IAQ and energy management
Indoor air quality (IAQ) and energy management are key in K-12 schools and higher education university buildings. This information will help to provide an efficient, effective HVAC system in a school or a university building.
- Understand the codes and standards that guide indoor air quality and energy efficiency requirements.
- Learn how to design HVAC systems to meet a building’s load requirements.
- Understand key equipment and controls interactions to improve energy efficiency.
IAQ and energy management are typically major concerns for any building operations and maintenance (O&M) staff. This is particularly true for K-12 schools and college/university buildings. In many cases, these two efforts are in direct competition with each other for budgetary dollars.
Facing shrinking annual budgets, facility managers are continually pressed to reduce annual energy operating costs while maintaining occupant comfort. The U.S. EPA estimates the annual energy expense for K-12 and higher education institutes is $8 billion and $2 billion, respectively. With this in mind, designers have an increasing responsibility to design HVAC systems that balance the owner’s requirements, up-front construction expenses, occupant comfort, and IAQ and energy savings.
Standards and guides
ASHRAE “writes standards for the purpose of establishing consensus for: 1) methods of test and classification standards; 2) design standards; 3) protocol standards; and 4) rating standards (in limited cases). Consensus standards are developed and published to define minimum values or acceptable performance whereas other documents, such as guidelines may be developed and published to encourage enhanced performance.”
ASHRAE Standard 62.1-2010: Ventilation for Acceptable Indoor Air Quality is the reference standard for IAQ. ASHRAE Standard 90.1-2010: Energy Standard for Buildings Except Low-Rise Residential Buildings is the reference standard for energy efficiency.
Standard 62.1 provides guidelines for the design of HVAC systems and equipment. It covers areas of IAQ management such as designing for air balancing, exhaust duct and outdoor air (OA) intake locations, filtration, moisture control, and ventilation system controls. Controls can be manual or automatic, but should allow the system to be operated to provide the required amount of OA for the building spaces whenever they are occupied. This can be problematic for O&M groups where either the staff or their collective knowledge is limited. Many of these individuals may resist systems that are new or perceived to be more complicated than the existing systems—or that may create increased costs to their maintenance or energy budgets.
The increased intake of OA can significantly impact the cost of energy through increased cooling and heating requirements dictated by the design of new or retrofitted HVAC systems. This energy impact can be quantified with energy modeling software or by specifically measuring the OA flow changes and then calculating the cost impacts based on utility rates. Energy changes, and thus costs, will be influenced by many factors such as the climate where the building is located, the building type and construction, the type and efficiency of the building’s systems (specifically the type of HVAC system), the occupancy and usage of the building, and ultimately how the building or systems are operated and maintained.
Standard 90.1 provides guidelines for building energy efficiency. It covers areas such as building envelope, building lighting, HVAC equipment efficiency, HVAC systems, service water heating, and system controls. Standard 90.1 sets the minimum energy efficiency requirements and system design requirements; similar to other standards, over the years it has adopted code language to increase state adoption and improve enforceability.
Each new edition of Standard 90.1 requires the Dept. of Energy (DOE) to issue a determination on whether the new edition will improve energy efficiency in commercial buildings over the existing edition. On Oct. 19, 2011, the DOE released a final determination that Standard 90.1-2010 would achieve 18.2% energy savings above buildings bound by Standard 90.1-2007. Every state has 2 years to adopt Standard 90.1-2010 or update its existing commercial building codes or standards to its requirements. With this 2011 determination, states had until Oct. 18, 2013, to file compliance certifications with DOE or request an extension. Check local codes or standards for state specific adoption and amendments.
Efforts to reduce energy consumption have led facility managers or the O&M staffs tasked with minimizing energy usage within their facilities to adjust controls setpoints, lockout/over-ride controls, turn off HVAC equipment overnight, or disconnect HVAC devices such as OA dampers (or close and shut them completely). Ironically, even though meant to save energy, we have found economizers and heat/energy recovery wheels disabled, primarily due to a lack of understanding of their function or the perceived increased maintenance efforts they require. These efforts often increase energy consumption and reduce IAQ.
