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PLC control or DDC control for general HVAC

Is there any literature out there in providing unbiased information regarding the decision whether to implement PLC control or DDC control for general HVAC plant?

By Curt Wuollet on 30 October, 2003 - 8:41 am

Probably not, but that doesn't mean what's available isn't
useful. I found some good stuff but it is rather biased. If
google doesn't help, I can look up some stuff.



You've asked for UNBIASED info. I'm afraid you'll find it hard to come by, especially on this web-site which is strongly PLC oriented.

Why not make your own evaluation? Here's a few points that may help:

1. The vast majority of installed HVAC digital control systems are DDC based rather than PLC.
2. Most all DDC manufacturers specialize in HVAC control.
3. The BACnet communications protocol, which has been approved as an ISO standard, was developed by ASHRAE, the largest professional society of HVAC engineers and designers. BACnet is a robust protocol highly supportive of HVAC functions and is enabled by most all DDC manufacturers.
4. DDC special purpose controllers are available for various HVAC equipment: air-handling units, chillers, fan-coil units, VAV terminal units, roof-top units, etc. They are less costly and easier to install, program, commission and maintain than centralized general purpose controllers. The increased diversity makes control system architecture easier understood and requires less sensor/acuator wiring.

For a good, unbiased insight on DDC systems and manufacturers, check out:

I hope this helps you.


By Bob Peterson on 30 October, 2003 - 8:37 pm

Unbiased is a subjective term. many of the HVAC specific controllers have builtin functions for common HVAC application issues that you would have to do by yourself in a PLC. Of course once you get it right in a PLC, you can just copy it for the next application, but the first pass through can be rough.

PLCs have a huge advantage in that you can do pretty much anything you want, and your maint people don't have to be trained on both PLCs and a bunch of HVAC only stuff.

By Peter Whalley on 31 October, 2003 - 12:18 am


Couldn't agree more. Further, there are a few companies that make both DDC and PLC products but when you ask them to tender on a HVAC control project they always offer DDC simply because it's easier to use and cheaper for them.

Unless you have some sort of industiral HVAC with extra high reliablity requirements or very wide environmental temperature range where the controls will be installed then the extra work involved in programming PLCs is not worth the effort.


Peter Whalley
Magenta Communications Pty Ltd
Melbourne, VIC, Australia
e-mail: peter*no-spam*
delete *no-spam* before sending

By Lloyd Firth on 31 October, 2003 - 12:14 am

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.


By Paul Bulmer on 28 June, 2010 - 3:09 am

> Is there any literature out there in providing unbiased information regarding the decision whether to implement PLC control or DDC control for general HVAC plant? <

I am interested in knowing what you found on this topic. We are considering using PLC for HVAC control.
Paul Bulmer.

Here is the simple truth. The necessity for a PLC over a DDC control system is speed and possibly industrial fortitude. For HVAC it isn't required to have a scan rate that a PLC is capable of. No matter what the control system you implement the building will still react as quickly as the equipment allows it. So it really comes down to price. In most cases I have been involved in DDC controls hardware should be about half the price of most PLC depending on options. In my opinion I/A series or Andover Controls can do it a whole lot cheaper for all the bells and whistles. I/A uses visio to program which is comparable to ladder logic where as Andover uses simple line by line plain English programing. Exposure is the only difference to difficulty.