Automation and controls for HVAC systems

Here are some best practices for designing building automation systems (BAS) and controls for HVAC systems.

11/17/2015


Learning objectives 

  • Assess basic design approaches for building automation systems (BAS).
  • Interpret codes and standards that guide BAS design requirements.
  • Understand key equipment for integration options to improve system efficiency.

The design of control systems for HVAC systems is generally based on the design engineer's philosophy and experience level. This article cannot cover all possible HVAC systems in use today, nor can it cover all the control methods that can be applied to many of them to enhance their operation and efficiencies. However, the reader should understand that the HVAC system itself, its controls components, and the building in which it is installed should all be considered together as parts of a single, whole design. Designers should remain flexible in providing the best possible system that can control, to various degrees of comfort required, different applications within the same building. The system should provide this control with reasonable costs at the least possible energy use.

An overall goal for the control system should be to establish a method, or a system, of operating and controlling one or more processes by automatic means, using various devices that reduce the need for human intervention. A process can be defined in several ways and HVAC systems can have numerous processes, but it's typically agreed that there are only a few basic elements of any control system: 

  • A process variable to be controlled
  • A desired control setpoint for the variable
  • A controlled device
  • A controller that coordinates decision making
  • A sensor to provide some type of feedback for a directed change.

Some of the control systems may be required for life safety reasons to prevent equipment from operating in an unsafe manner.

Each basic control system can be grouped together with others to form what can be considered a larger BAS for a building. In the case of a campus setting made up of more than one building, several different BAS could be gathered together in a larger energy-management and control system. Because each control system and/or BAS can be, and often is, proprietary or a "closed system" in terms of communication protocol used by the different individual manufacturers, it is often difficult to have "open" communication between control systems. System integration and interoperability have become more important as BAS options have developed, and this will continue to be a need in the future. BACnet protocol has been one of the driving forces for more open communication systems.

Regardless of whether the basic control system is stand-alone for a new piece of equipment (i.e., an air-handling unit [AHU] or a chiller) or it covers an entire building, an early review of codes, standards, and regulations is often necessary to allow for an expedient design and avoid conflicts that cost time and money to resolve. Groups such as ASHRAE, the Air Conditioning, Heating, and Refrigeration Institute (AHRI), the American Society of Mechanical Engineers (ASME), NFPA, the International Society of Automation (ISA), and many others all have standards to review for systems, equipment, and testing requirements.

A good primary resource for most engineers today is ASHRAE. The society's various technical committees write standards and guidelines to establish a consensus for such items as methods of testing and classification, design, protocol, and ratings for systems and equipment components of those systems. ASHRAE has numerous technical sources of information including a series of four handbooks that are updated every 4 yr.

Two of these handbooks, 2012 ASHRAE Handbook—HVAC Systems and Equipment and 2015 ASHRAE Handbook—HVAC Applications, contain several chapters filled with information and basic criteria needed to fully understand various HVAC systems. In addition, ASHRAE has published various other standards and guidelines to assist designers on designing control systems. These include: ASHRAE Standard 135-2012: BACnet, A Data Communication Protocol for Building Automation and Control NetworksASHRAE Guideline 13-2014: Specifying Building Automation SystemsASHRAE Guideline 36P-2015: High Performance Sequences of Operation for HVAC Systems; and Fundamentals of HVAC Control Systems.

ASHRAE Standard 90.1-2013: Energy Standard for Buildings Except Low-Rise Residential Buildings is the reference standard for energy efficiency. This standard illustrates minimum efficiency and control systems requirements along with commissioning for building envelopes, HVAC, power, lighting, and other equipment, all of which is included in a building system design.

Maximizing energy efficiency with controls

The first step in designing any efficient, effective HVAC system for a building, or a campus full of buildings, is to perform accurate building load calculations and energy modeling. ASHRAE 90.1 provides methods and guidelines for these tasks. The type of HVAC system designed and installed, and its configuration, will certainly require one or more control schemes. The constant interaction and changes in HVAC loads within a building, or between multiple buildings, on a chilled-water system loop, for example, should be part of the system considerations so all equipment can be sized and controlled properly to account for all the energy impacts.

The designer should become familiar with ASHRAE 90.1-2013, Section 6, which includes various requirements and exceptions that affect HVAC design. Section 6.4.3 is titled Controls and begins stating various requirements within the standard that must be considered and included in a BAS. Other sections provide additional information for control requirements. For example, there are some requirements, with necessary exceptions, for the following:

  • Off-hour controls on HVAC systems with automatic shutdown of HVAC systems for start/stop under different time schedules, or based on occupancies, or for life safety and security reasons.
  • Setback or other controls on heating and cooling systems with optimum start, which will also prevent mixing or simultaneously supplying air that has been previously mechanically heated or cooled, either by mechanical cooling or by economizer systems.
  • Motorized shutoff damper controls for outdoor air intake and exhaust systems, so they can be automatically opened or closed when the systems or spaces served are not in use and/or to reduce energy costs or meet code requirements.
  • Ventilation fan controls for fans with motors greater than 0.75 hp shall have automatic controls capable of shutting off fans when not required.
  • Controls to prevent simultaneous operation of humidification and dehumidification equipment.
  • Ventilation controls for high-occupancy areas using demand control ventilation (DCV).
  • In Section 6.5.3.2.3., there is a requirement to include variable air volume (VAV) pressure optimization in systems by using static pressure reset based on the zone requiring the most pressure. The static pressure control setpoint(s) are modulated to the lowest operable pressure to control the VAV terminal unit damper positions in the system, thus reducing fan power requirements.
  • In Section 6.5.3.4, multiple-zone HVAC systems must include supply-air temperature reset controls to respond to building loads, or outdoor air temperatures depending upon the existing climate zone.
  • Section 6.5.4.2 indicates a requirement to include pump-pressure optimization in systems where the total pump-system power exceeds 10 hp. This reduces pump energy by varying control valve positions in a hydronic system, thus providing variable fluid flows. 

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