Old Age - It’s time for you to think about replacement if your heater is 15 years or older. Heaters typically last 18 years on average. Trying to push it past its limit will only make your life harder.
Constant Repairs - You shouldn’t have to repair your heater more than once every few years. If each winter brings a heating problem, then you need a new unit.
High Energy Bills - Everyone's energy bill has increased, but if you notice that your usage has increased dramatically without any changes, then you may have a performance issue.
Unreliable Operation - Can you rely on your heater? If the answer is "no" or even a hesitant "yes" you should consider replacing your heater.