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Duct for air leaving an equipment

Duct to access fresh outside air only for economizing, not ventilation

Duct used to expel air back outside

Duct for exhausting combustion

Duct with air entering an equipment

Duct where fresh outside air and return air is mixed together

Duct to access fresh outside air for both ventilation and economizer

Duct returning air back to equipment

Duct to access fresh outside air only for ventilation, not economizing