How DDC Status Works – DDC and Status – Is the Device On or Off
This is a current switch sometimes referred to as a CT switch.
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.
VAV Box Types Fan Powered and Shut Off VAV Boxes - When attempting to name the different types of VAV boxes we find there are different names given to VAV boxes by different people and we find that no one is on the same page when VAV boxes are definitively defined.
We have fan-powered VAV and non-fan-powered VAV’s and we have series and parallel VAV’s and we have VAV boxes with a hot deck and a cold deck that mixes and one of the last projects I worked on some of the VAV’s were referred to as shut-off vavs. Variable Air Volume Boxes (VAV Boxes) regulates airflow into zones using a primary air source from a VAV air handler.
VAV Box Types Fan Powered and Shut Off
Cool only single ducted series
Fan-powered single duct series with reheat
Fan-powered single ducted parallel with reheat - uses plenum air before using reheat for energy efficiency
Aside from the control aspect of VAV boxes, pneumatic, electric, electronic, or DDC, there are many different variations to VAV Boxes and we will attempt to define each type of VAV including offering a diagram of each type and matching it with a specific application. VAV boxes are simply an assembly of a damper, damper controls, and sometimes reheat such as electric heat strips or a hot water coil. VAV boxes get their primary air source from a central VAV air handler and then distribute air throughout a specific zone the VAV box controls.
HVAC Control Transformer Bank - This bank of transformers is being used to power several VAV box controllers. One of many VAV boxes is powered by one of these transformers. In most VAV box applications, the power for the control system is at the box and a properly sized transformer for VA load is used to power the controller. In this application, the VAV boxes were not fan-powered and line voltage was not available in the boxes. So a central transformer bank was used and a wire for control power was pulled to each VAV box. (more…)
Magnehelic Duct Pressure Sensor - The Magnehelic measures duct pressure inside of ductwork. It is important, for many reasons, to know the pressure inside of ductwork. This Magnehelic measures the supply side pressure of the blower inside the air handler. (more…)
DDC Programming Logic Symbols 1 - DDC programming in HVAC Control can be rather complex especially to those who don’t understand software or programming. Some software and hardware manufacturers utilize symbols to build their logic programs for HVAC DDC control. Some of these logic blocks are rather complex but others are very simple. (more…)
DDC Programming Logic 3 - DDC control logic blocks have practically an unlimited way of arranging the blocks for HVAC control. Below is an example of a line of logic blocks arranged for static pressure control of a VAV air handler supply fan using a variable frequency drive. This same arrangement could also be used for static pressure control using a bypass damper instead of a VFD. (more…)
Programming DDC Logic 2 - 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 works 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 of 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 hardwired. This saves devices and wires necessary for optimum control.
Air Handler Filter Pressure Sensor - This is a pressure sensor. It tells a maintenance technician if the air filters are dirty and clogged. The pressure control is a low static pressure switch to trip the unit if the mixing box drops below the setpoint pressure. The pressure the switch is calibrated to.
In addition, the low static pressure sensor protects the fan and the ductwork including dampers from damage. If the economizer damper and the return air damper would fail closed (or another possible scenario) that would cause the unit to pull negative pressure in the mixing box. (more…)
York VAV Air Handlers | HVAC Zoning - This York air handling unit serves several VAV boxes and runs on a static pressure control set point determined by a DDC program. This program has a static pressure control setpoint and sends a signal to the variable frequency drive to speed up or slow down the blower to maintain the proper static pressure control setpoint. When the VAV boxes need more air the drive speeds the air handler blower up.
When the VAV boxes need little the drive slows the blower down. The VAV boxes are controlled by a thermostat in their respective zones.
York Variable Frequency Drives VFDs | HVAC Control - Variable Frequency Drives or VFDs have added a whole new dimension to energy efficiency and control. These drives, or VFDs, are hooked up to two different York VAV air handlers that have two three-phase motors a piece. (more…)
HVAC Tip - Ceiling Fans Save Energy: Ceiling fans installed strategically in your home or business can save you in air conditioning energy costs in the summer. The draft effect from the fan will allow you to raise the thermostat setting by up to 4 degrees and still remain comfortable. Make sure you purchase ceiling fans with energy efficient motors and blade designs. Some ceiling fans have inefficient motors and bad blade designs that will reduce the energy savings you can realize. A qualified electrician can install ceiling fans in your home or business and you can enjoy the savings and compliment your HVAC system.
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HVAC Control Transformer Bank - This bank of transformers is being used to power several 
Magnehelic Duct Pressure Sensor - The Magnehelic measures duct pressure inside of 
Air Handler Filter Pressure Sensor - This is a pressure sensor. It tells a maintenance technician if the 
York VAV Air Handlers | HVAC Zoning - This York air handling unit serves several VAV boxes and runs on a static pressure control set point determined by a 
York Variable Frequency Drives VFDs | HVAC Control - Variable Frequency Drives or VFDs have added a whole new dimension to energy efficiency and control. These drives, or VFDs, are hooked up to two different York VAV air handlers that have two three-phase motors a piece. 