1. Start with the age of the unit

A conventional tank heater runs about eight to twelve years; a tankless unit often reaches twenty. Find the serial number on the data plate — most manufacturers encode the build date in the first four characters. Everything else you decide follows from that number, because a repair on a two-year-old unit and the same repair on an eleven-year-old unit are completely different propositions.

2. Rusty water points at the tank itself

Discoloured hot water that runs clear on the cold side means corrosion inside the tank rather than in the supply. Sometimes that is a spent sacrificial anode, which is a genuine repair. Often it is the glass lining having failed, which is not repairable at any price.

3. Weigh the repair against the remaining life

A thermostat, a heating element or a pressure relief valve is a modest bill and worth doing on almost any unit that is not already past its expected life. A failed tank, a cracked heat exchanger or a gas valve on an older unit is usually a large enough share of replacement cost that you are paying twice.

The rough rule most plumbers use is simple: if the repair costs more than half a new unit and the heater is past two thirds of its expected life, replace it.

4. Efficiency belongs in the sum

A heater from a decade ago is meaningfully less efficient than a current one, and in a household using hot water daily that difference shows up on every bill for the next ten years. It does not justify replacing a healthy unit, but it does tilt a borderline decision.

“The most expensive water heater is the one you repair three times in its last two years and then replace anyway.”

5. A leaking tank ends the conversation

Water pooling under the unit from a fitting is a repair. Water seeping from the body of the tank is not — that is the tank wall itself, and there is no fix for it. Once a tank starts weeping it should be replaced promptly, because they tend to fail properly rather than politely.

If you find standing water under the heater, shut off the supply and the power or gas to the unit before anything else, then call someone. A tank that lets go holds a lot more water than most people expect.