Adapting DOAS To Your HVAC System

There is a proven method that will meet the challenges of complying with the ASHRAE Standards, delivering precise amounts of ventilation to spaces regardless of load size, and do it cost effectively.
Known specifically as a Dedicated Outside Air System, or DOAS, the outdoor air is conditioned separately from the air that controls the building’s space temperature (dry bulb). By having one system to provide and dehumidify all the ventilation air and a second system to control the space dry bulb temperature, both humidity control and space temperature control are improved.
By conditioning the outdoor air and recirculated air independently, a DOAS effectively separates the sensible and latent loads. The outdoor-air DOAS unit removes the latent load to control humidity, and the main HVAC unit removes the sensible load to produce a comfortable temperature. This is important because the primary source of building humidity in most climate areas is fresh outdoor ventilation air that has not been properly dehumidified. Additionally, the DOAS unit can assist the main HVAC unit by controlling smaller internally generated amounts of latent load that naturally build from occupants and other sources. It does this by providing air that is slightly drier than the target humidity level. Generally speaking, a DOAS provides “neutral” air of 70ºF to 72ºF @ 50% RH.
If desired, a DOAS unit can also provide the dehumidified air directly to the space at 55°F where it will offset some of the sensible load of the local HVAC unit. By delivering the air “cold”, this operation strategy doesn’t waste the sensible cooling byproduct performed by dehumidification but allows the local heating/cooling units to be sized smaller and requires less valuable floor space. A smaller main heating/cooling system means less energy consumption through smaller fans and compressors. A DOAS delivering cold supply air requires less reheat, but some reheat may be needed during periods of low sensible loads so the space is not “over-cooled” by the DOAS unit.
A DOAS doesn’t rely on totally new technology, but rather uses HVAC equipment configured to condition outdoor ventilation air separately from return air. The outside air conditioning system design consists of a cooling/dehumidification-reheat coil and supplemental heating system. The deep evaporator coil’s consist of 10 fins per inch/6 rows deep design, positioned in the draw- through air flow arrangement that provides the most effective moisture removal efficiency. It is this technique that differentiates it from conventional HVAC systems. This configuration will cool and dehumidify air in the summer and heat or cool it in the winter.
The operation is simple in design with the outdoor air first passing through an optional preheat coil (if used), which is sometime used for winter operation. When a heat exchanger is used, it brings the outdoor air closer to the temperature and humidity of the conditioned exhaust air.
A DOAS provides design engineers installation flexibility to meet the requirements of the application. Variables facing the engineer include, but are not limited to, if it’s new construction, retrofit or an installation having an existing system in place. Other considerations include the type of new or existing HVAC system installed such as constant volume, VAV and even the newer variable refrigeration flow (VRF) terminal units.
Delivering the conditioned OA from the DOAS to where it’s needed usually includes a separate ducting system running parallel to the HVAC supply air. For many climates, an independent duct system is considered the best choice because the ventilation air volume better meets the volume requirements of the project, and the DOAS ducting can be smaller than the conventional HVAC saving on the installation cost. Smaller ducting is also easier to manage in retrofit and existing HVAC installations.
A popular alternative ducting choice is a single duct system where the conditioned OA is blended with return air from the main HVAC system in a mixing box, or in a terminal unit that serves just one zone. If using a multi-zoned HVAC control system, individual zones are controlled separately and the DOAS will deliver the proper amount of outdoor air directly to each zone. In all cases, the DOAS can vary the fraction of ventilation to supply air, which can reduce the outdoor airflow rate by 40 percent by conditioning only the amount of air necessary for each zone.

The late sixties, early seventies ushered a new era into many facets of our lives. The rising cost of oil along with the uncertainty of supply made it necessary to evaluate what we had taken for granted for many years.
The October of 1973 Arab oil embargo sent oil prices rocketing while shortening oil supplies, causing building owners/operators to search for more reliable, less expensive ways to heat and cool large commercial spaces. The solution it seemed was to create a sealed building envelope thereby limiting the amount of infiltration and ventilation air to the minimum.
National energy conservation measures called for a reduction of outside air to 5 CFM per building occupant from 10 CFM. Most experts believed this would be sufficient ventilation to ensure adequate health and comfort, but they were quickly proven wrong.
The reduction in expensive OA resulted in a large increase in occupant complaints traced to their time at their workplace. Symptoms included nose or throat irritation, headache, dry cough, itchy skin, and sensitivity to odors, nausea, and eye discomfort. Sick Building Syndrome (SBS), as it was later known, caught the public’s attention with the sickening of 221 people and death of 34 others at an American Legion convention in Philadelphia through contamination in their air conditioning system.
Many studies have since proven beyond a doubt and established links between indoor air quality and human illness. When you consider the economic impact through lost productivity, lawsuits and increased insurance costs, building owners, HVAC design engineers and operators all take this issue very seriously.

OA is the Answer

ASHRAE Standard 62.1 – 2013—Ventilation for Acceptable Indoor Air Qualityforcommercialbuildings — quantifies the minimum ventilation rates and indoor air quality that will be acceptable to human occupants. 62.1 intends to minimize the potential for adverse health effects and has increased average ventilation rates from 5 CFM/person up to 20 CFM/person.
As a result, greater amounts of outside air must be introduced to the space, which also affects humidity levels. Humidity control becomes particularly important in the eastern half of the United States where mean dew point temperatures are 60°F e.g., 78°F/54% relative humidity, and higher during the summer.
These issues influence the need for an HVAC unit design capable of controlling ventilation, moisture levels and temperature in the space. To rectify the problem, traditional central station comfort cooling air conditioners must be “oversized” to handle peak latent load. To attempt to meet the new ASHRAE ventilation standards, a traditional air conditioning system generally requires 20%–70% more outside air than it was designed to cool, heat and dehumidify. Also, the traditional central HVAC must be set to provide the proper amount of outside air for the space with the greatest ventilation requirements. This ultimately causes over ventilating the rest of the building in the process and increasing the cost of conditioning that air.

The rapid increase of the cannabis industry within the United States creates a critical component needed in these facilities which is the proper HVAC equipment. Unlike most plants that are grown outdoors, marijuana is most often grown indoors which can be very tricky. It is critical that their HVAC needs are focused on cooling and dehumidification.
There are six main challenges that one will face when determining an HVAC system for a grow facility. Those six challenges are temperature, humidity, lighting, airflow, ventilation and smart technology. Grow room facilities typically need to be between 70°F-75°F with humidity between 50-60%. Your HVAC system should be able to maintain the specific temperature need to ensure growth for the plants along with quickly removing any additional humidity that may arise during the watering process of the plants. The lights needed in these facilities also produce a large amount of heat therefore needing a strong HVAC unit to keep the rooms cool and prevent overheating. Airflow is critical and your HVAC system should provide enough to create a “natural like breeze” to ensure the plants have constant movement and vibration. Your HVAC system also needs to provide an extensive filtration system to ensure no smells are leaked to the outside along with the ability to capture all dust and sediment to prevent mold spores from occurring. And most importantly, your HVAC unit must have smart technology in order to control all of the above mentioned or alert you if something goes wrong.
When it comes to selecting your HVAC unit for a grow room facility, you need to think of United CoolAir. United CoolAir can manufacture exactly what you are looking for in order to create a successful grow room. We have both the expertise and experience to provide you the best operational equipment in the USA.