#DOAS Archives | United CoolAir

August 13, 2020 by Kevin Stepp

Understanding DOAS and comfort cooling differences

While indoor air quality and ventilation have been major focuses of the HVAC industry for a long time, many misconceptions remain about what these applications require in terms of equipment and design. Today’s post will parse some of the most common questions about DOAS and comfort cooling equipment.

Purpose of DOAS

A dedicated outside air system (DOAS) or energy recovery unit (ERU) conditions air from the outdoors and delivers it to a space. It is essentially a dehumidifier in that its primary purpose is to remove moisture from the air, so when that air enters the space it is not adding additional load. This contrasts with comfort cooling equipment whose purpose is to maintain a consistent temperature in the space.

How

In order to achieve a cooling effect, a comfort cooling device must provide air to the space at a temperature much lower than the set point for that space. A DOAS or ERU, for its part, is not concerned with the temperature in the space. It monitors the temperature and humidity of the outdoor air and gauges how much capacity it needs to remove the moisture. The DOAS/ERU then discharges dry, room neutral air (roughly 70°-72°F and 30-40% RH).

When you need comfort cooling

A comfort cooling device operates only when the temperature in a space rises above set point. An outdoor air system, however, will typically operate during all occupied hours. ASHRAE 62.1 requires outdoor air be brought into in buildings at all times, so these machines rarely rest.

What is a DOAS unit?

Outdoor air equipment typically discharges air off of its cooling coil at 50-55°. The low leaving air temp effectively strips moisture from the air. However, constantly delivering low temperature air to a space will over cool it, so a modulating hot gas reheat coil is added to reheat the leaving air to a room neutral temperature around 70°.

DOAS and ERU equipment often feature some form of capacity control. Because outside air in cooling seasons can range from 65° to over 100°, these machines must be able to vary the amount of dehumidification they provide. Staged compressors, variable speed compressors, and modulating hot gas bypass are all options.

Space Temperature Control?

No, an outdoor air unit cannot replace a traditional air conditioner as the only device responsible for maintaining temperature in a space. The outside air unit operates constantly, meaning it cannot be turned off when the space is satisfied. Constant operation in a space temperature control design would lead to unacceptable temperature swings. More importantly, the mode of operation for a DOAS/ERU is determined by outside air temperature and humidity, not by space temperature. Space temperature control would become problematic when the space calls for heating, but it is 65° and raining outside. The unit must ignore the space and dehumidify instead, or the space humidity will rise out of control.

Not One or the Other

In order to properly apply a DOAS or ERU, it must be teamed with a comfort cooling device. This decoupling strategy allows designer to meet ventilation requirements while properly controlling space temperature and humidity. Thankfully, at United CoolAir we have solutions for both. Inquire today for more information on our comfort cooling equipment, DOAS, and ERUs.

June 29, 2020 by Kevin Stepp

What is DOAS?

With Independence Day weekend approaching Americans are making plans to celebrate with cookouts, swimming, and other outdoors activities. These simple pleasures become even sweeter knowing that, when outside, we are breathing clean, fresh air. What some don’t know is that when we return to work, we are often returning to buildings that have been consciously designed to bring in fresh outdoor air. This fresh air creates cleaner and safer indoor environments. You don’t need to be in the backyard in order to breathe fresh air!

We refer to HVAC equipment specifically designed to condition outside air as DOAS equipment, or a Dedicated Outdoor Air System. The goal is to flush recirculated air from a building and replace it with clean air from outdoors. Doing so reduces the quantity of unwanted particles in the air by diluting their concentration.

How does a DOAS unit work?

While technologies and applications differ, most DOAS equipment operates on the same basic principles. Outside air, which in the summer is hotter and more humid than indoor air, must be cooled and dehumidified before being discharged into the space. If it were not, temperature and humidity in the building would skyrocket. Air passes across a cooling coil that reduces its temperature to around 50°-55°F. This process removes much of the moisture from the air. Because 50°-55° is too low to comfortably blow into a room, DOAS units often offer hot gas reheat as an option to heat the now-dry air up to a more comfortable 68-72°. Now, fresh air is ready to enter the space.

While we will likely return to work still thinking about the fun we had outdoors over the holiday, we can do so knowing that some of the outdoors is still with us.

To learn more about United CoolAir’s DOAS offerings, click the following link to our Omega Air product or call us to discuss how we can modify our other products to fit your DOAS application.

