A dedicated outdoor air system (DOAS) is a type of heating, ventilation and air-conditioning (HVAC) system that consists of two parallel systems: a dedicated system for delivering outdoor air ventilation that handles both the latent and sensible loads of conditioning the ventilation air, and a parallel system to handle the (mostly sensible heat) loads generated by indoor/process sources and those that pass through the building enclosure.
Traditional HVAC systems, such as variable air volume (VAV) systems serving multiple zones, have potential problems in terms of poor thermal comfort and possible microbial contamination. Depending on the environment and the parallel system involved, in a DOAS setup the outdoor air system will handle some of the sensible load in addition to the latent load, and the parallel system will handle the rest of the sensible load. The main point of a DOAS system is to provide dedicated ventilation rather than ventilation as an incidental part of the process of conditioning interior air. DOAS is a term given to a system that has been used extensively in Europe and in various forms in the US. This article outlines the basics of DOAS, including the advantages and disadvantages of such a system, and the evaluation of energy and cost performance of DOAS.
William Coad proposed in 1999 to handle the OA (outdoor air) and return air separately in building HVAC systems. Gatley also describes the application of DOAS for delivering dehumidified air to buildings to improve the indoor air quality and thermal comfort. More recent research efforts have been conducted to study the basics of DOAS with emphasis on the potential advantages compared to the conventional HVAC systems. S.A. Mumma suggests that there are four main problems with conventional all air overhead mixing VAV HVAC systems. These issues of VAV systems highlight the corresponding advantages of DOAS systems. However, some disadvantages of DOAS include: potentially higher first costs, lack of use in the United States, and potentially higher complexity.
- Ventilation air in all air VAV HVAC systems: Designers and building engineers are unable to know exactly how the ventilation air that is mixed with the return air in a typical VAV system is distributed throughout the building. Issues such as air leakage, control setpoints, minimum air volume settings, and short-circuiting (e.g. exhaust air mixing with intake fresh air) can all affect the amount of ventilation air that reaches a space. A DOAS system solves this problem by providing a dedicated supply of 100% outdoor air.
- Need for excess outdoor air flow and conditioning in VAV systems: When the multiple spaces equation of ASHRAE Standard 62.1-2004 is used, generally from 20-70% more outdoor air is required in an effort to assure proper room air distribution in all air systems than is required with a dedicated outdoor air systems. Cooling and dehumidifying the high outdoor air quantities in the summer and humidifying and heating the air in the winter is an energy intensive proposition. The DOAS system is sized to meet the requirements, and does not require oversizing.
- VAV box minimums have to be set high to account for ventilation requirements: perhaps contrary to current practice, VAV box minimums must reflect both the ventilation requirements of the space and the fraction of ventilation air in the supply air. For example, a space requiring 5663 slpm (200 scfm) of ventilation air and served with supply air that is 40% ventilation air, will require a box minimum setting of 14158 slpm (500 scfm) (i.e. 200/0.4) rather than the conventional practice of 5663 slpm (200 scfm). When the box minimums are properly set to satisfy the ventilation requirements, the potential for considerable terminal reheat becomes an issue. Therefore, properly operating all air VAV systems will always use more terminal reheat than dedicated outdoor air systems supplying air at the same temperature.
- No decoupling of latent and sensible space loads: The inability to decouple the space sensible and latent loads leads to high space relative humidity at low sensible loads in the occupied spaces. Properly designed dedicated outdoor air systems can accommodate 100% of the space latent loads and a portion of the space sensible loads, thus decoupling the space sensible and latent loads. A parallel sensible-only cooling system is then used to accommodate the sensible loads not met by the dedicated outdoor air systems. There is therefore a strong incentive to control the space latent loads independently of the space sensible loads to avoid moisture related Indoor air quality problems.
For a typical DOAS ventilation system, the outside air system can accommodate around 0-30% of the space sensible load. In order to create a comfortable indoor environment, the balance of the space sensible loads must be accommodated by many other optional equipment choices as follows:
- Radiant ceiling panels
- A parallel all air variable-air-volume (VAV) systems
- Packaged unitary water source heat pumps
- Variable Refrigerant Flow (VRF) systems
- Fan coil units
Compared to other sensible cooling systems, radiant ceiling cooling panels are the best parallel system choice for use with the DOAS. Because the DOAS only accommodates the space ventilation and latent loads, it provides an opportunity to reduce the required floor-to-floor height by reducing the size of the duct system and the required fan power. There are numerous advantages of a radiant ceiling cooling system coupled with a DOAS.
There are two main ways to design a DOAS when using an air-based system as the parallel system.
In this setup, there is an outdoor air system that dumps preconditioned air (accounting for latent load and partial sensible load) directly into the space in its own duct/diffuser. There is a separate system (e.g. fan coil unit) that takes air from the space and conditions it to meet the remaining space sensible load.
Conditioned outdoor air is ducted to the terminal unit in the space. In this setup, the preconditioned outdoor air is ducted into the fan coil units directly, mixing with the return air from the space. This system is similar to a chilled beam setup.
With the increasing application of DOAS in many countries, there is also increasing demand for DOAS equipment, such as a total energy wheel that uses total energy recovery, a passive dehumidifier wheel, and other relevant equipment. The effectiveness of the total energy wheel is an important factor for improving the efficiency of DOAS.
The requirements in the design of a DOAS include:
- Separating the OA system from the thermal control system to ensure proper ventilation in all occupied spaces
- Conditioning the OA to handle all the space latent load and as much of the space sensible load as possible
- Maximizing the cost-effective use of energy recovery equipment
- Integrating fire suppression and energy transport systems
- Using ceiling radiant sensible cooling panels for occupant thermal control
Many studies have been conducted to demonstrate the energy and cost performance of DOAS in terms of simulations. Khattar and Brandemuehl simulated the parallel system and a conventional single system for a large retail store in Dallas, St. Louis, Washington DC, and New Orleans. The study demonstrated annual energy savings of 14% to 27% and 15% to 23% smaller equipment capacity for the parallel cooling system. Jeong et al. compared the energy and cost performance of a DOAS with parallel ceiling radiant panels to a conventional VAV system with air-side economizer for a nearly 3,000 square feet (280 m) office space in an educational building in Pennsylvania. A 42% reduction of the annual energy usage for the DOAS system with substantial savings in both fan and chiller energy use was reported in this study. Emmerich and McDowell evaluated the potential energy savings of DOAS in U.S. commercial buildings. The building model was developed to be consistent with typical new construction and meet the ASHRAE Standard 90.1 (ASHRAE 90.1) requirements. The simulation results indicated that the full DOAS resulted in the annual HVAC energy cost savings ranging from 21% to 38%. CUUUK
