MULTIZONE FAN SYSTEM The physical arrangement of most multizones 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 deck was controlled to 55°F constantly via the cooling coil for summer operation. The hot deck temperature was scheduled based on outside air temperature variation. The colder the outside air temperature, the hotter the hot deck. The thermostats blended air from the cold deck with air from the hot deck 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 zone damper motors, the heating and cooling are kept at a minimum value. If any zone opens its cold deck 100% and closes its hot deck 100%, that zone can gradually cause the cold deck temperature to be lowered. Conversely, if any zone opens its hot deck 100% and closes its cold deck 100%, that zone can gradually raise the hot deck temperature. The demand for cold deck is limited to a minimum of 55°F and the demand for hot deck is limited by an outside air reset schedule similar to original design. The system receives at least minimum ventilation during occupied mode. The gas sections of Wendell Statton PS and West Hill C.I. in Scarborough and the Administration Office and Henry Street HS in Durham are all multizones. The saving achieved with modern logic are illustrated later in this book. THREE ZONE MULTIZONE FAN SYSTEM RETURN FAN EXHAUST RETURN AIR NC AIR ZONE ZONE THERMOSTATS NO MOTORS COLD SUPPLY FAN DECK ZONE ZONE DAMPERS DUCTS OUTDOOR NC MIXED AIR AIR HOT DECK 7.52 M M M
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