One Mechanical Solution Does Not Fit All
HVAC system selection is one of a mechanical engineer’s most influential responsibilities in a project. The choice of system will drive factors such as occupant comfort, energy goals, operational costs, acoustics and aesthetics, equipment room sizing and location, and ceiling space requirements, to name just a few. The process of choosing the appropriate system is a team journey through myriad factors, needs, and concerns.
Humans spend about 90 percent of our lives indoors. We want to make it the best experience possible.
The science of how buildings operate with warm spaces, cool air, fresh air velocity, and positive and negative pressures and displacements creates many variations to create a hospitable environment. We can achieve those solutions through closed-loop refrigerant, ducted airflow, radiant flooring, chill beams, heat pumps—the possibilities are seemingly endless.
In order to sift through the opportunities and calculations, an engineer must first gain an understanding of the stakeholders’ goals. During the pre-design phase, we make sure to ask poignant questions such as:
- What is important to the design program, the facility, and its users?
- Is high efficiency, low-energy the utmost priority?
- Does equipment need to stay off the rooftop? Are there areas on the ground that need to be avoided?
- Is ambient noise a concern?
With an understanding of the goals, concerns, and limitations, system options can be filtered down into viable solutions and reviewed with stakeholders. We consider factors such as installation costs, maintenance responsibilities, limitations of the equipment, and other aspects that impact the final deliverable. Even the smallest factors should be addressed. According to the Consortium of Energy Efficiency, at least 25% of all rooftop HVAC units are oversized. This can directly affect first-installation costs, lost efficiency savings, and long-term operating and maintenance costs. Regardless of the final design, it’s important to understand as many contributing factors as possible.
Early discussions should include needs, feedback on options, and committed stakeholder decisions ranging from facility standards to site conditions that require explicit temperature and humidity levels. This will help the mechanical team make informed recommendations. Then the engineering team does what they do best: Design, size, and specify the best equipment and infrastructure to keep a building’s indoor environment comfortable, reliable, and supportive of the users’ needs.
Leveraging an integrated design team ensures that smart, experienced engineers are part of the early conversations shaping the course of a project. The Whole Building Design Guide estimates that the use of high performance HVAC equipment can result in considerable energy, emissions, and between 10 and 40 percent in cost savings. Alternatively, a whole building design—or what we refer to at DLR Group as integrated design—coupled with an extended comfort zone approach, can produce much greater savings of 40 to 70 percent. Early discussion of possible options and project factors facilitates team-wide endorsement of the chosen solutions and achievement of the highest level of performance.
CSE: What are the current challenges when specifying HVAC systems for educational facilities?
JR: Schools today are striving for flexible learning environments. Flexible and adaptive spaces present a distinctive challenge to a mechanical system. Defining space zones, how they’re used, and establishing acceptable comfort ranges is how equipment is sized. Controls systems drive the equipment to supply the spaces as they’ve been defined. Variety and adaptability are great for space planning and programs, but require versatility in a mechanical system and sophistication in its controls. This doesn’t necessarily mean complexity, but a system designer needs to be aware that target environments will change. These are not impossible requirements to provide—engineers are great at designing systems that can be dynamic and robust. These challenges simply need to be discussed, coordinated, and understood as the best process to accommodate needs for today, and in the future.
CSE: What unique HVAC requirements do such projects have that you wouldn’t encounter in other projects?
JR: Schools and the learning environment have a critical focus on noise. Our design must take into account solutions for sound attenuation. By performing acoustical analysis studies, we can implement mitigation strategies such as locating HVAC systems away from classrooms or adding additional sound barriers, so that systems can meet goals without disrupting education.
CSE: What are some of the challenges or issues when designing for energy efficiency insuch buildings?
JR: Energy recovery units are dependent on interior loads for redistribution or re-application. If spaces aren’t used with consistency or regularity, they aren’t good candidates to participate in an energy recovery scheme. Agua Fria Union High School District’s Canyon View High School is a facility with variable functions, programs, and times of use that can create limitations. The innovation in the mechanical design was to provide a variety of solutions, each selected for the portion of the facility based on its demand and tolerances. The project has a combination of direct expansion (DX) equipment, variable air volume (VAV) distribution, energy recovery units, and central plant-supplied fan coils. Each of the systems is at its respective best efficiency and effectiveness.
CSE: Have you specified variable refrigerant volume (VRF) systems, chilled beams, or other types of HVAC systems for a K-12 building?
JR: Specialty-type solutions like VRF, radiant slabs, and chilled beams require extensive early coordination and discussion with all of the stakeholders. VRF has great advantages, such as allowing simultaneous heating zones and cooling zones, reduced distribution space requirements, such as piping versus ducting, and easy expandability. But a project also needs to have experienced installers and a willingness to support a manufacturer’s proprietary system. Radiant heating and cooling systems can greatly reduce a building’s energy usage, but users need to be educated on the response times and shift familiarity with airflow. Chilled-beam systems tackle loads extremely well, but attention has to be paid to operable windows and varying dew points. All of these factors are easily managed and accommodated by effective design, but only when understood and embraced by the entire project team in the early stages. This eliminates surprises and helps prepare facility operators for their new systems.