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Conditioned Makeup Air
If you don't do a lot of commercial work, you may see a system like the one above and wonder what the heck it is. It is a 100% fresh air unit or a makeup air unit. You will see them more and more as codes start to reflect that balanced ventilation and conditioned outdoor air are a vital part of a healthy and comfortable building.
What are they for?
Most commercial buildings require that a good amount of outdoor air be brought into the building to dilute VOCs (volatile organic compounds), reduce CO2 buildup, and replenish oxygen levels.
In many cases, this fresh air requirement is accomplished through fresh air intake on the HVAC equipment. However, buildings are increasingly using dedicated fresh air machines as ductless, and VRF systems have recently become more popular.
The type shown above is most often used in kitchens, where there is significant exhaust air. When air is exhausted, you must add back outdoor air to achieve balanced ventilation. In the past, we would see many restaurants and grocery stores pump unconditioned makeup air into the kitchens. Then, we would see moisture issues and everything that entails in humid climates and hot kitchens. (To learn more about makeup air in restaurants from a fire safety perspective, check out Christopher Stephens's article about it HERE.)
By filtering, cooling, and dehumidifying the air as it travels through the makeup air intake, we can control the temperature and dewpoint of the air and reduce these issues.
The refrigerant side of this particular type of machine functions very simply. It uses an air pressure switch to “prove” sufficient negative air pressure, which indicates that the cooling equipment that sufficient airflow is present. It then uses a thermostatic control that measures the incoming air temperature to turn the equipment on and off based on heat load.
These can also be set up to work off of enthalpy—which is the total heat content of the air (humidity & temperature)—rather than the air temperature. It functions much like an economizer in this way; it can take advantage of the outdoor air when appropriate or run cooling and dehumidification where needed.