Dedicated Outdoor Air Systems (DOAS) are proving a viable alternative to all-air HVAC systems in green building design. They are auxiliary systems designed to supplement the ventilation and energy efficiency requirements of modern buildings. They can be particularly useful as a sustainable design application in humid climates for their ability to improve humidity control. They prove an energy efficient means of ventilation and air conditioning and used in conjunction with radiant cooling systems, they make for a more effective means of indoor climate regulation.
How Does a DOAS Work?
A dedicated outdoor air system is essentially a conventional HVAC system that is configured differently for its specific operation. All HVAC cooling systems have the job of removing two primary heat loads in their air conditioning function: that of sensible heat and latent heat. Sensible heat is the heat in a room caused by the heating of objects and is affected by outside air temperature. Latent heat is the heat in a room caused by a change of state from gas to liquid and is affected by humidity. These two heat loads make up what is known as the total capacity for HVAC systems to handle in their cooling and air conditioning function.
Conventional HVAC systems use only one system to perform the function of total capacity heat removal and controlling air flow back into the building. Dedicated outdoor air systems are advantageous in this regard in that they are designed with separately functioning systems to handle the different loads, increasing their effectiveness and efficiency in heat removal and controlling air flow.
The above diagram shows how outside air and return air are separated in the ventilation unit. By separating the two air streams, it allows the unit to accurately calculate latent and sensible heat cooling needs which then results in a more accurate delivery of ventilation requirements.
How Does a DOAS Unit Improve Efficiency?
According to a study done by ASHRAE, when properly designed advanced ventilation systems are used in parallel with a sensible cooling system (such as radiant cooling) can save a considerable amount of energy compared to a conventional HVAC system, while always supplying the correct amount of ventilated air.
It’s not the ventilation system on its own that contributes to energy efficiency, but the fact that the decoupling of latent and sensible loads in the outdoor and return air units respectively allows for a downsizing of HVAC units in the building. This, with the added ability of the outdoor unit to handle outdoor air dehumidification on its own, allows for a reduction of components in the partner HVAC system and less energy spent. In this way, having an auxiliary ventilation unit can save up to as much as 15-20% on total energy usage.
An interesting finding with regards to the implementation of dedicated outdoor air systems as green building design considerations is that their energy saving potential is that much higher with their application in humid climates. Coupled with this, combining this ventilation unit with a radiant cooling system has also shown to be the most energy efficient partnership for handling the total capacity in these climates. The reason for this is that with the particular function of DOAS units handling latent loads in dehumidification, the radiant cooling system overrides conventional HVAC system in the energy efficiency of handling sensible cooling loads in humid climates.
For their application in green building design, DOAS units come recommended to be used as auxiliary systems to radiant cooling systems for the purpose of energy saving and efficient ventilation.