HVAC Control and Direct Digital Controls 2

Table of Contents

HVAC Control and Direct Digital Controls 2 - An 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.

HVAC Control and Direct Digital Controls – The (Field) Technical Aspects of Direct Digital Control

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 (VFDs) on the return and supply blower fans. There is a mixed air damper that 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 freeze stat for safety reasons. This is the setup and we need to make it work. We need to get a point count and set up our inputs and outputs.

HVAC Control and Direct Digital Controls - Input and Output Types

Digital or Binary Outputs – HVAC Control and Direct Digital Controls

Compressor Stage One
Compressor Stage Two
Compressor Stage Three
Supply VFD Start/Stop
Return VFD Start/Stop

Analog Outputs – HVAC Control and Direct Digital Controls

Supply VFD Speed
Return VFD Speed
Mixed Air Damper

Binary or Digital Inputs – HVAC Control and Direct Digital Controls

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.)
Freezestat

Analog Inputs – HVAC Control and Direct Digital Controls

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.)
Humidity
Supply VFD Feedback
Return VFD Feedback

HVAC Control and Direct Digital Controls – DDC Inputs and Outputs Defined

Now, you’re 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 setpoint 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 control 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 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 contact 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 is 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, these 24-volts can be either AC or DC (read the instructions for the device). There may be optional settings on the transducer allowing you to send (from the device) either a milliamp or a voltage signal to the input of DDC. For simplicity’s 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 do the voltage coming from the device to DDC which will translate that into pressure.

HVAC Control and Direct Digital Controls – 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 wires will be in raceways or conduits. Above the 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 the ceiling and the electrical contractor had to move or remove the conduit the controls contractor would have to relocate the wiring.

HVAC Control and Direct Digital Controls | Conclusion

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.