These two services share a product and almost nothing else, so let us separate them plainly. Replacement — covered on our water heater replacement page — removes a dead or dying unit from an existing setup and puts a new one in its place, reusing the location, fuel, venting, and connections. Installation, this page, creates a setup where none exists: a new location, new supply and drain runs, new venting or electrical work, sized and placed as a fresh design decision. If a like-for-like swap is what you need, the replacement page will serve you better.
The distinction matters practically, not just tidily: installation involves design choices — where the unit lives, how far hot water travels to its fixtures, whether one unit or a point-of-use companion serves the layout — and rough-in work that replacement never touches. It is usually part of a larger project, a renovation or addition, and it rewards being planned alongside that project’s other trades rather than after the walls close.