Right-sizing HVAC

Improperly designed HVAC systems can be a source of headaches. Too big for a project and they can lead to huge bills, both at the outset and over years of operation; too small, and they don’t provide adequate heating and cooling for the facility. Here, industry experts offer advice on tailoring HVAC systems just right, every time.

By Jenni Spinner, Contributing Editor December 6, 2010


  • Roger Chang, PE, LEED AP, Principal, Director of Sustainability, Westlake Reed Leskosky, Cleveland
  • Robert F. Sty, PE, LEED AP, Associate, SmithGroup, Phoenix
  • Peter D. Zak, PE, Principal, GRAEF, Milwaukee

CSE: What is right-sizing HVAC?

Peter D. Zak: Proper system sizing involves establishing the correct design criteria, used to determine actual building loads. Matching HVAC equipment to actual loads reduces first costs and operating costs.

Roger Chang: The goal is to select equipment to match realistic heating, ventilation, and cooling demands as closely as possible. We try to apply this approach to all projects, with close consultation with the owner. For example, the typical assumption for lighting power density in a museum gallery is 6 W/sq. ft. On a recent project, by carefully working with the exhibit designer and leveraging current lighting technology, we were able to design to 3.5 W/sq. ft. comfortably.

CSE: What are the benefits of right-sizing HVAC?

Chang: Capital can be transferred to more efficient equipment, an improved building envelope, or perhaps just nicer interior finishes. Laboratories, in particular, benefit from significantly reduced energy consumption, due to the tremendous amount of conditioned air that is typically moved to maintain either thermal comfort or air quality.

Zak: Proper sizing will permit smaller equipment, which creates a lower first cost. Properly selected equipment improves overall system efficiency, which will avoid equipment cycling and inefficient part load performance. Maximizing efficiency and minimizing equipment cycling reduces maintenance and operating cost expenses.

Robert F. Sty: Many times HVAC systems are oversized in an effort to be prepared for the “unknown” conditions—i.e., better safe than sorry. What happens is that the systems may end up being many times larger than what is really required, and the corresponding construction bill is greater than what the owner was expecting. Imagine applying a safety factor of 10% to each zone in an office building floor plate after performing a detailed heating/cooling load calculation and adding a 5% overall safety factor, or rounding up the size, to the air handling units (AHUs) which serve the zones. By establishing extreme temperature setpoints in the space in addition to this, you could end up with an air system that could be 20% larger than what the load calculation had called for. Carrying this through multiple air handlers, pumps, chillers, boilers, etc., can yield large cost overruns in the mechanical system and downstream cost implications for electrical and structural systems.

Operational costs can spike if equipment is operating at a point of decreased efficiency due to oversizing. Peak loads happen only 1% to 2% of the year in a typical office building, so even though equipment is sized to properly match the peak load, it is actually operating at part load most of the year.

CSE: What tools, techniques, or technologies work best? Which ones don’t work?

Sty: As load calculation and modeling software become more advanced, engineers are able to model heating/cooling loads and predict the performance of equipment at various operation points more accurately. Any calculation program, however, is only as accurate as the information entered by the user. The engineer should truly understand the building construction, orientation, internal loads, and required space temperature/humidity setpoints to calculate accurate results. This requires working closely with the architectural team early in the design to develop an energy-efficient building envelope, and with the owner to best identify the use of the space, setpoint requirements, and occupied hours. Revisiting projects that have been in operation for 2 to 3 years and examining historical trending data from the building management system can provide information which can be applied to similar projects in the future.

Zak: The best programs are computer models for energy calculators, and equipment selection programs which allow you to evaluate full and part load performance.

Chang: Develop a very detailed basis of design that captures owner requirements, climatic design assumptions, code guidance, and best practices. Design assumptions should be refined by modeling actual lighting power, equipment loads, and envelope thermal properties. There is no substitute for robust calculations based on industry standards. ASHRAE Standard 90.1 has really driven down heating and cooling loads, which makes older rules of thumb inappropriate. Calculations have traditionally been very conservative by assuming no internal gain for heating and no variation in internal heat gains over the course of a day for cooling calculations. Dynamic thermal simulation tools, such as IES-VE, EnergyPlus, or Ecotect, can be employed to right-size based on more realistic operating conditions.

CSE: What are the most common mistakes made? What should engineers avoid?

Chang: Client expectations themselves must be right-sized. Communication is extremely important. The client needs to understand design assumptions and safety factors. Many institutions have facility design guidelines that mandate very specific sizing criteria. Right-sizing may violate those criteria and lead to conflict after construction. Because the design process for a building can be very dynamic, it is important to align calculations with architectural design at regular intervals. Be conservative with building envelope assumptions, particularly given the complexity of details we are seeing with buildings today. Engineers should not feel pressure to reduce equipment sizes to the absolute minimum. It’s easier to reduce sizes, rather than have to go back and increase, due to associated costs, impact on electrical service, and space requirements.

