How to choose the right HVAC System: Code and efficiency considerations

HVAC insights

• Project-specific criteria influence HVAC system selection and design.
• Air handling units (AHUs) and associated components are critical in maintaining indoor air quality.

The process of selecting and sizing heating, ventilation and air conditioning (HVAC) systems includes several steps that a designer needs to complete for any given installation or application. The final selection of an appropriate system option is based on the designer’s experience, the owner’s requirements and the overall project constraints.

While providing comfortable HVAC systems has always been important for design engineers, energy efficiency has become a particular focus in recent years. The owner’s project budget and many other factors will impact a building’s overall energy use. As building codes and standards have become more stringent, owners, property managers and/or those in charge of budgetary responsibilities for projects should fully understand how design decisions will impact energy use.

Codes, standards and guides

Local, state and federal codes, standards and/or regulations dictate requirements that can affect an HVAC system’s design, installation and operation. Location, building type and occupancy dictate the specific sections of the codes and standards that will ensure safety, efficiency and compliance with regulatory requirements. As the design of a system includes multiple components related to its specific application, a thorough review of the codes, standards and/or regulations necessary to finish a design must be completed to avoid conflicts.

Codes vary from state to state but typically include safety, structural stability and energy efficiency requirements. To ensure compliance throughout the design, construction and operation phases of a project, the entire team of professionals must familiarize themselves with the relevant codes and standards.

ASHRAE is one of the most useful organizations that coordinates and develops consensus standards to establish minimum values for acceptable performance levels. ASHRAE publishes standards and guidelines specifically related to HVAC systems, indoor air quality (IAQ), energy efficiency and thermal comfort. These include ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings. This is the reference standard for Energy Efficiency with the purpose, “to establish the minimum energy efficiency requirements of buildings, other than low rise residential buildings, for 1) design, construction and a plan for operations and maintenance and 2) utilization of on-site, renewable energy resources.”

Figure 1: This illustrates a plan view of a hydronic supply and return water boiler plant system using multiple boilers and pumps in a primary / secondary pumping arrangement with N+1 redundancy. Courtesy: Submitting Engineering Firm

Of the International Code Council codes, the International Building Code  systems, in residential and commercial buildings. The IECC mandates energy efficiency requirements for HVAC systems and the IMC provides requirements for the design, installation, operation and maintenance of various mechanical systems. It covers ventilation, duct systems, heating and cooling equipment and energy efficiency.

Finally, the Unified Facilities Criteria (UFC) covers all types of systems and components for the different military branches and some federal buildings. One example is UFC 3-410-01 for HVAC Systems which provides design requirements for system sizing and selection and types of HVAC systems based on building occupancy, usage and climatic conditions.

Understanding system application, selection and constraints

It may sound simplistic, but system selection is greatly dependent upon the project being developed. A new building being designed can have almost limitless options, depending on the owner’s budget for both upfront costs and long-term operating costs. The options might be considerably reduced for a building upgrade or retrofit project due to space constraints or budget, often dictating a similar system selection with some conversions to more advanced components.

The owner and designers must work together to define and prioritize project goals and criteria to meet the design. Designers should present the advantages and disadvantages of various system types, while the owner should relate how the systems will fit within the project’s financial and operational goals. Project criteria to consider include local climate and comfort, building load calculation and energy model, proposed and future equipment capacities, system reliabilities and redundancies, serviceability and sustainability.

Figure 2: A preliminary dual duct air handling unit (AHU) selection’s elevation view. Courtesy: Stanley Consultants

Once the project criteria and goals are identified, any system constraints must be evaluated with the different possible system selections. Constraints may include the building architecture and functionality (occupied tenants, process and ventilation requirements, etc.), available space for equipment, limitations on capacities and performance (temperature, humidity, efficiencies), applicable local codes and available utility sources.

The primary goal for HVAC designers is to provide a system that can meet the building’s comfort requirements at a reasonable cost while minimizing maintenance costs and energy use. IAQ, energy use and occupant thermal comfort are a few of the concerns faced by building operations and maintenance staff. Due to project constraints, and specifically the owner’s budget, potential HVAC system choices are typically narrowed down to one or two options. These selections will typically also consider a primary system that creates a medium for the heating/cooling requirements and one or more secondary systems that deliver the conditioned medium to the individual spaces for environmental control.

