How to engineer systems in mixed-use buildings: HVAC and controls
When working on mixed-use buildings, engineers must address many needs in one building. This reviews aspects of HVAC, automation and controls, and energy efficiency.
Mark Crawford, Principal engineer, Southland Industries, Las Vegas
Christopher M. Kearney, PE, LEED AP, Project manager, exp, Maitland, Fla.
Brian McLaughlin, Associate, Los Angeles, Arup
CSE: What factors do you need to take into account when designing building automation systems (BAS) for a mixed-use building?
Kearney: Numerous factors should be considered when designing a BAS. Generally, the first factor is the owner’s project requirements. Some owners desire to have many of their building systems, such as fire alarm, lighting control, miscellaneous equipment, etc., integrated with their BAS, while others may not. Integrated building management systems require a lot more coordination between the BAS design engineer and the product selection engineers to ensure the desired communication protocols (BACnet, LON, Modbus, etc.) are specified with the product as well as the communicated points. In the absence of strong owner requirements, the criticality of the facility should be considered. Critical facilities or portions of the facility that are considered critical that have low tolerance to equipment or system failures or environmental changes may require more monitoring from the BAS but possibly more or less sequence of operations complexity. The sophistication of the systems also can play a factor in designing the BAS. Unitary based systems generally have limited interface options with BAS, and the general system design may not warrant a sophisticated controls overlay. Budget will also come into play as the more equipment and systems integrated with the BAS, the more the installed BAS will cost.
CSE: How does implementing BAS in an existing building differ from designing controls for a new building?
Kearney: With existing systems, the BAS design engineer needs to understand the owner’s requirements or expectations as they relate to integrating the new BAS control with the existing BAS control system. If the owner desires to have the BAS associated with the new work integrated with the existing, the design engineer needs to understand if the owner wants to extend the existing system with the existing BAS provider and forgo a competitive scenario or have competition and potentially let another BAS provider in its facility. If the latter, the BAS design engineer needs to do some investigative homework to understand the existing system and its capabilities to integrate with other systems, both as the primary control system or potentially the secondary system. The engineer needs to help the owner understand that generally both control providers will need to cooperate with each other to effectively integrate the systems and with this realize that if another BAS provider is awarded the new work, the existing BAS provider will need to be contracted to assist with data transfer and possibly graphics. Additionally, functionality of the existing BAS, including graphics, may be desired for the new work and the BAS design engineer will need to convey this functionality in its documents.
CSE: What types of energy management systems are you specifying on mixed-use building projects? Are these part of full-building control systems?
Kearney: Generally, partial building control systems are designed for mixed-use projects. Public areas, office areas, conference space, central plants, and back-of-house areas are areas where BAS is typically designed. Many times for cost savings, condominium and hotel rooms are provided local control with minimal or no integration with the house BAS. Depending on the owner preference, a guest check-in system that transitions the room HVAC equipment from unsold, sold but unoccupied, and sold and occupied modes for the hotel rooms may be incorporated into the design. In addition, systems serving retail areas that are leased out are generally not tied to the BAS, with the exception of house HVAC equipment that provides ventilation.
CSE: How have changing HVAC codes and standards affected your work on mixed-use buildings?
Kearney: Code and standards updates have, for the most part, made positive improvements to the building industry. This has come through sustainability initiatives, increased energy efficiency, and more strict life safety requirements (i.e., smoke control).
CSE: What organizations or resources do you turn to most for codes/standards in mixed-use buildings?
McLaughlin: This is entirely dependent upon the location of the project, the authorities having jurisdiction (AHJs), and the owner requirements. Codes in Los Angeles differ greatly from the regulations in the United Arab Emirates. Also, for large corporations that may use a global insurance company, the insurer may strive to apply a consist standard regardless of the location of the project. Over the last several years, I have had to incorporate NFPA 5000, the IBC, Abu Dhabi Civil Defense codes, and various FM Global requirements on mixed-use projects.
Kearney: ASHRAE, NFPA, ASPE, IEEE, and International Building Codes are the most common.
CSE: What unique requirements do mixed-use building HVAC systems have that you wouldn’t encounter on other structures?
Crawford: Mixed-use buildings have unique user requirements that affect HVAC and plumbing system application and size. System diversity (and therefore peak load) will vary in the hotel tower based on the podium functions. For example, a convention center in the complex could drive the peak domestic hot water load to a short peak period during the week, and a heavy restaurant load may shift the load to weekends at night. As the balance of these loads varies, so does the load profile and plant sizing. There are great opportunities for innovative sustainable design in mixed-use facilities. Simultaneous heating and cooling is common, which may allow efficient use of technologies such as heat recovery chillers. Water reclaim may be more lifecycle effective because of the increased number and frequency of reuse options. The options are as varied as the mix of building occupancies that are put together.
