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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 outdoor air ventilation system that handles latent loads and a parallel system to handle sensible loads. The conventional HVAC systems, such as variable air volume (VAV) systems serving multiple zones, have some potential problems in terms of poor thermal comfort and possible microbial contamination. Depending on the environment and the parallel system involved, 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 part of conditioned air. DOAS is a term given to a system that has been used extensively in Europe and in various forms in the US. This page outlines the basics of DOAS, including the advantages and disadvantages of such a system, and the evaluation of energy and cost performance of DOAS. == System overview == 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.〔http://doas.psu.edu/doas.html Accessed 11/15/2010〕 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 IAQ problems.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dedicated outdoor air system」の詳細全文を読む スポンサード リンク
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