PLC vs. HVAC type controllers

If I gave you a spec sheet for a PLC and a building equipment controller, and neither of them had the brand name, or a controller identifier on them, could you tell the difference? How would you know that one of them is a PLC and the other is not?

I have very limited experience with PLCs, and extensive experience with Building Automation controllers.

As I understand it, they both have the ability to monitor digital and analog inputs, control digital and analog outputs, and accept custom programming to accomplish a desired sequence of operations. Both can be networked to each other, and to a central control and monitoring station...

If it is a matter of "One is more suited for this and one is more suited for that.." Please explain WHY one is more suited for this and one is more suited for that... etc.

If it is a matter of "One is more robust than the other..." then I must ask - Define robust? and where is the line on 'robust-ness' that differentiates between a PLC and a building controller?

Thanks in advance for your input!

John

PLCs generally have a larger range of sizes, from half a dozen I/O in a unit to hundreds. Scan time in PLCs can be much faster in the millisecond range. Functions such as motion control are seldom seen in HVAC controllers.

Having worked with both types of controllers I would say a properly chosen PLC could do any building automation task, but I doubt you could find a HVAC controller that would be able to do every machine control application that PLCs do.

Often HVAC controllers have programming languages that are "directed" to building control applications to make programming less time consuming. Also many HVAC controller manufacturers offer sensors for temperature, pressure, humidity etc. to go with their equipment. Generally this equipment is "commercial" as opposed to industrial duty (NEMA 1 enclosure versus NEMA 12) temperatures under 150 deg F and so on.

A number of HVAC controllers offer analog pneumatic outputs for controlling actuators but I have not seen a PLC that offers this type of output... Many HVAC controllers offer battery back up which I have not seen
incorporated in PLCs Very few HVAC controllers support industrial networks such as Modbus, Data Highway, Profibus

James Bouchard

John,

I work in the industry of HVAC automation. I have automated HVAC equipment using both a PLC as well as a controller designed for HVAC. For the most part, the difference I have seen between the two in terms of control is that the PLC reaction times are generally a lot faster. The PLC can knock the bad beer bottle off the assembly line as 10,000 of them are flying by where the HVAC controller generally cannot. The question becomes: Do you need that kind of speed for the application? How quickly do I need a PID to respond to control mixed air temperature on an Air Handling unit? In this scenario, the PLC far surpases the needs of the application.

In terms of programming I have noticed that the HVAC controller will generally group blocks of data together to cut down set up and programming time. This functionality passes on to the end user. This will be true for any manufacture although the functions and group types will vary. For example a discrete output is bound to a discrete input to make a tag which will both command a field device and confirm status. With this internal structure alarms, totalization as well as other functions call also be easily bound to this tag identity. Again, this ability will vary by vendor.

A PLC on the other hand is truly a blank slate. All of the desired functionality in terms of the groups mentioned above has to be coordinated between the PLC and HMI software you use. Being a blank slate also means that it has greater programming possibilities as it is up to the end user to decide control structure.

To address your question on PLC robustness. I have seen a PLC soaked, dried and continue to operate. None of the HVAC controllers I have worked with can make that claim.

Jonathan

Short answer: I'll bet HVAC is easier to do with the HVAC box than the PLC and any other application is harder.

The HVAC controllers probably come with most algorithms pre-programmed that you will need for HVAC control (take the input from a direct acting thermostat, put through a PID function, output the result to a VAV box, keep track of all VAV box positions for chilled and hot water setpoint reset, etc.).

PLC's can be, and have been, used for HVAC equipment and system control, but the value of any specialized system is that development time will be reduced because of the precanned algorithms mentioned earlier, including "nice to have" functions that you wouldn't take the time for if you were writing the code from scratch. You can concentrate your development time on programming the features which actually are unique to your project, instead of reinventing the wheel all the time. The disadvantage? The tools and instructions are probably missing or less powerful than a PLC in areas not typically required in HVAC applications (high speed motion with encoder inputs, high speed discrete control with interrupts, etc.)

Startup and checkout is another advantage with such a system. In a machine building environment, significant amounts of time are used in making sure the system meets the requirements, which will be much less time consuming in a system with tried and proven modules.

Finally, some of the signal formats used in HVAC are not very common in the industrial world, and HVAC controllers will have I/O to connect to these sensors and actuators, while PLC I/O in those formats is harder to come by.

John, I too have extensive experience with HVAC controls, and some limited experience with PLCs. The difference as I see it are as follows:

1: PLCs are normally designed for control in one room or building. HVAC controls are normally designed for large buildings or multiple buildings.

2: PLCs are typically designed for a large amount of points (I/O) in one location, with the processing/programming for fast response. HVAC controls usually require a smaller point count, and the points are normally spread-out and do not nornally require fast processing.(Exception: fire shut-down, etc.)

