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VAV with Zone Control of AHU Heating

Modified prototype variable air volume system to control heating in the air handling unit based on zone return air temperature.

This variable air volume system uses zone return air temperature to determine air handling unit heating discharge temperatures. The heating coil is set to heat to 85°F under cold zone return air conditions and proportionally resets to 55°F when the zone return air is warmer. Each layer of the multiplex with this controller active will have a ‘vote’ for the discharge temperature of the air handling unit heating coil. Whichever vote requires the warmest temperature will prevail.

The air handling heating coil controller is configured so as to mimic the zone heating coil controller, though it controls system air temperature and not zone air temperature. This eliminates the need for heating happening at the zone level when heat is delivered by the air handling unit.

This system is fully autosizeable but the temperature controller for the heating coil in the air handling unit requires manual updating. Any changes made in System Parameters to setpoints, schedules, or discharge temperatures will be updated in the zone heating coil controller. Changes in discharge heating temperature will need to be entered into the air handling unit heating coil controller by the user.

In order to minimize simultaneous heating and cooling in the air handling unit, the temperature discharge controller for the air handling unit heating coil is interlocked with the energy recovery controller. It is NOT necessary for the system to utilize energy recovery for the controllers to function in this capacity. By default, the energy recovery controllers sense return air temperature and determine whether the system is in heating operation or cooling operation. If the system is in cooling operation, the air handling unit heating coil controller – and thus the air handling unit heating coil – are disabled.

Because this air handling unit responds to zone return air temperature it is expected that there may be over heated hours. When one space requires significant heating, all of the spaces served by the air handling until will receive the warmest air. Because the variable air volume controls sequence air temperature ahead of air flow in heating operation, the spaces not requiring the warmest air handling unit air should be receiving minimum airflow. It is expected that even at minimum flow, some rooms will receive air significantly warmer than required to meet their heating load. As such, those spaces will be overheated.

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26th February 2018