VAV with System Return Air Control of AHU Heating

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

This variable air volume system uses system return air temperature to determine air handling unit heating and cooling discharge temperatures. The cooling coil is set to cool to 55°F under warm return air conditions and proportionally resets to 65°F when the system return air temperature is cooler. The heating coil is set to heat to 85°F under cold return air conditions and proportionally resets to 55°F when the system return air is warmer.

The air handling heating coil controller is configured so as to mimic the zone heating coil controller, though it senses system return air temperature and not zone air temperature. This minimizes the heating happening at the zone level by maximizing the heat 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 cooling coil controller and 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 prevent simultaneous heating and cooling in the air handling unit, the temperature discharge controllers for both the air handling unit cooling coil and air handling unit heating coil are interlocked with the energy recovery controllers. 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 heating operation, the cooling coil controller – and thus the cooling coil – are disabled. If the system is in cooling operation, the air handling unit heating coil controller – and thus the air handling unit heating coil – are disabled. The controllers are configured such that the system is always either in heating operation or cooling operation when the fan is running.

Because this air handling unit responds to system return air temperature it is expected that there may be unmet cooling hours. A zone level heating coil provides additional heating to a space or zone when the supply air from the system does not sufficiently meet the load. As there is no zone level cooling device, it is possible that the amount of cooling requested based on system return air temperature will not be sufficient to meet the needs of every individual space or zone. Likewise, users should expect some unmet cooling hours when using this configuration.

Installation Instructions

Un-pack the .ASP file into your project directory or the HVAC Template library folder in the IESVE installation folder (…/apps/Templates/HVAC/Library)

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