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 and you will see them more and more as codes start to deal with the fact that balanced ventilation and conditioned outdoor air are an import part of a healthy and comfortable building.
What are they for?
Most commercial buildings require 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 intakes on the HVAC equipment but increasingly building are using dedicated fresh air machines as ductless and VRF becomes 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 in unconditioned makeup air into the kitchens and then we would see moisture issues and all that entails in humid climates as well as hot kitchens.
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 acts as an indication to the cooling equipment that sufficient airflow is present. It then uses a thermostatic control that measures the temperature of the incoming air 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 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.