DDC Programming Logic

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HVAC DDC Programming Logic Block Arrangements

DDC control logic blocks have practically an unlimited way of arranging the blocks for HVAC control. Below is an example of a line of logic blocks arranged for static pressure control of a VAV air handler supply fan using a variable frequency drive. This same arrangement could also be used for static pressure control using a bypass damper instead of a VFD. The input and output logic blocks, here the analog input and the analog output, are hard wired to control devices. The input logic block in this case is hard wired to a static pressure transducer which is placed in the duct work and reads static pressure. The analog output in this control logic arrangement is hard wired to a variable frequency drive and sends the appropriate signal to the VFD depending on what the program calls for in output. Secondary inputs are required in the program and are linked via the program. The secondary input requirement for this program is the fan status input which is on a completely different line (not shown). The fan status input is hard wired to a CT which is at the supply fan and reads either a 0 or 1. 0 being off and 1 being on and this depends on the contacts on the CT and whether they are closed or open.

There are many other configurations on the board and in the program which a trained DDC Start-up Technician will be familiar with and set prior to energizing the DDC controller for the first time. There are jumper settings at the DDC controller and often times at the devices themselves. Variable Frequency Drives also have jumper settings and a program array which must be configured to be compatible with the DDC control signal. There are also parameters inside the drive that must be matched up to the motor which include voltage, amperage, and service factor information. These settings depend on the VFD manufacturer and their requirements for settings. All of these settings and parameter changes need the attention of a qualified DDC Start-up Technician or Engineer before any DDC Controlled equipment is put into service. Additional a qualified testing and balancing company should balance the entire DDC system and HVAC systems before the system is released to the owner for use.

Program Configuration

Each block in most programs are configurable by either navigating to a separate page or by clicking on the block. Clicking on the logic block will give you a pop up window where additional parameters including physical input/output assignments can be made. It is important when assigning these inputs and outputs that the parameters are verified and correct. A DDC start-up technician should set all the parameters necessary for the proper functioning of the program including tuning of the program to ensure efficient and proper operation of the HVAC equipment.

This arrangement of DDC control logic blocks is for static pressure control of a supply fan on a VAV air handler unit. Reading from left to right:

• The analog input for the duct static pressure transducer which is mounted in the duct work in a strategic location according to control engineering parameters. This number feeds into the PID logic block along with the static pressure set point and the input from another line of logic for fan status which is a binary input.
• The PID logic block processes the input it receives and makes a calculation for the necessary amount of output for the supply fan VFD. It sends this output to the next logic block.
• The low limit analog signal logic block is the minimum percentage that the fan should run when there is low demand. If the setting on this logic block was 20% it would allow all analog signals over 20% but if the PID logic block sent it a 10% input the low limit analog signal block would only allow a minimum of a 20% output.
• The digital switch will change according to fan status input. If the binary value of the fan status goes to zero then this switch will turn off giving all output to the right of this block a 0% signal.
• The ramp up analog block will limit the signal based on time so the fan doesn't go to 100% in 1 second if the program calls for 100% fan output signal to the next block.
• This is the analog output signal to the variable frequency drive which controls the speed of the fan.

There are many other arrangements of DDC logic blocks that are used in DDC controls programming and this is just a sample of the arrangements of blocks and the unlimited control one can have by utilizing such an amazing system of HVAC DDC control.

DDC Related Links

DDC HVAC Control Home 1 | DDC Technical Aspects 2 | DDC HVAC Control VAV's | DDC HVAC Control Economizers | DDC HVAC Control Programming Logic Page 1 | DDC HVAC Control Programming Logic Page 2 | DDC Controls Technician Descriptions | Energy Recovery Wheels | Variable Speed Drives | DDC HVAC Control Logic Page 3 Top

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