on the front.
A tankless unit runs longer — commonly fifteen to twenty years — because there is no steel tank sitting full of heated water waiting to corrode. It buys that life by needing to be descaled, and a tankless unit that is never serviced does not reach the top of that range.
Those are service-life ranges for the appliance in general. We cannot tell you how long the one in your garage has left without looking at it, and anybody who quotes you a number over the phone is guessing.
Check the date. Most manufacturers encode it in the serial number on the data plate, usually as the first four characters — two digits for the week or month, two for the year. If the label is gone, the install permit or the previous owner's paperwork may have it.
What shortens them in Pasco County
Hard water. Most of this county runs on hard water, and the dissolved calcium and magnesium drop out as scale every time the water is heated. In a tank it settles as a layer of sediment on the bottom. In a tankless unit it coats the heat exchanger.
Sediment is not just dead weight. It insulates the bottom of the tank from the burner, so the steel runs hotter than it was designed to while the water above it heats slower. That is how a ten-year tank becomes a six-year tank.
No expansion tank, or a failed one. A closed plumbing system with a backflow preventer has nowhere to put the pressure when water expands as it heats, so the tank and the relief valve absorb it every cycle. Thermal expansion control is required on systems like that for a reason, and a lot of older installations either never had it or have one with a dead bladder.
An anode rod that was never changed. The rod is a sacrificial piece of metal that corrodes so the tank does not. Once it is consumed, the tank is what corrodes next. On hard water it goes faster, and almost nobody replaces it.
Garage installs. A water heater in an unconditioned Florida garage cycles through heat and humidity all year, and the fittings and jacket show it.
Sediment on the bottom of a tank does not slow it down, it cooks it — the steel runs hotter than it was built for every time the burner fires.
The signs it is near the end
Rusty or brown hot water that runs clear on the cold side. That is the inside of the tank arriving at your tap. It means the lining is gone.
Water that runs out before it used to, or never gets as hot. Sediment takes up volume and blocks heat transfer, so the tank holds less usable hot water than the label says.
Rumbling, popping or a kettle sound when it heats. That is water boiling in pockets under the sediment layer.
Any moisture at the base. Condensation happens, but a damp ring that comes back, rust streaks down the jacket, or staining on the pan means the tank is seeping. A seeping tank is not repaired. It is replaced.
A relief valve that discharges on its own. Sometimes that is the valve, often it is pressure the system has nowhere to put. Either way it wants looking at, not capping.
Age on its own. A twelve-year-old tank with no symptoms is still a twelve-year-old tank, and the failure mode is a floor full of water.
Why a failure is a flooding problem
A tank holds forty, fifty, sometimes eighty gallons. When the shell finally lets go, that volume hits the floor at once, and the cold supply keeps feeding it until somebody shuts a valve. An unattended failure can run for hours.
Most Florida homes put the heater in a garage or an interior closet on a slab. A garage will usually drain. A closet in the middle of the house will push water into drywall, baseboard, cabinets and flooring, and that damage costs far more than the appliance did.
A drain pan plumbed to somewhere useful handles a weep. It does not handle a tank letting go. If yours sits over finished floor, that is worth knowing before it matters.
Whether any of that is covered is between you and your carrier. Policies commonly treat the failed appliance and the water damage it caused as two different things, and we do not know what yours says. Photograph everything, dated, before anyone starts pulling up flooring.
Getting the years you paid for
Flush the tank annually. On hard water that is the single thing that changes the outcome, because it takes the sediment out before it can insulate the bottom of the tank.
Have the anode rod checked partway through the heater's life and replaced if it is consumed. It is a cheap part doing the work the tank would otherwise do itself.
Confirm the system has working thermal expansion control, and that the bladder in the expansion tank is still alive. A dead one looks exactly like a live one from the outside.
Descale a tankless unit on the schedule the manufacturer sets. Skipping it is how a twenty-year unit fails in eight.
If the water is hard enough — and in much of this county it is — treating it upstream protects the heater, the fixtures and the appliances at the same time.