DDC (Direct Digital Controller) is similar in function to a small PLC with capabilities of handling limited number of digital and analog input/ outputs and is dedicated to HVAC systems. A PLC (Programmable Logic Controller) is a much more versatile control system capable of handling digital/ analog inputs/ outputs and is used for control of complicated machinery as well as entire process control systems. They could be as small as a DDC with as few as 8 I/O and very large with a few thousand I/O when the application is for a large and complicated PLC. Unlike the DDC they are scalable and have much superior reliability and security features
There is also DCS (Distributed Control Systems) which is functionally similar but follows a distributed architecture for the logic across multiple controllers instead of a single processor and is the A380 among control systems
Generally if the HVAC system is a small one, a low cost PLC such as a Koyo could be cost effective as far as hardware is concerned. However the programming would not be. The HVAC Controls companies have canned algorityms for their DDC units that basically only require i/o addressing to set them up. Therefore the programming is much less expensive that the PLC programming.
For large Systems, PLCs are not usually viable. THe HVAC Controls companies have large numbers of dedicated modules to control the various types of HVAC equipment. THese modules communication with their DDC units and require very little setup to do so.
The DDC units use either a propriatry network or in some cases Bacnet to comunicate with each other or with unit controllers. You would need to write drivers to do or perhaps find some commercially.
These companies also have programs available for scheduling equipment, trending alarming etc. You would have to find or write all these programs.
SOme large industial companies have already used PLC’s in HVAC control and the installed cost ran 2 to 4 times the cost of dedicated HVAC controls systems
Hope this brief note sheds some light for you.
LHF