IAQ and energy management
- 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.
Key energy savings opportunities
Though many different HVAC systems and control strategies exist, the following items can create an impact on energy costs and system efficiency. Review Standard 90.1 and the AEDG for climate zone specific requirements and select the heating and cooling equipment efficiencies based on the climate specific tables in either document as deemed appropriate.
Chilled water systems can be designed for high temperature differentials of 12 to 18 F delta T, low supply water temperatures (38 to 40 F), and variable flow with modulating valves. Selecting a chiller for a higher delta T can reduce equipment cost and energy use when compared to the traditional 10 F delta T. This design strategy can reduce pump energy (lower flow) and piping installation cost (smaller pipe sizes); however, lowered leaving water temperature does use more chiller energy that may not be offset by perceived gains in pumping and fan energy savings. The manufacturer’s minimum chiller flow rate should be maintained when setting the minimum pump flow. The total annual system energy use must be considered.
Another aspect of selecting a lower supply water temperature is that it may increase occupant comfort by allowing for a reduction in the supply air temperature and dew point at zone equipment. Low-temperature chilled water systems allow the supply air temperature to be lowered from the traditional design temperature of 55 to 48 F or lower. This type of air-side design is now called a cold air system. The lower supply air temperature requires less airflow, yielding a smaller fan, duct, coils, etc. Another energy savings opportunity is to implement a chilled water temperature reset schedule. The temperature can be reset based on outdoor air temperature, zone cooling demand, or both. The engineer must take care to avoid “dumping” cold air on the occupants by selecting high aspirating diffusers or using fan-powered terminal units to provide tempered mixed air.
HVAC heating water systems designs should be centered on high-efficiency condensing boilers with design temperature differentials of 30 to 40 F. Condensing boilers achieve higher efficiencies by condensing water vapor in the flue gases and reclaiming this waste heat to preheat the return water. Most condensing boilers require return water temperatures of 140 F or less to achieve efficiency levels above 85% dependent on firing rate. The designer must carefully select the entering supply water temperature to ensure that the return water temperature is correct. Again, this design results in lower pump energy, and lower installation costs. Similar to the chilled water system, the heating water system should use a temperature reset schedule.
DOAS coupled with water- or ground-source heat pumps, fan-coil units, or single-zone VAV systems can reduce energy consumption by removing the ventilation OA conditioning and dehumidification load from the zone heating and cooling loads. A separate DOAS unit will heat, cool, and dehumidify the OA to deliver dry, neutral air to the space that has the added effect of offsetting the space latent load. DOAS configurations may include direct exchange (DX) coils, chilled water coils, indirect gas-fired heating, hot water coils, steam coils, and an energy-recovery device. DOAS can be used in conjunction with single zone or multiple zone systems. A designer can use the following strategies to further reduce DOAS system energy costs.
Consider supplying cold OA, rather than neutral temperature air, directly to the zone. This can reduce reheat energy and partially meet the zone sensible cooling load. The terminal HVAC equipment should then be right-sized to account for the reduced cooling load. Please note that there are many design paths that can be taken and many other factors such as space humidity should also be considered during the design process.
Incorporate demand control ventilation (DCV) with modulating dampers and airflow measuring stations in the DOAS. DCV can use a combination of space carbon dioxide (CO2) sensors in heavily occupied zones and occupancy sensors in normally unoccupied/limited occupancy zones. Moreover, the occupancy sensors can control the lighting and set back the VAV box airflows to minimum and ultimately control the main HVAC system. The VAV boxes can receive building automation inputs including building schedules (see Figures 3 and 4 for VAV box schematics). Standard 62.1 outlines when a system must use DCV. The control sequence of operation can be complex, but a general guide is presented in Standard 62.1. Remember to account for building pressurization; the DCV minimum OA setpoint must be equal to or greater than the total exhaust airflow.
Exhaust air energy recovery is used to recover energy from the exhaust airstream and to the OA stream. This can be achieved with sensible heat exchange devices (sensible energy transfer only) or total energy exchange devices (sensible and latent energy transfer). During cooling conditions, the OA is precooled and/or partially dehumidified. During heating conditions, the OA is preheated and/or partially humidified. Commonly used air-side energy recovery devices are run-around loops, plate heat exchangers, total energy wheels, and heat wheels. Refer to Standard 90.1 Table 18.104.22.168 that provides exact conditions when an HVAC system requires energy recovery. The requirements are based on climate zone, percent OA, and design supply airflow. When an energy recovery device is required, the system must have a minimum 50% effectiveness.