3: PLCs can have PID loop control, but it is intended for simple individual control of items like VFD's, for flow, pressure, etc. HVAC controls typically have PID control loops as required for control of heatpumps, fancoils, VAVs, in small controllers to be mounted on each unit.

I could go on & on, but basically they are designed for different purposes. A process control system is more of a cross between the two, with the larger point count spread-out in a large facility. Any of these could do the job of the other, but it would require more work or time.

I hope this helps.
Clint
http://members.cox.net/clintlee/hvac

PLCs can and are used for HVAC.

Clint stated that PLCs are designed for a large number of I/O in one place. Not true. PLCs have very good and robust remote I/O systems such as Device Net that can have all manner of devices on the line eg: drives, temperature controllers, HMIs, digital I/O, analogue I/O etc etc.

Some PLCs even have fuzzy logic modules (Omron were one of the first). Omron also have 2 degree of freedom auto tune PID loops built in. Turn on the auto tune bit, the PLC does the calculations and then turns off the bit.

Some of the latest PLCs have many hundreds of functions to make programming easier. Programming languages such as FBC, SFC, Ladder, Instruction List, Mnemonic, all are now available.

John,

I think everyone for the most part has made valid points regarding the differences between HVAC controllers and PLCs... However, when it comes right down to it they are one in the same. As for "Robustness" well... that's a little hard to explain but I will try to sum it up. Will it last and give good service in the bottom of a coal mine or only in a clean HVAC mechanical room? That is the way to determine robustness.

PLCs are designed to take the abuse, vibration, electrical noise and contamination dished out on manufacturing plant floors everyday. From a control standpoint the bottom line is that both are computers capable of analyzing various inputs and controlling outputs based on the user programming.

The key difference in the two aside from "Robustness" is flexibility. HVAC controllers as the name states are designed with one function in mind, control of HVAC systems. PLCs on the other hand have no specific function in mind, therefore, you buy them based on memory capacity, communications support, control features (higher math capabilities), and the numbers and types of I/O required to do a specific task. Being an Electrical Engineer at a chemical plant that manages a huge networked PLC control system with 30,000+ points of analog and discreet I/O would tend to make me lean toward using PLCs for everything. This is only because I can purchase as much or as little as I need and if the project gets scrapped the controllers can always be used for another project no matter what it is.

For you on the other hand you have to look at a few basic things. #1 is unit cost. I would assume that these HVAC units are fairly pricey. But in fairness this cost has to be evaluated on a per I/O point basis so that you compare apples to apples. #2 is ease of programming. Electrical people tend to love PLCs because they program for the most part like electrical industrial drawings are drawn. HVAC contractors on the other hand may find the HVAC controller interface easier to use due to the way the programming components interact. Most of them use some form of Visual Basic symbology rather than ladder logic as far as I can gather. #3 is flexability and scalability. If you do only small commercial systems then HVAC controllers may be the answer. If you are like me and deal primarily with industrial applications PLCs may be a more viable option.

To give you an example: I have a glycol chiller system with a maximum capacity of 640 tons. I'm not sure but I don't think you are gonna find an off the shelf HVAC controller that can sequentially control 8 80ton chillers based on load demand, run hour load balancing, etc... and in turn control all the pumps, tower fans, water levels, analog and digital valves, temp and pressure transmitters, thermocouples, RTDs, start/stop stations, indicator lights, you get the picture... without a lot of hassle. The key is flexability... If a cheap generic HVAC controller will do the job use it. On larger more complex systems PLCs may be the answer.

Hope this helps,

David
[email protected]

P.S. I am available for consultation for a small nominal fee! HEHEHE just kidding unless ya really want something... drop me an email sometime if you have any more questions...

Nice topic.

Can somebody give some brand names and models of HVAC controllers which are popular? I am quite new to these types of controllers.

regards,
Suseka

Suseka,

Check out this site: http://www.ddc-online.org
It's comprehensive and impartial!

Good luck,

Zvi

David,

We do this kind of thing all the time with "off the shelf" DDC controllers...and with a lot less hassle than it would take with a PLC!

How about a 128 I/O controller, 32 bit CPU, 2 MB flash memomry, 16 bit A/D inputs with built-in conversion tables for RTDs, 128 PID controllers with auto-tuning, over 200 built-in trend and run-time logs with time-date stamping, 80 weekly and annual operating schedules, high level programming language, TCP/IP multi-user networking, and much more for the price of a medium-size PLC?

If you've only been working with PLCs it may take you a day or two to make the transition, but you won't go back.

Charles Marcus
[email protected]

PLC's have a faster data sampling rate. All other issues tend to vary by manufacturer. I can control any process with a building controller, or a PLC. Building controllers are much easier for most applications, but PLC's are lots faster. Most high speed production lines are controlled by PLC's due to the required response time. Overall install costs, lifecycle costs, and meeting requirements of the system it is designed to control are the onlly things most good engineers look at.