HVAC system equipment and integration

Centralized, or decentralized primary HVAC systems will produce the medium air, water or refrigerant for the cooling/heating requirements for a specific application and deliver it in a constant or variable volume format. Afterward, one or more secondary systems will ultimately deliver the medium to building spaces (e.g., pumps, air handling units) for the control of each individual environmental requirement. Designers have multiple options to employ in HVAC systems but, in general, there is always a heat absorption or removal component and a transfer of that heat within the medium (water, refrigerant) to a heat rejection component (a condenser, heat exchanger) where the waste heat is “dumped or wasted,” and the medium is recycled back to the original component.

Cooling systems may consist of various technologies such as chillers, direct expansion systems or evaporative cooling that provide equipment for selecting and designing these systems to meet specific cooling loads and conditions. Most primary cooling systems employ refrigerants in a vapor-compression or absorption cycle. They can also include chillers (air- or water-cooled), heat pumps (air, ground or water source) and other refrigeration-based systems that may be centralized, remote, split or portable. Some systems are evaporative, using water evaporation and swamp coolers to cool air in either smaller or larger applications, including cooling towers. Other systems use chemical-based substances like desiccants to control temperature and humidity by absorbing moisture from an air stream. No matter what kind of system is employed, there will be additional components and possible devices such as pumps, compressors, expansion tanks, air separators or eliminators, control valves and more.

Figure 3: A retrofit project for housing historical documents included the replacement of an existing air handling unit (AHU, on the right) and a new DOAS unit (on the left) to assist in space humidity and temperature control. Courtesy: Stanley Consultants

Primary heating system types may include boilers and furnaces that provide heat by circulating hot water or steam through a network of pipes or heated air that is distributed throughout the building via ductwork. The distributed medium temperatures are controlled either by utilizing various types of fuels or with heat pumps and refrigeration-based systems. Just like in the cooling systems, there should be an emphasis on selecting the most efficient and appropriate system while keeping in mind that there may be similar additional components and devices. Designers should select the equipment at high part load efficiencies to maximize energy savings while considering any mission-critical functions that might require redundancy provisions incorporated in the design.

A pump is required in any hydronic system composed of piping, fittings, heat exchangers or other equipment through which a fluid needs to be delivered or transferred. The process of selecting and sizing pumps includes several steps for any given installation.

Items to consider in selecting a pump include overall system layout, building floor space and headroom, requirements of the pumping scheme, code issues, intended life of the system, up-front costs of the pump versus maintenance costs and overall energy use. The piping distribution system may have more than one pump and may operate in series or parallel.

Air handling units

Air handling units (AHUs) are comprised of several components that must be integrated to efficiently condition a mixed air stream of both outside air (OA) and return air (RA) within the building. In terms of functional definitions, most HVAC air systems fall into two major types. A constant-volume, variable air temperature system provides constant airflow and varies the temperature of the air delivered. Conversely, a variable-volume, constant air temperature system will vary the amount of air delivered to the space and keep the temperature the same. With new technology and increased sophistication of building automation systems (BAS), modern HVAC systems can be a hybrid.

AHUs typically contain added equipment or components for control, isolation, safety and static pressure gain to perform a combination of the four basic psychrometric processes of cooling, heating, dehumidification and humidification. Common components are fans or blowers, cooling and heating coils, filters to remove particulates and contaminants, energy recovery devices, dampers, controls to regulate and circulate the various airstreams and inlet and discharge plenums. AHUs can come in any number of sizes and capacities ranging from airflows of a few hundred cubic feet per minute (cfm) to tens of thousands cfm.

AHUs are used to condition and/or circulate air within an overall HVAC system. They can be small terminal types used in local environmental spaces that include minimal components such as fan coil units or blower coil units. Slightly larger AHUs selected for outdoor use, located on grade outside or the roof, are usually referred to as packaged units or rooftop units. Other units of similar sizes can be located in buildings, typically inside a mechanical room, or maybe in an elevated mezzanine area. In addition to the components noted above, these units will typically have control dampers and serve larger areas or multiple zones within a building. They are much more defined in terms of total heating and/or cooling capacities.

