Question: how do you know when you need to replace your water heater?
Answer: An Old Unit. If your water heater is older, you can find its exact age by looking for the serial number on the manufacturer's sticker near its top. ... 
Rusty Water. If your hot water is rusty, your water heater could be rusting on the inside, and it might start leaking soon. ... 
Not Enough Hot Water. ... 
Rumbling and Noise. ... 
Leaks.