I am a big fan of DOAS – Dedicated Outdoor Air Systems.
Traditionally, a building is served by an air handler which is a big metal box with fans, coils, and various dampers that are supposed to do several opposing tasks at the same time. Often, they can’t handle it—pun intended.
DOAS is a paradigm shift. Mechanical engineers design systems to control both sensible and latent heat. It would require a course in thermodynamics to fully understand all the nuances, but sensible heat refers to the number on the thermometer and latent heat refers to relative humidity. Comfort wise, this explains why a 90-degree day in dry Phoenix is more tolerable than a 90-degree day in humid Louisiana. When you get the air in your building at the ideal temperature and humidity, then an air handler can just circulate it, lowering or raising the sensible temperature as needed.
Outside air, however, complicates this with its wide variety of temperature and relative humidity changing hourly. Buildings are not perfectly airtight—toilet room fans exhaust air, buildings perform better when slightly positively pressurized, people come and go, and the mechanical code requires a minimum of fresh air for human health (think sick building syndrome).
In response, traditional air handlers use a system of outside air dampers, mixing boxes, fans, and controls to try to dump the right amount of outside air with its latent heat into the building, condition it, and mix it with the recirculated air. Moreover, you need to make sure to get rid of some of the existing air since blowing up the building like a balloon is not desirable.
DOAS separates this process. A simplified air handler is installed to circulate pre-conditioned air in the building, adding or subtracting sensible heat to keep occupants comfortable. A separate dedicated outdoor air system is incorporated, which adds just the right amount of air to the building, removes the right amount to keep pressures correct, adds or removes relative humidity, and often captures some of the energy from the exiting conditioned air.
The ideal concept building would be a tightly sealed envelope with fixed windows and one door. Open the door during the ideal ASHRAE day (70 degrees with 50% relative humidity), trap the air, and then weld the door shut once this ideal ASHRAE air is contained in the building. Henceforth the heating and cooling system can then maintain this ideal environment.
Now, only sensible loads require mechanical intervention. Energy-saving technologies that reduce fan energy such as chilled beams and radiant heating would be possible. Unfortunately, humans would eventually batter down the door and insist on occupying the space—bringing their latent loads and annoying habit of requiring outside air for breathing.
Architects like me have neither the patience nor aptitude for complex mechanical engineering solutions, so I like the inherent simplicity of one system to handle the sensible heating and cooling loads environmentally placed on the building; another system to handle the latent and ventilation loads placed on the building by the humans that inhabit it.
Bring in the DOAS system. Latent loads from breathing, sweating, going potty, and brewing coffee can be addressed; and with exhaust functions from these activities balanced, outside air can be introduced, and building pressurization can be controlled. The two separate systems can be optimized to do their jobs without treading on the other’s turf.
Why do we combine the two functions in an air handler? Like the song from Fiddler on the Roof, “Tradition,” I believe the reason we combine the two basic functions in modern air handlers is similar. They both involve moving air. Yet the two main tasks of conditioning indoor air and conditioning outside air are very different.
I will make an analogy to the modern residential bathroom. We do two basic functions in our bathrooms. One function is making our bodies clean, refined to an invigorating and sometimes social ritual by the Romans, Greeks, and Japanese. The other is eliminating urine and feces from our bodies. Two activities that are more dissonant are hard to imagine. The only common element is the use of water. In an ideal world, we would bathe in one room and eliminate waste in another, but we have co-evolved the two activities in modern life because the plumbing is easier and cheaper if placed in one location.
DOAS systems are not a panacea for all building mechanical system challenges. Usually, they are more expensive and require more talented mechanical engineers to design. Breaking down functions, however, almost always improves both comfort and energy use.
Their use might herald an acronym change. Instead of HVAC (heating, ventilation, and air conditioning), we should update to HCCV (heating, cooling, conditioned ventilation).
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