When your water heater begins to leak, you have to replace it fast. We'll show you how to install your own gas water heater in less than a day. Even if you don't need a new water heater right now, chances are you will within the next few years. Water heaters tend to last seven to 15 years. If yours is getting old, this article is also for you. Replacing a water heater isn't difficult if you're handy with basic tools and have a bit of experience soldering copper.