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The importance of outdoor air In air conditioning systems


Mark Knowles BSc. (Hons), Mechanical Engineer • Email: [email protected]

Air Conditioning has become ubiquitous in modern living both in residential and commercial settings. The benefits of Air Conditioning are also well understood, both in terms of comfort and health benefits, as well as on productivity in the workplace. However, in consideration of Air Conditioning for any particular application, there is usually a significant distinction between a residential application and a commercial application. In fact most building codes are very specific to distinguish between residential and commercial guidelines.

One of the major differences between the design guidelines for Air Conditioning is with regards to Ventilation or more specifically “Outdoor Air Requirements for Ventilation”. To grasp the importance of this it is useful to understand one basic principle behind Air Conditioning (AC), and this is that most AC systems are re-circulating systems. This means that the air in a space is circulated by a fan in the space and passed over a cold surface which cools it and sends the cooled air back to the space. This means that the air is circulated over and over in the space keeping it cool. However, during the circulation process other substances accumulate in the air,
these include carbon dioxide and Volatile Organic Compounds (VOC’s). Volatile Organic Compound is a general term that describes any organic substance that can exist in gaseous form mixed with air. VOC’s can come from adhesives, fabrics and other items in an occupied space, which emit small quantities of substances such as Acetone, Ethanol, Ammonia and Formaldehyde. The concentration of these substances as well as Carbon Dioxide can increase over time because of the recirculation of the air by the AC system.

In a residential environment, it is normally assumed that small openings in windows and construction joints, etc. will allow air to infiltrate into the occupied space and this outside air will dilute the contaminants in the air and maintain an acceptable air quality. For this reason and also because of the relatively small size of most residences, most building codes do not mandate ventilation rates and mechanical ventilation for single family residences.

For commercial buildings, however, codes normally mandate a ventilation rate. This ventilation takes the form of outdoor air which is drawn in or forced into a building and mixed with the re-circulating indoor air. Mechanical exhaust is also normally used to remove part of the re-circulating air and balance the system. Contaminants will build up more quickly in a commercial building because of several factors, including more occupants per floor area, larger floor areas and tighter construction.

Increased concentrations of Carbon Dioxide can cause headaches, fatigue, changes in pulse rate and general feelings of ‘stuffiness’ to occupants. This is in addition to several other possible negative health effects associated with VOC’s. We can thus appreciate that outdoor air ventilation is essential to an efficient building and to the health and safety of the occupants. Any gains in occupant efficiency attributable to the AC systems cooling and de-humidifying effects can quickly be eroded if the ventilation is inadequate. Occupant discomfort and health effects will lead to losses in commercial activities.

Some building operators disconnect, disable or block off the fresh air intake into a building, effectively reducing the ventilation to a minimal amount. This is sometimes done with the misguided aim of trying to increase the cooling effect of the AC unit, with the assumption that if the unit makes the space cooler, the occupants will be more comfortable and productive, or to reduce the energy consumption of the system. This often has an unseen negative effect as
noted above. Negative human occupant effects will far outweigh any energy cost reduction if this strategy is employed. It is often better to allow a slightly higher indoor temperature than to compromise the ventilation in a building.

Arguably, the best approach to reaping the best benefits of your AC system is to have the
AC system properly engineered, with calculations done in advance to define the system to be
installed. This will allow proper ventilation rates and ensure a building that operates with the
highest level of efficiency.

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