Large Diameter Hose (LDH) is used for which primary purpose?

Answer: A — Relay operations and long lays

Large Diameter Hose is used for relay operations and long lays. The big idea here is moving large volumes of water efficiently over distance. LDH has a larger internal diameter, which reduces friction losses as water travels through the hose. That means pumpers can deliver higher flows with less pressure drop when the hose runs are long or when the water source is far from the fire scene. In practice, LDH is laid out between multiple pumpers (relay operations) or over extended distances from a hydrant or draft site to the attack engine, enabling sustained supply without losing pressure at the nozzle. It’s not typically used for standpipe testing, irrigation, or interior ventilation/drying tasks, and firefighting isn’t limited to residential incidents. LDH’s strength lies in supply operations where volume and distance matter, which is why relay operations and long lays are its primary purpose.