DDC Programming Logic

HVAC Control DDC Logic Quickpage Jump links:

Analog Network Input

Binary Network Input

Ramp Step Up Control

Ramp Step Down Control

Reset Limit Logic Control

Runtime Logic Block

Digital Switch Logic Block

DDC Control Related Links

HVAC DDC Programming Logic and Symbols Page 2

Basic HVAC Control DDC Program Logic 101 Page 2

Analog Network Input

The analog network input is similar to an analog input except this input is not hardwired to the local controller but on a remote controller in the network. It is very important that the communication work effectively for this analog network input to work. An example of this is the outside air temperature which is hardwired to a DDC controller in the penthouse of a multistory building. The building engineer only wants the boilers to fire if the outside air temperature is less than 55° F. Instead having an outside air temperature device hardwired to the boiler controller an analog network input can be placed into the boiler controller program and linked to the outside air temperature reading which is in the penthouse controller where the outside air temperature thermistor is harwired. This saves in devices and wire necessary for optimum control.

Binary Network Input

The binary network input works the same way as the analog network input except it receives a binary input from the network from a remote controller over the comm trunk. An example of this is: There two boilers in a large boiler room and only one combustion air damper. The end switch to prove that the damper is open is wired to one boiler controller so it gets permission to fired from the binary input which the end switch is wired to on its controller and the other boiler can get its permission from the binary network input which is read over the network from the other boiler controller.

Ramp Step Up Control

The Ramp Step Up logic block allows you to slowly change the analog signal to a piece of equipment when it is starting. An example of a Ramp Step Up logic block would be: You have an air handler that has a VFD for the blower and you know when the blower starts the program is going to call for 100% because the static pressure is zero. You don't want the blower coming on and going to 100% until all the VAV boxes send their requests for airflow through the program so you set the Ramp Step Up logic block to increase the program analog signal to 10% every 30 seconds so even if the signal from the program is 100% this block will only allow it to go 100% after a specific period of time (whatever the parameters are set to within this block).

Ramp Step Down

The Ramp Step Down logic block works in the same manner as the Ramp Step Up logic block except it works when shutting down a device like a pump, motor, or damper. Based in increments of seconds or minutes you would use the Ramp Step Down logic block any type of analog signal which you would want to slow the process of stopping or slowing down the signal to the device.

Reset Limit Control Logic Block

The Reset Limit logic block allows one to set a reset schedule such as set point control for heating or cooling. If you wanted base the boilers set point on outside air temperature you would link the left side of this block to the outside air and put the outside air temperatures you want the set point to be based on. On the right side of this block you would put the set point numbers. For example, if the outside air temperature were 65° F. and you wanted your lowest set point for this outside air temperature to be 120° F. and at 40° F. outside air temperature you wanted your set point to be 180° F. then the Reset Limit would change your set points based on a linear curve of these numbers you put into the Reset Limit block. Any outside air temperature above 65° F. would give you a set point of 120° F. and any outside air temperature below 40° F. would give you a set point of 180° F.

Runtime Logic Block

The Runtime logic block is a great tool for keeping track of the runtime hours on a motor or other equipment for maintenance scheduling purposes. The Runtime block can be set for a specific number of hours and then it will generate a maintenance notice. This makes it easy for maintenancing of all equipment and its based on actual runtime of the equipment.

Digital Switch

The Digital Switch logic block is just like a plain switch except it is digital and relies on input from another source to trigger it to on an off. When in the off position an analog value can be assigned to this block for a minimum output from the Digital Switch logic block.

There are many other types of DDC logic blocks that are used in DDC controls programming and these are just a sample of the blocks and the unlimited control one can have by utilizing such an amazing system of 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 Controls Programming Example | DDC Controls Technician Descriptions | DDC HVAC Control Logic Page 2 Top

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