The (Field) Technical Aspects of Direct Digital Control
A HVAC technician who knows nothing about HVAC control systems would wonder how these hvac control systems work. To understand hvac controls, one must understand basic electricity and circuits. Starting with a large commercial VAV air handling unit with three stages of DX cooling and add all the necessary hvac ddc control points. We have variable frequency drives (VFD’s) on the return and supply blower fans. There is a mixed air damper which closes off the return and opens up to outside air for economizing and CO2 control. Static pressure control will be achieved with a duct pressure transducer and the HVAC control program through the frequency drives and blower motors. Direct Digital Control needs to monitor supply air temperature, mixed air temperature, and return air temperature along with outside air and outside humidity. We will also monitor the duct smoke detectors and a freezestat for safety reasons. This is the set-up and we need to make it work. We need to get a point count and set-up our inputs and outputs.
Input and Output Types
Digital or Binary Outputs
- Compressor Stage One
- Compressor Stage Two
- Compressor Stage Three
- Supply VFD Start/Stop
- Return VFD Start/Stop
- Supply VFD Speed
- Return VFD Speed
- Mixed Air Damper
Binary or Digital Inputs
- Smoke Detector
- Supply Fan Status
- Compressor Status (Only one compressor status for the first stage. We assume this compressor has two unloaders for the other two stages.)
- Supply Air Temperature
- Return Air Temperature
- Mixed Air Temperature
- Duct Static pressure
- Outside Air (this point along with humidity can be networked across a comm trunk from a distant controller. If the network option is chosen these two points would be analog network inputs and hard wiring at this controller would be unnecessary.)
- Supply VFD Feedback
- Return VFD Feedback
DDC Inputs and Outputs Defined
Now, your asking yourself what analog, and binary, and digital inputs and outputs are?
- Digital or Binary Outputs – Digital and binary are the same. It simply means either on or off, 0 or 1. With DDC control these are either dry contacts or triacs. So with our compressor start/stop above we simply use HVAC controls to energize a relay that starts or stops the compressor starter. These contacts are typically rated for 24 volts but may also use 120 volts depending on the manufacturer and the ratings of the dry contacts and/or triacs. Again with the VFD’s we are simply energizing a relay that starts and/or stops the drive(s). These (digital or binary) contacts can also be used for Pulse Width Modulation (PWM) control.
- Analog Outputs – This is simply a modulating signal from the controller to a particular device or piece of equipment. Our drives, depending on how they are set up in their local parameter programming will need a signal to tell it how fast to go. We want these drives to speed up and slow down according to what our static pressure set point is set at. There are three different typical signals we can send to these drives from the controller. These are a 4-20 Milliamp signal, a 0-10 volt DC signal, or a 2-10 volt DC signal. To keep it simple we’ll set both drives and the controller up for a 0-10 volt DC signal. This is an output signal from the controller and an input signal to the drive. If we send 0 volts to the drive it will run at minimum, if we send 10 volts to the drive it will run at maximum. If we send it 5 volts it will run at half speed and so on and so forth depending on what the program calls for the output to send to the drive. These analog outputs can also be used to control SCR’s for resistive heating loads.
- Binary or Digital Inputs – Again, binary and digital are the same. Some controls manufacturers use binary and some use digital when they describe their inputs and outputs. This is where the program monitors some dry contacts like the smoke detector. These contacts are normally closed and when the detector detects smoke the contacts open. DDC sees this and stops all blowers and sends alarms to the appropriate sources. (We don’t necessarily rely on the direct digital control to stop all the blowers. There should be a hard wired interlock that will stop all blowers if the smoke detector alarms.) The supply fan and compressor status inputs can come from either contacts on a CT or from aux contacts on the appropriate starters. Depending on how the program is set up, DDC simply wants to see an open or closed position from these sources.
- Analog Inputs – Again, an analog signal is a modulating signal. In this case, we have analog inputs. Input to the controller from a device or piece of equipment. Our temperature sensors are usually thermistors. Depending on the temperature, the resistance of a thermistor will change. The controller sends a small amount of current through the thermistor circuit. The program will take the input current coming back through the thermistor circuit and translate it into a temperature. This is done in the program through tables in the database which are matched up to the rated resistance/ temperature of the thermistor. For the duct pressure transducer we are going to get a different type of signal back to the controller. We are going to power the transducer up with a separate 24 volts. According to different manufacturers, this 24 volts can be either AC or DC (read the instructions for the device). There may be optional settings on the transducer allowing you send (from the device) either a milliamp or a voltage signal to the input of DDC. For simplicities sake we’ll select a 0-10 volt signal from the device to the input of the controller. There should be range settings on both the device and in the program. These settings should be synchronized. Let us say for example that the range setting is 0-10 inches H2O. Therefore, with a 0-10 signal setting at the device and configured in the program, when we get a 0 volt signal back from the device we have 0 inches H2O pressure. When we get 5 volts back from the device we have a pressure of 5 inches H2O and so on and so forth as the pressure changes so does the voltage coming from the device to DDC which will translate that into a pressure.
DDC Wiring Standards
A good HVAC control installation crew will use wiring installation standards to
ensure all wire used meets appropriate codes and everything has consistency. In mechanical rooms all wire will be in raceways or conduit. Above ceiling plenum installations will use plenum rated wire and tie wraps to secure the wire in a high location. Securing wiring in a high location is important and prevents someone from haphazardly cutting the wire or snagging it and pulling it loose from the controller or device. Additionally, above ceiling plenum wiring should never be tied off to other trades piping or hangers. If the wiring were tied off to all the electrical conduit above ceiling and the electrical contractor had to move or remove the conduit the controls contractor would have to relocate the wiring.
Where consistency really counts in wiring standards is what color is wired where. An example of this is communication wiring. Communication wiring is generally 24 gage 18-2 conductor shielded wire. The two conductors, generally one is white and one is black, should be terminated consistently throughout the project such as white is positive and black is negative or vice versa (terminations at the controllers on a network). It is super important that every technician responsible for terminating completely understands these wiring standards and what color is used as positive and what is used as negative. This will save lots of headaches and communication troubleshooting in the future.