How does a dry pipe system differ from a wet pipe system?

A dry pipe system is designed to address the risk of freezing in colder environments. Unlike a wet pipe system that is constantly filled with water, a dry pipe system maintains pressurized air or nitrogen within the pipes. This pressurized air prevents water from entering the pipes until an activation occurs, such as a fire triggering a sprinkler head. When a sprinkler head opens due to heat from a fire, the pressure drops, allowing water to be released from the dry pipe into the system through the open sprinkler head.

This feature makes dry pipe systems particularly useful in unheated areas like warehouses or garages where temperatures can drop below freezing. The need for air pressure to keep water out is what distinctly characterizes these systems, setting them apart from wet pipe systems, which contain water at all times.

The other options do not accurately represent the characteristics of a dry pipe system. For example, a dry pipe does not use fiberglass pipes specifically, nor is it inherently less expensive to install than a wet pipe system. The uniqueness of the air-containing aspect is what confirms the correctness of the chosen answer.