In which scenario are two water heaters typically installed?

The installation of two water heaters in parallel with similar pipe lengths is beneficial for several reasons. When water heaters are installed in parallel, they work simultaneously to provide a greater volume of hot water to meet demand. This is particularly advantageous in situations where high hot water usage is anticipated, such as in larger households or commercial settings. Ensuring that the pipe lengths are similar helps to balance the flow between the two units, promoting efficiency and reducing wear on each water heater.

In contrast, installing water heaters in series might not maximize capacity effectively, as one heater may heat water before it reaches the next, leading to potential inefficiency. Vertical alignment could be relevant for space-saving designs, but it doesn’t inherently provide the benefits that parallel installation does. The concept of "only when necessary" suggests a more situational approach that doesn’t leverage the benefits of increased hot water availability that parallel installations provide.