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All air systems can be single duct or dual duct. Single duct systems provide cooling and heating using the same duct, but cannot provide them simultaneously. Dual duct and multi-zone systems can provide heating and cooling simultaneously. A dual duct system employs two air ducts, a cold air duct and a warm air duct, which run parallel to each other. In an air water system, both air and water are used for temperature control in the condition to spaces. The air and water are cooled or heated in a central plant. The air supplied is called primary air, and the water is called secondary water. Ducting systems with fans distribute the air and water pipelines, and pumps distribute the water.