ASHRAE’s Advanced Energy Design Guides (AEDG) series also provides design and energy efficiency recommendations for various building types. While the AEDG series was developed based on previous ASHRAE 90.1 Standards, the recommendations can still be applied to buildings designed to ASHRAE 90.1-2010 for additional energy savings. These AEDGs provide recommendations on building envelope, fenestration, lighting systems, HVAC systems, service water heating, and plug/process loads arranged by climate zone. Although the AEDGs are centered on new construction, the recommendations can be applied to renovations. While many of the AEDG recommendations are simply selecting between systems, the owner should be brought in to the design process to ensure that goals are being met and that maintenance staff has the expertise to service the systems.
With the exception of very few areas, the K-12 and higher educational facilities generally have higher ventilation rates than most other areas. This has been studied and shown to assist in providing healthy environments and contributes to fewer days missed by students and teachers, as well as improved learning.
HVAC system types and design considerations
The type of HVAC system installed and the amount of OA ventilation required play a large part in the overall building energy usage. When designing new, or retrofitting existing, HVAC systems, the interaction between OA ventilation requirements and the energy needed to condition that amount of airflow should be part of the systems’ considerations so the equipment can be sized and controlled properly to account for all the energy impacts. These systems not only require bringing in the required ventilation air, but the designer also must ensure this air is delivered into the individual occupied spaces when needed.
Many different types of HVAC systems are used in K-12 and higher education buildings today. The building system types vary from packaged rooftop units to central station air handling units (AHUs), with single or multiple zone variable air volume (VAV) terminal units. Figure 1 shows a typical AHU schematic, and Figures 2 and 3 VAV box schematics. Some systems use water or ground-source heat pumps and/or fan coil units (FCUs) with a de-coupled AHU for OA ventilation needs. These de-coupled AHUs are sometimes referred to as dedicated outdoor air system (DOAS) units, which provide reliable OA with improved energy efficiency in most cases. The more segregated the systems are, the easier it may be to provide the required ventilation air to the spaces.
Design HVAC load calculations, equipment selection
The first step in designing any efficient, effective HVAC system is to perform an accurate building load calculation and energy model. Whether the project is new construction or a renovation, a thorough understanding of the building environment is critical. Many components affect HVAC loads and energy consumption including building envelope, fenestration (glazing and doors), lighting, plug loads, occupancy, and sequence of operations, to name a few. The 2013 ASHRAE Handbook-Fundamentals Chapter 18 and Standard 90.1 provide methods and guidelines for developing HVAC load calculations and building energy modeling. Remember, heating and cooling load calculations are not the same as building energy modeling. Energy models analyze the proposed design energy requirements as the system operates over an extended period of time, typically 1 year or more. Load calculations measure the energy the HVAC system must add or remove from the zone to maintain the design conditions.
Most commercially available load calculation software produce peak and block load estimates. Peak loads assume every zone is at the maximum cooling or heating load simultaneously. Typically, equipment selected for peak loads will be oversized and does not require any rule-of-thumb oversizing. However, the engineer must consider the accuracy of existing building information and apply oversizing carefully to ensure proper equipment selection.
Consider sizing main equipment (such as air handlers, chillers, and boilers) based on zone block loads while sizing terminal units, piping, and ductwork based on peak loads. Right-sized main equipment reduces equipment costs, reduces energy consumption, increases dehumidification performance, and increases occupant comfort. Sizing some equipment such as terminal units, piping, and ductwork based on peak loading may increase energy efficiency by reducing fan or pump power.
Accurate HVAC load calculations lead to accurate equipment sizing. The designer should apply equipment safety factors carefully to avoid unnecessarily oversized equipment. Oversized equipment may short-cycle due to limited turndown ratios, reduce dehumidification capacity, or lower equipment life.