DUAL DUCT FAN SYSTEM The physical arrangement of most dual duct systems is illustrated above. Original design caused a constant mixed air temperature of 55°F. When the outside air exceeded 70°F the mixing dampers returned to their minimum ventilation setting. The cold duct was controlled to 55°F constantly via the cooling coil for summer operation. The hot duct temperature was scheduled based on outside air temperature variation. The colder the outside air temperature becomes, the hotter the hot duct becomes. The thermostats blended air from the cold duct with air from the hot duct in a mixing box, usually in the ceiling of the room with the thermostat, to achieve a supply air temperature satisfying the individual zone. Modern logic knows the maximum demand for cooling and heating from all the zones. If all zone signals are within the range of the mixing boxes’ damper motors, the heating and cooling are kept at a minimum value. If any zone opens its cold duct 100% and closes its hot duct 100%, that zone can gradually cause the cold duct temperature to be lowered. Conversely, if any zone opens its hot duct 100% and closes its cold duct 100%, that zone can gradually raise the hot duct temperature. The demand for cold duct is limited to a minimum of 55°F and the demand for hot duct is limited by an outside air reset schedule similar to original design. The system receives at least minimum ventilation during occupied mode. The set up and logic for a dual duct system is similar to that for a multizone. RETURN FAN EXHAUST AIR NC RETURN AIR NO COOLING COIL COLD DUCT SUPPLY FAN MIXING OUTDOOR MIXED BOX TO SPACE AIR N C AIR (TYPICAL) M HOT DUCT THERMOSTAT HEATING (TYPICAL) COIL 7.53
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