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.
From there, the Germans dug trenches. When the British and French forces tried to pass these trenches, they were killed by machine guns and chemicals (chlorine gas). The Allied forces could not pass through those trenches effectively, and therefore, dug their own trenches. **************************************... Because of this, the warfare became slow because neither forces were capable of penetrating the opposing force's trenches. Thus, TRENCH WARFARE.