October 2012

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BAS vs PLC for HVAC Control

Some interesting points about the pros and cons of using industrial programmable logic controls (PLC) over commercial Building Automation Systems (BAS)

Paul Ehrlich, Ira Goldschmidt & Angela Lewis
Building Intelligence Group

As published
Engineered Systems 
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The August issue of Engineered Systems focused on some of the unique challenges faced in Data Centers. One of the articles “Commercial vs Industrial Automation in Data Centers” raised some interesting points about the pros and cons of using industrial programmable logic controls (PLC) over commercial Building Automation Systems (BAS).  In a critical environment, such as a data center, this becomes an interesting discussion, after all uptime is key, and the added investment in a PLC solution should be able to be readily justified.

So if industrial systems make more sense in this type of critical HVAC application does the same logic follow into other critical areas?  For example would you use it in a critical manufacturing environment?  How about a lab or research center?  Or what about a hospital or a large chiller plant?  All of these projects have been done successfully (and arguable unsuccessfully) with both PLC and BAS based systems.  How do you go about deciding what kind of system will work best?  Here are some key issues to help you decide:

Parameter  Description PLC  BAS
Critical Uptime Dual processors, redundant operation ++ -
Contractor Expertise Expertise in HVAC applications +/- +/-
Speed Ability to run fast loops (millisecond or faster)  -
BACnet and LonTalk Protocols Open protocol support - BAS - +
Modbus and IP Support
Open protocol support – PLC + -
Cost Cost for installation and support - +
Support Ease of support by owner and contractor +/- +/-

Based on this scorecard it would appear that other then cost and building protocol support that PLC’s are the better solution.  But this really doesn’t tell the whole story.  For most HVAC solutions the control system doesn’t need to be any faster or more reliable then the underlying system you are controlling.  This means in most cases that BAS systems are more then adequate, better supported, and less expensive then the comparable industrial solution.

The same cannot be said for critical electrical control.  Management of switching, power quality, and distributions need to be done at control rates much higher then for HVAC.  This makes the use of PLC based solution the only viable solution. 

About the Authors

Paul and IraPaul and Ira first worked together on a series of ASHRAE projects including the BACnet committee and Guideline 13 – Specifying DDC Controls. The formation of Building Intelligence Group provided them the ability to work together professionally providing assistance to owners with the planning, design and development of Intelligent Building Systems. Building Intelligence Group provides services for clients worldwide including leading Universities, Corporations, and Developers. More information can be found at www.buildingintelligencegroup.com  We also invite you to contact us directly at Paul@buildingintelligencegroup.com or ira@buildingintelligencegroup.com


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