The exhaust and outdoor airflows should be balanced as near as possible to maximize energy transfer and to maintain building pressurization. Bypass dampers must be installed around the energy recovery device when an HVAC system uses an air-side economizer. It is imperative to downsize the heating and cooling equipment based on the adjusted design loads with energy recovery. Right-sizing the heating and cooling equipment will have a cascading energy savings (such as reduced pumping power, downsized chillers, and boilers).
Air-side economizers provide free cooling when outdoor conditions are able to fully meet or partially meet the cooling load. A typical starting point in a cooling predominate climate would be equivalent to the selected design discharge air temperature of the building AHUs such as 55 F, but there may be instances where the designer could select other temperatures to meet the project’s needs. Standard 90.1 does not require economizers in climate zones 1A or 1B because of limited operation hours in these hot, humid climates. All other climate zones require economizers on systems with a cooling capacity greater than or equal to 54,000 Btu/h. In more humid climates, the designer should use an enthalpy-based control sequence to minimize unwanted moisture entrainment.
Control sequences are a key element in achieving energy management and savings. In an effort to standardize control sequences and aid in the design process, ASHRAE developed a set of control sequences for commonly used HVAC systems. These sequences provide a good starting point for the designer to expand ASHRAE’s sequence to suit the particular HVAC system and state codes/standards, and to meet the owner’s requirements. Furthermore, Standard 90.1 requires that spaces be grouped into similar thermostatic control zones controlled by a single thermostat. For example, exterior zones and interior zones cannot be zoned together.
Standard 90.1 requires that building automation systems (BAS) employ time-of-day schedules and have night setback/setup temperature setpoints. This is preferred over programmable thermostats because the occupants cannot override the zone setpoint. The AEDG suggests using optimal start controllers to determine the time required for each zone to meet the occupied temperature setpoint and delay system startup as long as possible. Standard 90.1 requires optimal start controls for individual air systems with a supply air capacity greater than 10,000 cfm. Optimal start controls save energy by reducing the HVAC system run-time hours.
Multiple-zone VAV systems must employ a supply air temperature reset schedule based on OA temperature, zone cooling demand, or a combination. See Figure 5 for supply air temperature reset. For example, the BAS monitors OA temperature and resets the supply air temperature up or down. Overrides are typically included to reset the supply air temperature to the minimum if the zone humidity exceeds an upper limit setpoint. Additionally, interior zones and telecom rooms must be designed to meet their cooling loads at the warmest supply air temperature. Failing to do this will result in undercooling when using outdoor temperature reset or the supply air temperature will never reset when using zone demand control. While this strategy increases fan energy use, it decreases both cooling and reheat energy consumption.
Furthermore, Standard 90.1 requires systems with direct digital control (DDC) of individual zone boxes that report to the central control panel and have a static pressure reset schedule based on the zone requiring the most pressure. Typically, the static pressure is controlled so that one zone damper is 90% open. This requires the fan be equipped with a variable frequency drive (VFD). The VFD will modulate the fan based on the system’s cooling demand and will reduce the building electrical load when the spaces are not fully occupied. The VFD lower limit should be set based on the motor manufacturer’s lower limit.
Providing ventilation air for IAQ and maximizing HVAC system designs for energy savings is interwoven with owner requirements, interdisciplinary coordination, equipment selection, and design iterations. As equipment efficiencies approach the law of diminishing returns, overall system efficiency and building efficiency will become the subject of future standards and codes.
Randy Schrecengost is a project manager/senior mechanical engineer with Stanley Consultants. He has extensive experience in design and project and program management at all levels of engineering, energy consulting, and facilities engineering. He is a member of the Consulting-Specifying Engineer editorial advisory board. Gayle Davis is a mechanical engineer with Stanley Consultants. He has experience in the design of HVAC systems, boiler plants, compressed air systems, plumbing systems, steam distribution systems, central heating, and cooling plant design. He is also experienced in commissioning and retro-commissioning.