How DDC Status Works – DDC and Status – Is the Device On or Off
The basics of DDC include learning about the status of a fan or a compressor or pump. How does a DDC controller read the status of the device it is monitoring? There are several ways to prove a device is running and some pros and cons of each type of device used to monitor on or off status. This is a basic DDC guide or reference to help a person understand the basics of DDC status and how it works. When one learns how DDC status works they can improve their troubleshooting skills. Too many maintenance technicians and others including HVAC technicians don’t understand DDC systems or building automation systems and are therefore crippled when it comes to troubleshooting DDC systems. High Performance HVAC hopes to change that and help HVAC techs and HVAC maintenance staff learn the basics of DDC systems including how DDC status works. Once someone understands the basics about DDC a light will turn on and they will see that DDC is not really rocket science.
DDC Status Type 1 | How DDC Status Works
This is the basic binary (on or off) CT or current transformer and making an experienced educated guess the most common type of status. It is really nothing more than a switch that closes or opens its contacts when it senses current through a wire. Simply put this over a wire going to a load like a fan motor or pump and when the pump energizes the contacts open or close (depends on whether the termination to the CT is on NO (Normally Open) or NC (Normally Closed) contacts) in the CT or current transformer. The DDC controller is wired to this CT and the wires terminate at the CT and back at the controller on an input terminal. The DDC controller input terminal can monitor the switch to tell if the switch is open or closed and depending on the programming the DDC controller processes the input and uses it in a logical manner for a response depending on the program. Of course, this will most likely be shown on a GUI or a Graphical User Interface. If it is a fan then it will likely show a motor turning a squirrel cage blower or some other sort of blade (maybe a propeller blade) that should either be in motion or not in motion depending on the input or CT.
The CT is like a magnet that is energized by the current flowing through the wire going to the load. The down side to this type of status is that the motor or load can still be running however, the fan or pump can have a broken belt or coupling and one would continue to read status unaware of the real problem. This can actually cause some real problems such as: The DDC program will lock out the compressor if there is no status for the air handler fan. Let us say the air handler fan is motor is running however, the belt is broken so the blower is not providing air flow across the DX coil.
What happens? Any HVAC tech will tell you the DX coil will freeze up with no airflow. For a pump coupling that is broken the technician can easily see there is a problem with the pump simply by looking at the discharge temperature in the piping downstream from the pump. These types of statuses can be cheaply installed but there are down sides to the CT status as mentioned above.
These types of status devices help with a basic input for an on or off status but one is likely better off with an analog type of CT where you can read the amp draw on the motor. The average load of the motor can be taken into account and anything outside of the average can be set to alarm so you will know when the blower is malfunctioning or a coupling is broken.
DDC Status Type 2 | How DDC Status Works
The Analog CT or current transformer. This type of CT is attached to the wire of the load the same as the binary CT is attached. This type of CT also reacts to the current of the wire going to the load except this type of CT also reads the actual amperage in the wire. The CT should be calibrated upon installation and a range should be set for the normal operating range of the load and expected current for different loads. The program can be set to accept certain tolerance ranges so if the load goes too far out of tolerance for the expected amperage range and if the load goes out of tolerance an alarm is raised to let the operators or maintenance staff know there is a problem. These types of CTs are more expensive to install and require increased maintenance to check the calibration of the CTs from time to time however these have definite advantages over the binary CT.
DDC Status Type 3 | How DDC Status Works
The DP (Differential Pressure) type of status uses pressure to make a contact closure usually using a bellows type of contact closure. This makes the DP status a binary (Open or Closed) type of status. The DP type of status can be found on pumps with a tap on the suction side of the pump and one on the discharge side of the pump. The DP type of status can also be found on fans and used for filter statuses to tell if the filter needs changing by having the contacts close at a certain pressure across the filter bank. It is important that these DP or differential pressure sensors be calibrated to ensure the DP responds to the appropriate pressure. This can also be a factor in future maintenance schedules to ensure the DP switch is responding correctly. Some VAV Boxes with electric reheat along with some duct heaters commonly use DP switches to prove airflow before the heat can be energized. The predecessor to the DP switch in these applications is the sail switch where a sail switch was placed in the duct work and when there was airflow the sail was pushed down and closed the switch.
DDC Status Type 4 – Usually not available in all circumstances but some relay or contactor auxiliary contacts can be used to read a status of a load. When the relay or contactor is closed the aux contacts can tell the input at the DDC controller whether the load is energized or not. Again this is not available in all circumstances and this method has drawbacks as with DDC Status Type 1.
DDC Status Type 5 – Again, this is usually not available in all circumstances as this method requires the load to be controlled by a VFD or variable frequency drive with feedback and or status indicator inside the drive. This method can combine Type 1 and Type 2 methods of status input but will require one binary input and one analog input on the controller. That means an extra point upon installation and extra points cost, however, this is probably the most effective way to monitor status.
DDC Status Type 6 - Paddle switch which opens and closes based on the flow of water or some other medium. An example of this is a hot water boiler system. Before the boiler will fire the water needs to be flowing through the loop. Chillers use this same type of status before the compressors kick on to chill the water in the chiller evaporator barrel. The paddle is inserted into the pipe and when the water is flowing it pushed the paddle. This action causes the contacts to close in a switch built into the paddle switch. This is a binary point or simply open or closed contacts reading back to the DDC controller.
How DDC Status Works - Methods and Types | HVAC Control - Conclusion
These are the main types of status that can be installed and used in most DDC systems. I am certain there are other types and ways of reading status but these are the most common.
How DDC Status Works? DDC Status Methods and Types
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