McLaughlin: The challenge is not necessarily individually unique requirements but rather the confluence of the special requirements of each portion of the mixed-use building. Issues such as condensation, acoustics, security (biohazard/physical), natural life (insects/birds), controls for motorized equipment, lighting/mechanical system zoning, and smoke exhaust all need to be addressed in a cohesive manner to create an efficient building.
Kearney: Many different types of unique airside systems might be found in mixed-use venues. For example, fan coil units in apartments or hotel rooms, under-floor air in office areas, constant volume air handlers, and possibly central station VAV air handlers in public spaces with varying loads. Most mixed-use projects we are involved with include a central energy plant. A unique aspect of the HVAC systems in mixed-use buildings may also come from the use of different waterside systems. For example, chilled water for hotel, office, and public spaces; and condenser water heat pump systems in residential units to simplify HVAC metering requirements.
CSE: How can automated features and remote system control benefit mixed-use building clients?
McLaughlin: Automation and remote control certainly assist with the optimization of mixed-use facilities by, for example, allowing motorized windows to open when the outdoor air temperature and humidity are ideal for thermal comfort. However, along with the automation, overrides would typically be incorporated in the design to enable the client to forego optimization temporarily for security or other specific needs.
Kearney: Automated features in a mixed-use building coupled with variable volume waterside systems help improve energy efficiency. With occupancy controls and time-of-day schedules, building tenants are able to limit unnecessary energy usage. On the owner’s side, understanding the planned building use and installing appropriate controls could allow for significant diversities in the central energy plant bringing first costing savings. The diversities could come from occupancies with opposite occupancy schedules (e.g., residential and office).
CSE: What is the most important indoor air quality (IAQ) issue you typically address in these projects, and how do you address it?
Kearney: Project location will play a role, but humidity and how it can contribute to mold and mildew is always a concern. We have developed a tool we call the “M&M scale” that we use to determine the likelihood of mold and mildew within each project. The scale looks at building envelope materials, ducted versus plenum returns, fresh air delivery methods (i.e., preconditioned versus unconditioned), and continuous versus intermittent exhaust. Concentrating on the fresh air delivery method during design will not only improve IAQ, but also potentially limit energy usage. The conditioning of fresh air within a mixed-use building is likely one of the largest HVAC loads. By integrating energy recovery and CO2 control into public spaces, offices, etc., within a mixed-use facility can help ensure IAQ is maintained at the lowest operating cost possible.
CSE: What software or systems do you use to model the energy consumption of the building?
Crawford: We typically model in Trane Trace 700, the latest version, to model building energy consumption.
Kearney: Visual DOE 4.0, IES, Equest, Energypro, and Trace 700 are the most common within our company. Visual DOE, Equest, and Energypro all run using Dept. of Energy (DOE2) software. We only use Energypro in California because it is also the state’s code compliance software. Visual DOE and Equest are very similar, have 3-D views, and are fairly easy to use. Trace seems to do the best job of updating new HVAC system types, but it is more cumbersome to use than Visual DOE and it is not 3-D. The 3-D capabilities help us with quality control (e.g., looking at the 3-D view shows if the windows look right, if the daylight sensors are located properly, etc.). If you make typos on dimensions in programs like Trace, it is very hard to notice these errors without a model to review.
CSE: In general, what payback period do building owners expect from a mixed-use building?
Crawford: Oftentimes only paybacks up to 5 years will be considered.
Kearney: We typically see expectations of less than 5 years; however, to help the owner understand potential opportunities we have created a tool we use to identify early design technologies/concept alternatives. We group these opportunities into three economic tiers for four categories—energy, water, waste, and transportation—as follows:
- Tier I: Just the right thing to do
- Tier II: These have reasonable paybacks
- Tier III: In spite of the cost, I just want to do it.
After brief discussion with the owner, Tier I items are usually incorporated into our designs because of their proven short paybacks (1 to 2 years). After an agreement in which Tier II items should be considered, our design team will develop payback analyses for those items. These studies along with other pertinent information will help the owner and the design team make a final decision of whether it is right for that specific project. Return on investment parameters vary for different type of owners. We typically categorize items with an expected payback between 2 to 6 years in this group. Finally, Tier III items will be listed for the owner’s review and comment, but from years of experience, we know these items most likely will not be incorporated unless the owner has a special corporate or marketing reason to force its use.