August 31, 2018 by Matt Hanson

Dedicated Outside Air Systems (DOAS) part 2

Adapting DOAS To Your HVAC System

There is a proven method that will meet the challenges of complying with the ASHRAE Standards. This method delivers precise amounts of ventilation to spaces regardless of load size, and do it cost-effectively. Known specifically as Dedicated Outside Air Systems, or (DOAS), the conditioned outdoor air separates from the air that controls the building’s space temperature (dry bulb). By having separate systems providing dehumidified and ventilation air and another for dry bulb temperature, there is an improvement of both humidity and space temperature control. By conditioning the outdoor air and recirculated air independently, Dedicated Outside Air Units effectively separates the sensible and latent loads.

The DOAS unit removes the latent load to control humidity, while the main HVAC unit removes the sensible load. The result is a comfortable temperature for the space occupants. This is important because the primary source of building humidity in most climate areas is fresh outdoor ventilation air is improper dehumidification. Additionally, the DOAS unit can assist the main HVAC unit by controlling smaller internally generated amounts of latent load. This latent load naturally builds from occupants and other sources. It does this by providing air that is slightly drier than the target humidity level. Generally speaking, a Dedicated outside air unit provides a “neutral” air of 70ºF to 72ºF @ 50% RH.

What About Dehumidification

A DOAS unit can also provide the dehumidified air directly to spaces 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. This results in the local heating/cooling units sized smaller and require 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 could happen during periods of low sensible loads. In this way, there is not “over-cooling” by the DOAS unit.

A DOAS doesn’t rely on 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 a supplemental heating system. The deep evaporator coils consist of 10 fins per inch/6 rows deep design, positioned in the draw- through airflow arrangement. 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 design with outdoor air first passing through an optional preheat coil (if used), is sometimes used for winter operation. By using a heat exchanger, bringing the outdoor air closer to the temperature and humidity of the conditioned exhaust air is achieved.

Dedicated outside air units provide design engineers with installation flexibility to meet the requirements of the application. Variables facing the engineer include; new construction, retrofit, or installations 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.

All About Ducting a DOAS

Delivering the conditioned OA from the DOAS to where it’s needed usually includes a separate ducting system. This system runs parallel to the HVAC supply air. For many climates, an independent duct system is the best choice because the ventilation air volume better meets the volume requirements of the project. Furthermore, 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. For this, we use a mixing box, or in a terminal unit that serves just one zone. If using a multi-zoned HVAC control system, individual zones are controllable separately. In this way, 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. This is due to only conditioning the amount of air necessary for each zone.

January 3, 2018 by Matt Hanson

Healthcare-Associated Infections: How can we help?

Indoor air pollutants can affect the health of children, the elderly and those with health problems and lead to infections. As people spend an extreme amount of time indoors, the air we breathe in closed-in spaces can further worsen health problems. For example, each and every day we are open to indoor impurities such as chemicals, gases, dust, and mold. Many health problems can occur from poor indoor air including sore eyes, nose burning, sore throat, headaches, and fatigue. These pollutants can be found in homes as well as schools offices and even healthcare facilities.

Healthcare-Associated Infections

Healthcare-Associated Infections (HAI) are infections that patients can get while having treatment in any healthcare facility. Most of these HAI’s could be preventable with proper indoor air quality. Meanwhile, pollutants can cause or worsen existing allergies and breathing illnesses such as asthma, heart disease, cancer, and other conditions. Furthermore, the hospital’s physical environment can have a momentous impact on patients’ health. If the air is dry (as it is in most hospitals) this can create a home for microorganisms. In addition, this can worsen the indoor conditions that lead to infections and allergens to humans. An indoor relative humidity between 40-60% will help to protect patient health and decrease excess health care costs.

Indoor air hydration will support patients in a variety of ways including skin and respiratory tract defenses. This also includes immune cell functioning wound healing and total body fluid balance. In other words, all of those are natural defenses against HAI’s. Healthcare facilities need to ensure they are making patients their number one importance by focusing on indoor air quality. With proper indoor air quality, you will see a decrease in re-admissions, shorter inpatient stays, and lessening of non-reimbursable hospital expenses. One of the best ways to control IAQ and improve health is to have a dedicated outdoor air system (DOAS) in place. United CoolAir produces many units designed to bring in the fresh air and improve the health of buildings and patients.