Gonna be another long one, haha. Again, this will be simplified for ease of typing and time.
A DDC system is a series of controllers tied together through a common network allowing for control of a system or systems. More often than not there is a “main” controller that allows the user or users to interface with the controllers for programming, setpoint manipulation etc… and there are sub controllers that do the actual work.
Sub Controllers are broken up into a couple different sections. They have inputs and outputs. Inputs and outputs have a couple different ways to be utilized… I’ll list a couple commonly used examples.
Inputs
Analog/universal input - (0-10VDC and 4-20mA was not supposed to be here. Temps are read in resistance/ohms… Sorry. Typing too fast)
space temp reading
Well sensors/clamp on sensors for pipe/water/steam temp
Probe sensors for air temp
Pressure transducers for pressure
Potentiometer setpoint reading
(Basically anything that involves changing numbers… Temps, override setpoints…)
Digital input - (dry contact)
Current sensor
Push button override
Alarm feedback
Basically anything that is on/off, start/stop.
Think of these as 0 or 1
Outputs
Analog output (0-10VDC or 4-20mA)
VFD speed
Temp setpoints
Valve position
Digital output
Closing of a relay
Valve pulse (not typically seen unless on steam)
Using these inputs and outputs (and many more depending on the system) you have as much control over the system as you want/customer wants to pay for.
Let’s say you have a hot water boiler and loop pumps that feed an air handler that takes care of a conference room and a few auxillary offices with convectors that are controlled by zone valves.
Starting at the boiler loop
- Boiler start stop including warm weather shutdown (digital out)
- boiler loop temp for outdoor reset control on convectors (outdoor reset meaning as outdoor temp goes up, loop temp comes down [analog output and input])
- loop pump start stop (digital out)
- boiler firing rate (to avoid overshoot of loop setpoint [analog out])
- loop pump speed to vfd (needed due to mixing valve and zone valves on convectors[analog out])
- differential pressure (needed due to zone valves and mixing valve to calculate vfd speed [analog in])
AHU
- conference room space temp (analog in)
- mixing/three way valve position (analog out)
- return temp (analog in)
- supply temp (analog in)
- mixing box temp (if equipped with economizer[analog in])
- outdoor air temp (analog in)
- outdoor air humidity (analog in)
- coil pump start stop (digital out)
- freeze stat (if equipped with economizer[digital in])
- economizer damper actuator setpoint (analog out)
- occupied override (room is occupied without schedule being set[digital in])
- differential pressure readout for clogged filter alarm (analog in)
Convectors
- space temps (analog in)
- zone valve position or on/off (analog/digital out)
Using the controllers, you can turn the boiler on, set the loop temp, read back differential pressure and calculate pump speed, start the air handler, use the mixing valve to get proper discharge air temp to the conference room, allow outdoor air for ventilation if needed, open zone valves etc etc.
