Colorado Real Estate Journal - September 2, 2015
Buildings are not just neutral environments to shelter our activities and protect our belongings. They have a direct impact on our physiology and our psyche. This means that buildings are tools to help us improve our health and wellbeing. So if we can make people better by making better buildings, how do we make better buildings? Conventional HVAC system design is meant to deliver “acceptable” air quality and “acceptable” thermal comfort. How do we achieve superior comfort and indoor air quality while still lowering energy use? Let’s first look at conventional building design and acceptable air quality and comfort. One of the most conventional HVAC systems is the all-air, variable air volume system. VAV systems combine ventilation and space conditioning into a single system and have flexibility to address multiple thermal zones. A significant amount of air is required, because the air is meant to condition the space (about 80 percent of the air), as well as ventilate it (about 20 percent of the air). All of this air is cooled to 55 F, typically delivered overhead, and to satisfy the room setpoint the air in the space needs to be well mixed. VAV systems have cost effectively addressed past conceptions of acceptable air quality and comfort for decades. However, with growing expectations for superior building performance, VAV systems have several shortcomings: Thermal comfort: • Relies on air temperature and convective transfer to provide thermal comfort, but human physiology strongly favors radiant thermal exchanges for maintaining comfort. • Areas near the 55 F air supply diffusers can have uncomfortable cold spots when the diffusers are not adequately specified. Energy: • The large volume of air requires significant fan energy. • The large volume of air is cooled to 55 F, requiring significant cooling energy. • When heating is required the air is “reheated” after being cooled to 55 F. Indoor air quality: • Varies the air delivered based on meeting space temperature setpoints, making it uncertain if ventilation requirements are always met. • The fully mixed air approach inadvertently circulates contaminants and airborne pathogens throughout the space and into the breathing zone. • Recirculated return air can potentially reintroduce contaminants back into the space. A new approach to building design is emerging, which pairs methodical control of thermal loads and air contaminant sources with next-generation HVAC systems that break from the all-air approach of VAV and can deliver on the ever-increasing performance demands for exemplary energy efficiency, high IAQ and enhanced thermal comfort. These next-generation HVAC systems are characterized by decoupling ventilation from space conditioning using a dedicated outdoor air system to meet ventilation needs with 100 percent outside air. In humid climates the DOAS can also handle the latent loads, leaving sensible cooling and heating to be handled separately with greater efficiency and comfort. A DOAS-based system is typically paired with an ultra-efficient heating and cooling system such as radiant, variable refrigerant flow, geothermal heat pumps or chilled beams. These system combinations also typically provide reduced equipment footprint, reduced noise and lower life cycle cost. One of the high-performing system combinations is pairing DOAS with radiant slabs or panels, and one of the best ways to deliver the ventilation air is through displacement ventilation or under-floor air distribution. From a performance perspective, DOAS-based systems provide many benefits to building owners and occupants: Thermal comfort: • Depending on the terminal system used moderate supply temperatures do not create cold spots at supply diffusers. • Paired with radiant heating and cooling, can provide superior comfort through radiant thermal transfer. • Compatible with natural ventilation allowing for the thermal comfort (and energy) benefits of a mixed-mode system. Energy: • Reduced air volumes and moderate supply temperatures reduce cooling and heating energy for tempering ventilation air. • Energy efficiency is enhanced with energy recovery. • Reduced air volume reduces fan energy. • Utilizing zone specific demand control ventilation with CO2 sensors can further reduce energy use. • Paired with a parallel system for heating and cooling such as radiant, VRF, water source heat pumps or chilled beams saves energy by moving heat throughout the building with water or refrigerant, which is a much better thermal conductor than air. Indoor air quality: • Ventilation air is not recirculated building-wide and 100 percent outside air is used. • Air delivery is not tied to space conditioning and temperature set-points, assuring that ventilation requirements are always met. • Can be configured as displacement ventilation or UFAD. By delivering cool air at the floor level and returning warm air at the ceiling, the air stratifies rather than mixes. This means that the air moves in one direction, removing, rather than recirculating, any contaminants or pathogens in the space. Market drivers like LEED and the new WELL Building Standard are moving the industry toward more sustainable, healthy buildings with lower energy use and increased comfort and indoor air quality. DOAS-based HVAC systems, fully integrated with architecture that controls both thermal loads and air contaminant sources, can help the industry meet these ever growing expectations. The next-generation of building design is poised to make people better by making better buildings.