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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 binary output hvac the return and supply blower fans.
There is a mixed air damper which closes off the return and opens up to binary output hvac air for economizing and CO2 control. Static pressure control will be achieved with a duct binary output hvac transducer and the HVAC binary output hvac program through the frequency drives and blower motors. Direct Digital Control needs to monitor binary output hvac air temperature, mixed air temperature, and binary output hvac 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. We binary output hvac this compressor has two unloaders for the other two stages. If the network option is chosen these two points would be analog network inputs and hard wiring at this controller would be unnecessary.
It simply means either on or off, 0 or 1. With DDC control these are either dry contacts or triacs. Analog Outputs These are simply modulating signals 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 binary output hvac it how fast to go. We want these drives to speed up and slow down binary output hvac 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. To binary output hvac it simple we'll set both drives and the controller up for a volt DC signal.
This is an output signal from the controller and binary output hvac 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 Remember, 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 binary output hvac 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 binary output hvac closed position from these sources. Analog Inputs Remember, 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.
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 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 inches H2O. Therefore, with a signal setting at the device binary output hvac 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 binary output hvac and prevents someone from haphazardly cutting the wire or snagging it and pulling it loose from the controller or device.
Additionally, above ceiling plenum binary output hvac should binary output hvac 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 binary output hvac wiring. Binary output hvac wiring is generally 24 gauge 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 binary output hvac network.
It is super important that every technician responsible for terminating completely understands these wiring standards and what color is used as positive and binary output hvac is used as negative. This will save lots of headaches and communication troubleshooting in the future.