Class A office building system design: HVAC
Class A office buildings are among the toughest projects an engineer can work on—complex structures, demanding clients, and advanced technology. HVAC systems are a key component of these office buildings.
Daniel G. Dowell, VP Energy Performance Contracting Sales, ABM, Raleigh-Durham, N.C.
Kurt Karnatz, PE, CEM, HBDP, HFDP, LEED AP, President, ESD, Chicago
Lance Kempf, PE, Director of Mechanical and Electrical Engineering, LEO A DALY, Minneapolis
Brian Michelson, PE, MEP Design Phase Manager, Mortenson Construction, Minneapolis
Joseph H. Talbert, PE, ARM, Project Manager, Aon Fire Protection Engineering, Lincolnshire, Ill.
CSE: What unique HVAC requirements do Class A office building projects have that you wouldn't encounter in other buildings?
Michaelson: Class A office space by itself is not unique. It's when the client has space needs that fall outside of this. A cafeteria and conference center in an office building, 7 floors of office above a parking ramp, or a basement-level data center tend to make these buildings unique.
Karnatz: Office buildings are increasingly being expected to provide higher degrees of flexibility, controllability, measurability, and feedback. For example, more office tenants are operating during hours outside of the typical workday. Systems must be able to efficiently turn down to support both localized and after-hours loads. Office occupants also expect more control and autonomy in determining when systems run and what temperature their space is. Measurement, verification, recording of, and feedback from building HVAC systems also has become a common expectation of tenants. Verification of air quantities, ventilation rates, air quality, and energy consumption are typical data that office tenants are increasingly requesting.
CSE: What changes in fans, variable frequency drives (VFDs), and other related equipment have you experienced?
Karnatz: Electronically commutated motors and direct-drive fans arranged in multiple modular arrays is the new norm. This approach results in high-efficiency systems with very fine controllability and turndown capability.
Michaelson: When I started my career, VFDs were large, expensive, and unreliable—but it didn't take long for the size and cost to lower and reliability to improve. Today, it is rare to not have them in use for all types of systems. Pumps and fans are now available with the drives built-in and eliminate the need for a separate drive altogether.
CSE: When retrofitting an existing Class A office building, what challenges have you faced and how have you overcome them?
Michaelson: The toughest challenge I have seen is installing new air handlers that will be used while the existing air-distribution ducts remain. Careful planning was required to make certain the new equipment would fit and that there was a way to bring it into the building. We couldn't rely on the record drawings and had to survey the space ourselves. It is even more challenging when the client only has a small window of time during which this can be done. In this case, the air handling systems had to be disassembled outside, moved inside in sections, and then reassembled in the final location.
Karnatz: The greatest challenge typically in a retrofit is maintaining business continuity in an occupied building during the construction. Very often, systems need to be paralleled and cut over from old to new many times through the process. Engineering the process, the logistics, and schedule requires careful and thorough planning including contingency planning for recovery from disaster or unintended consequences.
CSE: Have you specified any combined heat and power systems on a Class A office building? If so, please describe the system.
Karnatz: Not recently. In fact, we have decommissioned and retrofitted several in 2015.
CSE: What indoor air quality or indoor environmental quality challenges have you recently overcome, especially in a mixed-use office building? Describe the project, and how you solved the problem.
Karnatz: Controlling stack effect in our very tall buildings is always a challenge. Mixed-use occupancies complicate the controllability of indoor air and environmental quality. Beyond compartmentalization and pressure controls, we typically use zoned and dedicated outside-air systems, enhanced pressure, and pollutant measuring and catalytic filtration.
CSE: Have you recently specified more alternative HVAC systems on Class A office building projects? This may include displacement ventilation, underfloor air distribution (UFAD), variable refrigerant flow (VRF) systems, chilled beams, etc.
Karnatz: As engineers and good consultants, it is important that we serve our clients well by carefully analyzing system options and guiding them to make the most appropriate solution to satisfy their business case. What is right for one building is usually not so for the building next door. We have designed more than 20 million sq ft of UFAD systems and think very highly of the approach, but it is simply not right for certain climates. Similarly, chilled beams are a great solution for specific applications, such as high sensible cooling environments and low floor-to-floor heights and retrofits, but not for a building that demands a high degree of flexibility. VRF is also a first-choice solution for certain applications, but also has regional code-compliance complications.
Kempf: Cooling for the majority of the new SAC Federal Credit Union headquarters is provided by an active chilled-beam system. The new system will provide more comfort and operate more efficiently, and will be easier to operate than a standard mechanical configuration. LEO A DALY estimates the chilled-beam system to be 41.8% more efficient (than a conventional mechanical system), lowering SAC's future utility costs by almost 30%.
The heart of the system is a large heater/cooler composed of five water-to-water heat pumps. The unit rejects heat to water when cooling and absorbs heat from water when heating. This water is pumped to a geothermal well field comprised of 218 300-ft-deep wells. The well field is on the south side of the property. Hot and cold water is pumped in and out of the heater/cooler at levels regulated by the heating and cooling needs of the building. To cool the building, water travels from the heater/cooler to chilled beams that are integrated into the ceiling tiles. Dehumidified and cooled air is then distributed at a high velocity in the chilled beams, inducing room air across cooling coils, thereby cooling it before quiet recirculation back into the space. To heat the building, water is pumped through the heater/cooler to perimeter radiators. Again, the amount of heat in the water is regulated by the heater/ cooler.
CSE: Describe a challenging building envelope project you recently designed in a Class A office building.
Karnatz: A building is a system of systems, and we view the envelope as one of them—especially in terms of performance, modeling, code compliance, and thermal/photometric quality. Engineers need to be very careful with the performance data that is provided by envelope systems manufacturers when creating energy, thermal, and photometric models. Unfortunately, we often find that actual installed performance is worse than modeled. For this reason, we typically require that physical thermal tests be conducted in a mock-up condition during design or prior to construction.