The next level of AHU is considered semi-custom, highly flexible and cataloged. They can be selected to meet almost any commercial, institutional or industrial applications. Many manufacturers have a line of these types of AHUs that can be modified to meet a designer’s specific job requirements for new or existing building projects. These units use building-block or modular construction methodology for a wide range of standard and custom-engineered modules or splits. There are standardized components available such as fans, coils, filters and control packages with optional components.

Figure 4: An HVAC layout within a dorm room utilizing 4 pipe FCUs along with pretreated OA (DOAS supply air) being provided directly to the room. Courtesy: Stanley Consultants

Lastly, designers can specify custom AHUs for special applications or capacity requirements of a project that goes beyond standard manufactured equipment, often due to physical size constraints. These units are engineered and designed so their size, material type, wall thickness, insulation and internal components can be altered to meet specified performances. For example, a large semi-conductor manufacturing plant might need several AHUs sized at 150,000 cfm with a 200 horsepower fan motor and a preheat coil, a primary and secondary chilled water coil, a glycol coil, a reheat coil and a humidifier section to precondition and treat outside airstreams.

One other subset of AHUs is a dedicated outdoor air system (DOAS). As the name implies, a DOAS conditions 100% of the outdoor air brought into a building for the use of ventilation. This outside air can be delivered to all the occupied spaces directly or in coordination with local/zoned AHUs or terminal equipment to serve the building’s HVAC requirements. A DOAS unit will use familiar equipment to treat the building’s outdoor air stream independently and completely before it enters the building’s systems and mixes with other supply and/or return airstreams. A DOAS unit can be successfully applied across a wide geographical range within several different climate zones.

AHU components

An AHU is a metal box that comes in different sizes depending on the components or appurtenances necessary for the applications. They are typically constructed around a framing system with insulated roofing, flooring and side panels, either single or double skin. They are sometimes built in modules, sections or splits as required for the overall configuration.

AHU control damper assemblies should be corrosion-resistant and automatic for mixing and controlling the outside and return airstreams. Both parallel or opposed blade dampers can be used, but pressure relationships and sizing for wide-open and/or modulating pressure drops need to be considered. Dampers should be specified to meet the leakage requirements of ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings and the International Code Council.

Filters should be placed in the mixing plenum or filter section of an AHU ahead of other components, such as fans and coils. Their primary function is to filter out dirt and other contaminants and protect the AHU’s other components. Filters can be arranged in one or more layers or sets within a holding frame assembly or racking system configured in flat or angular bank arrangements. Filters can be a variety of types and sizes, from throwaway 2-inch-thick type to reusable 36-inch-deep type, based on the application. Filters are rated by ASHRAE Standard 52.2 test methods and classified by the minimum efficiency reporting value ratings.

Figure 5: An HVAC system layout within a mechanical penthouse illustrating a combination of an ERU and DOAS equipment. Courtesy: Stanley Consultants

Another important component of AHUs is the supply air fan. Depending upon the size and application, this supply fan can be composed of a centrifugal fan, one or more plenum or plug fans or a vane axial fan. The AHU can also have a return air fan to assist in overcoming static pressures to get the return air back to the AHU. The ASHRAE Handbook – HVAC Systems and Equipment, Chapter 21, covers fans and provides information on selection criteria, such as the desired design operation point for the airflow required and the static pressure of the system.

Coils are used in AHUs to provide sensible heating or cooling and/or in conjunction with humidification and dehumidification for various psychrometric applications or processes. Coils are primarily constructed of copper tubes with copper or aluminum fins pressed or extruded on the external surface for several heat transfer processes. Chilled water and refrigerants for are often used for cooling and hot water, steam and refrigerants are typically used for heating.

BAS are typically involved in all HVAC systems as well. Using direct digital control system components, the BAS will provide enhanced control and monitoring of HVAC operations using equipment and component control sequences to achieve energy management and savings goals. These sequences improve system efficiency and provide real-time data for maintenance and troubleshooting efforts. An overall control sequence can manually enable the system’s components or automate the process through a BAS. Control schemes for a HVAC system will usually vary with the size and complexity of the system.

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