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Unitary Automation Controller on Air Handling Units

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  rev. 2008-11-25        

Existing Conditions

The existing equipment is under basic pneumatic <electric> <manual> control that is not working well.  With this type of control, equipment runs longer than necessary to cover all possible expected operating conditions.  These controls are subject to tampering when they fail to provide comfort in the space, and cannot be set up with the energy-saving strategies available with modern digital controls.

Retrofit Conditions

We recommend replacing existing controls with a 'unitary' automation controller.  This type of controller is a standalone unit with input and output capability and a small LCD display and keypad.  It would be programmed at the time of installation according to energy-conserving strategies specified by the design engineer.  The building operator would be able to use the keypad to change basic settings such as occupancy schedules and temperature setpoints.  Generally these controllers are members of product families that can be networked together into facility-wide DDC (Direct Digital Control) systems.  DDC systems can include many safety and convenience features such as full graphic terminal operation, remote access via phone line, dial out of alarms, security monitoring, and trending of energy and equipment data.

For the purpose of this study, we are recommending the most cost-effective approach, which is the installation of small local controllers intended to save energy.  The decision to connect several units together into a single network can be deferred until clear operating benefits are identified.

For this unit, we recommend the following strategies:

•  Scheduled start/stop with adaptive optimized startup.

•  Night setback of space temperature, both heating and cooling.  Setpoints adjustable at local terminal.

•  Unoccupied mode with fan cycling to maintain setback temperature range and no ventilation.

•  Timed setback override (adjustable period) <for each zone>.  Simple override activation from space sensor.

•  PID control of supply air temperature, reset from space.  Reset curve endpoints and room sensor authority adjustable from local terminal.

•  PID control of free cooling economizer, mixing to 13°C operator-adjustable setpoint.

•  Economizer dry bulb override to minimum position when outside air temperature is above 20°C.

•  <Scheduled seasonal lockout for electric baseboard heat.>

•  Holiday scheduling.

•  Night purge during cooling season, when nighttime outside temperature allows.

•  Auxiliary output for scheduling associated exhaust fans.

•  Text alarm messages displayed at local terminal.

•  <<Individual control of zone space temperature through <volume control boxes> <zone dampers>.>>

•  <Air handler bypass, controlling to duct pressure.>

•  <Discriminator strategy, to reset both hot and cold deck temperatures based on greatest zone requirements.>

Further Benefits



Application Details


Issues and Concerns






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