Despite the enormous changes in building heating- and cooling-load patterns in modern buildings, HVAC systems continue to be configured very much the same way as they were generations ago. Then, the building envelope, lighting, and plug loads dominated the thermal balance in commercial structures. In today’s more efficient commercial buildings, the thermal influence of envelope, lighting, and plug loading has fallen so dramatically that in many efficient buildings occupants now dominate the thermal load.
When control capabilities were limited and non-occupant loads dominated building heating and cooling, it made sense to set up building-wide occupancy schedules, and apply HVAC resources to condition and ventilate buildings more or less uniformly during scheduled occupancy. But with the changes in technology and thermal loads, and the requirement for more efficient buildings, our industry sorely needs new HVAC systems!
So let’s start by considering the paradigm shift possible in HVAC systems as the occupants themselves become the dominate element in the thermal equation of building spaces. The need for general space conditioning fades and it becomes possible to direct both thermal and ventilation resources entirely to the building occupants - whenever and wherever they are in the building. Modern controls make it easy to determine when and where occupants are in the building. The only sticking point is that most current HVAC system components available today are intended for general building conditioning. It is both difficult and expensive to reconfigure these components for more specific occupant-based conditioning. What’s really needed is an entirely new kind of HVAC system.
Low Air Volume Systems
While I’ve seen a number of new ideas, my own research and analysis makes me believe the best solution for the next generation of HVAC is the developing Low Air Volume (LAV) system concept. This system delivers 100% outside air to provide both ventilation and conditioning. And rather than providing general building conditioning, the LAV system at all times channels these resources directly to the building’s current occupants.
The LAV system operates similarly to traditional variable air volume (VAV) systems except the primary air volume is very low. A LAV system can be configured to provide only the needed ventilation air which is then conditioned and volumetrically controlled at each zone to satisfy both the thermal and ventilation requirements of the occupant(s). In an LAV system, the zone size is much smaller than traditional HVAC system zones.
A LAV zone would be a single workstation or office. Since traditional HVAC system diffusers work to mix air more generally within large spaces, new types of air distribution approaches are employed to provide more locally focused air delivery and mixing. Such localized air distribution and mixing concepts permit occupants at adjacent workstations to request and experience somewhat different thermal comfort levels leading to much improved occupant comfort. In the LAV system concept, buildings are not started and shut down each day. Instead each unoccupied zone remains in a “standby” condition awaiting the start of the next occupancy.
Cost Effective and Integrated
In many aspects, the LAV and other similar system concepts are simpler, smaller, and less expensive than traditional HVAC systems. But from a control standpoint, they are much more integrated and complex than present day systems. These new HVAC system configuration concepts need to be implemented as a system, not through the one-of-a-kind piecemeal component-by-component configuring process that is used for most HVAC systems today. This is perhaps the greatest challenge and obstacle to getting such ultra-efficient HVAC systems in buildings because it requires a change in the process that manufacturers, designers, controls integrators, and even building operators currently employ to design, procure, construct, and operate buildings.
The approach also requires interdisciplinary coordination through design, construction, and into operation to ensure actual space loads do not exceed design levels. Integrated design processes are not new, but the integrated effort must be much more rigorously applied through the entire building life cycle to be successful with LAV systems.
The benefits are substantial. Distributing both ventilation and individually adjustable thermal conditions directly to each occupant guarantees a fresher and more comfortable environment for all. And the energy implications are enormous. With LAV, one could today design and operate office buildings with vastly improved comfort and air quality that consume under 10kWh/sq.ft. (less than 35,000 BTU/sq.ft.) total energy annually.
In a few years this energy requirement is expected to fall to as low as 5 kWh/sq.ft. annually. Or, in newer terminology that I’d like to see used, office buildings will soon be able to operate at less than 0.5 kWh /occupant-hour. The transformation from VAV to LAV technologies will fully satisfy society’s need for the level of sustainable buildings envisioned for this century and can be part of an effective retrofit strategy for existing buildings. But such a transformation will challenge our industry’s desire and ability to adapt new practices throughout the design, construction, and operation phases to ensure it works correctly over time.
At this point the industry needs to learn more about these developing HVAC concepts, showcase them in example buildings, and ready ourselves to work together to make this necessary HVAC system transformation successful!
I invite your thoughts, comments, and good ideas about this article. Please contact me at firstname.lastname@example.org.
An internationally recognized expert on the use of advanced building controls and control networks, Thomas Hartman, PE, is the principal of The Hartman Co. in Marysville, Wash. He holds several patents for efficiency and comfort-enhancing technologies and has been a member of the HPAC Engineering Editorial Advisory Board since 1991.