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Dedicated Outdoor Air System Provides Comfort and Savings

Key Points
  • DOAS systems condition the incoming outside air with energy recovered from the exhaust air.
  • Series and parallel DOAS designs are available; each has its advantages and disadvantages.
  • A DOAS system can reduce tonnage requirements for space conditioning by 30 to 40 percent.

Air conditioning systems work hard. Often their toughest job is handling the moisture content of the outdoor air. Conventional variable-air-volume (VAV) systems reduce the air temperature and humidity for both outdoor and indoor in one process step, which is challenging to do efficiently. Dedicated outdoor air systems (DOAS) may provide a more cost-effective alternative.

Comparing DOAS systems with conventional HVAC

VAV cooling systems condition both the return air and outdoor air in a single unit, as shown in the following diagram. They dehumidify the air by lowering the temperature to the point where the water vapor condenses out. However, the air temperature may be so low that reheating is required before it's introduced into the space; this takes extra energy.

doasconventional

DOAS systems condition only the incoming outdoor air, with help from energy recovered from the exhaust air. The return air is cooled separately by terminal devices, such as radiant cooling panels, fan coil units or chilled beams. Outdoor air is dehumidified by the outside unit and exhaust air is used to condition the incoming air. So there are basically two supply air loops for the space.

There are two types of DOAS arrangements:

Series. All outside air is conditioned and introduced upstream of the terminal devices. A reduction in requirements for cooling capacity and reheating is typical. However, all of the terminal units must run continuously during the day to meet ASHRAE 62.1 fresh air requirements.

Parallel (illustrated below). The conditioned outdoor air is supplied directly to the space, bypassing the terminal devices. The outdoor unit reheats typically 55°F dehumidified air to room temperature, similar to VAV systems. Because the terminal devices only operate when the room temperatures rises, energy is reduced compared to series systems. Also, fresh air requirements are adequately met by conditioned air from the outside unit.

doasdoas

The benefits of DOAS systems

In the right application, DOAS systems provide a number of potential benefits compared to conventional systems:

  • Easier to comply with ASHRAE fresh air requirements
  • Improved space humidity control
  • Handles unoccupied spaces efficiently
  • Provides building pressurization to reduce unwanted warm air infiltration
  • Typically 30 to 40 percent reduction in required cooling tonnage capacity
  • Up to 20 percent operating cost reduction at peak load

Standards are available to help you choose high-performance systems. AHRI Standard 920 Performance Rating of DX-Dedicated Outdoor Air System Units provides system testing and rating requirements. Also, ASHRAE 90.1 2016 requires DOAS systems to meet a minimum efficiency levels and to use 75 to 90 percent site-recovered energy to reheat dehumidified air to the neutral condition.

DOAS provides healthy savings

Purdue University's Health and Human Science Building is a 127,000-square-foot, multi-use facility. A series DOAS system supplies 52°F dehumidified air through the return connection of fan coil units and active chilled beams, not directly to the space. Fan coil units serve large conference rooms and entrances, while chilled beams serve labs, classrooms and office spaces. The DOAS system uses 26 percent less energy than a standard VAV system.

Marian University's Michael A. Evans Center for Health Sciences is a new, 140,000-square-foot building. A reheat coil takes the hot gas from the evaporator coil of a water-source heat pump and uses it for reheating dehumidified DOAS air. All high occupancy rooms are controlled by a demand-controlled ventilation (DCV) system. The parallel DOAS system uses 43 percent less energy than a conventional HVAC system.

If you have extreme climate conditions or your facility requires a constant supply of fresh air, a DOAS may be the answer.

 

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