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Installing a new water heater begins with the old heater. If you've got an heater, replace it with an heater, unless you're willing and able to run line and exhaust vents. If you've got a heater, stick with, unless your breaker box has room for and you're willing to install a new heater.
Install the water heater in an area where it won't be cramped. Leave at least 6" of clearance around it for ventilation. Don't set it next to flammables.  Turn the water heater so that access to the burner and controls is unobstructed. Place a level on the side of the water heater and plumb it with plastic shims. Use Teflon tape on the pipe threads. These fitting are directional and must be installed properly. Both have arrows showing the correct direction for installation. Tighten using two pipe wrenches or adjustable pliers.