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Dual Duct

Dual Duct

Dual Duct

(OP)
thread403-205523: DUAL DUCT SYSTEMS
The question was asked about sizing ductwork for a dual duct constant volume air handler.  The duct sizes will be a result of the heating and cooling loads and depends on where the facility is located on the planet and how the building envelope insulation is constructed and the internal load of the space.  These conditions determine the space load. Generally it takes about twice as much cool air as hot air to satisfy the same zone.  The cooling duct size for a dual duct constant volume system should be about  twice the area of the heating duct.  The cold and hot decks follow this premise.  The total fan volume is fixed at max load and doesn't care what size the ductwork is but fan volume should vary to maintain constant duct static pressure set point in order to maintain proper airflow at the mixing boxes as the call for the volume of cold and hot air varies to satify the thermostat set point.  

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