Sty: Rules of thumb are gross estimates at best, and should only be used for preliminary equipment sizing in the very early stages of a project to help set basic parameters needed by the design team. Engineers should work with architects to understand the impact of building envelope construction and shading strategies on HVAC equipment sizing. Avoid working in a bubble, and become more integrated in the whole building design process.

CSE: What factors are commonly overlooked?

Chang: There is no such thing as a perfectly constructed building. While a designer’s intent can be shown carefully, issues can arise during construction that lead to changes in duct and pipe routing, which can impact fan and pump sizing. The use of 3D modeling platforms helps reduce this, but each building project needs to be evaluated on its own for the level of risk where design intent may not be met.

Zak: Each building has its own energy footprint. To efficiently match the HVAC system, building loads need to be evaluated on an individual basis.

Sty: The amount of energy required for reheating after dehumidification of the airstream sometimes can be significant and easily overlooked. This is especially true in lab buildings which can be ventilation-driven versus equipment or occupant loads. The designer should examine the use of enthalpy wheels or “runaround” energy recovery systems in addition to traditional means of reheat.

CSE: What calculations are needed to right-size these systems?

Sty: Heating and cooling load calculations, duct and pipe pressure loss calculations are performed as they should be in all projects. These calculations should be run for not only peak load values but to also investigate how the system reacts when the facility is operating at part load conditions. Conditioning outside air to meet ventilation requirements can account for a large percentage of the energy use and equipment size, so understanding what is truly required for the building for proper indoor air quality and pressurization is very important. Performing calculations based on ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality, can help define proper ventilation rates for the building.

Chang: ASHRAE handbooks provide guidance on assumptions required for sizing systems. Ongoing research, particularly at national research labs and the New Buildings Institute, is focused on better understanding receptacle loads (computers, copiers, printers). Designers should use tried-and-true computer-based heating and cooling load programs. For laboratories and data centers, the challenge is predicting heat gain or fume hood diversity with an appropriate level of comfort. Obtaining submetered data of major equipment in comparable facilities can help designers achieve a higher level of comfort with design assumptions. Designers need to develop a careful inventory of equipment. Ultimately, more progressive codes and industry-approved research will give designers more comfort to right-size. Another related example is natural ventilation—until the adaptive thermal comfort standard was released in the 2004 version of ASHRAE Standard 55, designers were more hesitant to completely eliminate HVAC systems in buildings.

CSE: What are the long-term considerations, and overall return on investment?

Zak: Long-term considerations should include the added value of carefully matched systems, and potential savings from operating and maintenance costs. Generally, our client base relies on a ROI of five years or less.

Chang: Future flexibility is the most important consideration. A right-sized system today could be an undersized system in the future. Applying modular approaches to design is preferred.

Sty: The long-term considerations for right-sizing are the costs of maintenance and extending the life of the equipment. Right-sizing equipment will allow operation at the most efficient points, thereby reducing unwarranted stress and early failures. Investing the proper design time up front to perform the required calculations and modeling can pay off with correctly sized equipment, reducing energy and operating costs.

CSE: Describe the environmental and indoor air quality (IAQ) considerations.

Sty: There has always been a balance in attempting to provide proper IAQ and maximizing energy savings in facilities. Systems which do not provide enough ventilation air can lead to serious problems such as “sick building syndrome,” but providing too much can unnecessarily drive up a building’s energy/operating cost. In a mixed air system if the overall ventilation requirement is established by the requirements of a single space, the addition of a dedicated outside air unit may improve energy efficiency and downsize the main AHU. The design engineer should pay close attention to the space and humidity temperature setpoints established during design. Is it reasonable to maintain an office space at 74 degrees during the summer, or is it more realistic that 76 to 77 degrees will be maintained? A few changes in the degree setpoint can have a major impact on equipment size and energy use.

Chang: Dehumidification performance, particularly of DX equipment, is significantly compromised by oversizing. Deliberate undersizing of systems still has its place. Consider San Francisco, a city that experiences only a dozen days where natural ventilation is not possible. Does an owner install a system sized for the hot days or just accept that people might have to go home on those days? The designer must align thermal comfort expectations with the design. Work at the Center for Built Environment is focused on better understanding what drives comfort. While 80% to 90% satisfaction is considered standard of care, if the CEO wants a 70°F environment, and systems aren’t sized for that, there will still be a problem.

Zak: Indoor air quality and environmental considerations are controversial in my opinion. ASHRAE acknowledges that there is no absolute temperature and humidity level that will appease all of the occupants. There is a range or comfort zone in which a majority of the subjects were satisfied. As to air quality, again, we typically rely on CO2 as a measurement of air quality; this of course can be compromised by background CO2 levels of the outside air being introduced. Other considerations are potential contaminants within the building itself. In general, the codes will define minimum requirements; an open, honest dialogue with the owner will further define the ventilation requirements.

