If dry system piping is located in the attic, and the attic is not heated, what is the only type of pendant sprinkler that can be utilized on the system?

In this scenario, using a dry-type sprinkler is the best choice for a dry system located in an unheated attic. Dry-type sprinklers are designed specifically for environments where the temperature may be significantly below freezing, as they contain a thermal element that allows them to activate when the ambient temperature rises above a certain threshold, typically indicating a fire.

In an unheated attic, the risk of freezing is significant. Dry-type sprinklers utilize air in the system rather than water, which prevents the water from freezing within the pipes. This characteristic is essential for maintaining operational readiness in cold conditions.

The other types of sprinklers are not suitable for this type of system due to their design and intended use. For instance, special sprinklers may not have the necessary capabilities to operate effectively in below-freezing temperatures. Open nozzle type sprinklers are typically used in applications where water is always present, and could lead to freezing in unheated areas. Hip attic sprinklers, while they might be designed for specific attic configurations, may not provide the necessary protections in a dry system located in a cold environment. Thus, dry-type sprinklers emerge as the only feasible option to ensure fire safety without the risk of freezing in an unheated attic space.