Example - for a 3-bedroom, 2,000-sq.ft. home, with an exhaust-only ventilation strategy, and a target ventilation rate of 50 CFM you would need to install a 50 CFM fan with an adjustable-speed DC motor for continuous operation.
Quick Answer. Trenches were built during World War I to protect stalemated troops on both sides from artillery and rifle fire. Although the war began with rapid movement of the German army, when the Allied forces stopped the Germans, both sides dug trenches to help avoid losing territory they had gained.
Trenches are geological features formed by convergent boundaries. When two tectonic plates converge, the heavier plate is forced downward, creating a subduction zone. This process results in the formation of a trench. The Marianas Trench is an example of a trench formed by the convergence of two oceanic plates.