CSE: With regard to fans, blowers, etc., do variable frequency drives (VFDs) play a role?

Zak: VFDs do play a role in operational efficiency—that is, of course, assuming you are varying the motor speed to respond to a fluctuating load. Too often I see systems which have drives installed on constant load systems in lieu of a starter. The drives are used as a means of balancing the system. In some cases the speeds have been set at substantially lower levels than 85%. To me this suggests improper sizing. Some will argue that there is no substantial cost difference between a good starter and a drive, and in fact there may be some increase in efficiency in using the drive. I will argue that if there is a device that can be manually sped up or slowed down, and the maintenance staff is not properly trained or qualified to operate the building, the device speed will be adjusted, which will eventually create some level of imbalance within the HVAC system.

Sty: VFDs offer great opportunities to save energy, and should be installed wherever possible. When selecting equipment, say for example chilled water supply pumps, it is important to view the performance of the pump curves during part load application. A pump that is selected with only the peak performance in mind will operate there only 1% to 2% of the time throughout the year, and may not be the best selection for the majority of operating hours. Pumps should be selected at the most efficient operation under the largest range of run hours, and then verify that the pump is sized sufficiently for the peak loads.

CSE: What other building improvements will help ensure the HVAC system is efficient?

Sty: HVAC systems do not create the load; they respond to the load. Providing a tight, energy-efficient building envelope with shading will help reduce the size of the systems. Building orientation and footprint also play a large role, and if possible the mechanical engineer should provide input to the architectural team. Proper daylighting strategies can reduce the internal heat gain from lighting loads without adding solar gain to the building. System control strategies and reasonable environmental setpoints must be established during the design, commissioned, and constantly monitored during occupation.

Chang: A high-performance building envelope allows an HVAC system to be sized to accommodate a smaller variation of heating and cooling during the course of a day. Use an integrated design process that involves the owner, architect, engineer, and contractor early on.

Zak: The purpose of an HVAC system is to respond to changes in building thermal, ventilation, and internal loads by adding or subtracting heat, moisture, and properly filtered air. Static building components such as the envelope should be selected to minimize the transfer of energy between the indoor environment and the outdoors. Dynamic loads such as varying ventilation and occupancy or process loads require accurate and reliable controls to ensure a proper response from the HVAC system.

CSE: What controls are installed on these systems?

Zak: Typically electric or electronic controls are used in basic systems. More sophisticated building automation systems will use a computer interface to coordinate the operation of the building HVAC system and other building functions.

Sty: The modern control system for larger buildings is a direct digital control (DDC) responding to a building automation system. These systems allow the equipment to be controlled to achieve maximum energy efficiency. They also allow trend logging of data, which will allow the building operators to make adjustments as needed to increase efficiency and avoid mistakes such as simultaneous heating and cooling of spaces. In smaller buildings, fully programmable thermostats may be a cost-effective measure.

Chang: A modern DDC system with energy measurement functionality allows a building operator to understand how each HVAC component is operating. Because a right-sized system does not have spare capacity, it’s necessary for systems to be able to shift heating, cooling, and ventilation to zones with actual demand quickly.

CSE: What prescriptive program, such as the Whole Building Design Guide (WBDG), do you suggest, and why?

Chang: The Whole Building Design Guide contains a wealth of information and links to other references that will assist any designer. There is no single prescriptive design path available. The best approach is to carefully apply standards and codes and communicate assumptions clearly.

Zak: You cannot provide a set guideline to maximize efficiency; a prescriptive guideline should be just that—a list of suggestions, ideas, or guidelines to achieve a performance goal. However, each building is an individual entity and should be treated as such to achieve anticipated goals.

CSE: Anything to add?

Chang: One of the unfortunate things in the industry is the fear of litigation. Until there is a greater understanding of the intent of right-sizing, many designers will be hesitant to eliminate safety factors. Owners need to be engaged throughout the design process proactively. Engineers and architects need to communicate their design intent clearly. HVAC engineering is likely the most challenging of all building engineering disciplines, because of the pressure to right-size and the thousands of inputs that are required to size a system. Many of these inputs are nonprescriptive. The reality is that many projects are still provided with significantly oversized systems because the amount of analysis required to right-size with comfort doesn’t align with design fees.

Sty: With sustainable, energy-efficient design becoming the expectation of our clients, the manner in which we size our systems and equipment is of great importance. Rule-of-thumb application and overlapping safety factors will lead to equipment that will meet load requirements but may not provide the best environmental conditions and will certainly not perform in the most efficient manner. Understanding the loads of the building will help the design engineer develop a system that responds properly without oversizing and wasting energy. Right-sizing equipment will lower first costs (which we can all appreciate in our current economy) and provide long-term energy and cost savings throughout the life of the system.