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to the return grille, and recommended that the return be placed close to pollutant sources and
away from occupied workstations.
We did not find any studies that examined the influence of workstation characteristics on
the performance of floor-mounted mixing systems. It is possible that effective mixing could be
inhibited by workstation partitions, particularly for those workstations not located above a
diffuser. This hypothesis, however, has not been empirically studied.
Overall, empirical evidence suggests that mixing air distribution systems are capable of
producing a relatively uniform indoor environment in office settings, including open-plan offices.
In addition, when used appropriately mixing systems can achieve acceptable levels of ventilation
efficiency, pollutant removal efficiency and thermal conditions.
IRC-RR-161 7 Office Air Distribution Systems and Environmental Satisfaction
2.2 Displacement Systems and Physical Conditions
A number of studies have examined the IAQ and thermal conditions arising from
displacement air distribution systems in office settings (e.g. Akimoto, Nobe, Tanabe, & Kimura,
1996; Gan, 1995; Jiang et al., 1992; Nielsen, 1996; Olesen, Koganei, Holbrook, Seelen, &
Woods, 1994; Tanabe & Kimura, 1996; Xing, Hatton, & Awbi, 2001).
Empirical evidence indicates that displacement systems produce two horizontal zones
within office spaces. The lower zone exhibits an upward convective flow, with air temperature,
pollutant concentration and age of air all increasing with height from the floor. In the upper zone,
these displacement characteristics are less pronounced, and the airflow pattern tends towards
mixed air and more uniform conditions. The height of the lower zone is primarily dependent on
the supply airflow rate, the amount and distribution of heat sources within the space, and the
thermal plumes created by these heat sources (ASHRAE, 2001; Yuan et al., 1998). Studies
reviewed by Yuan et al. (1998) have also suggested that displacement airflow patterns are
influenced by factors such as the temperature of external walls and windows, the difference
between supply and room temperature, and the ceiling height.
Research suggests that, under certain conditions, displacement systems can achieve better
IAQ and contaminant removal efficiency in the occupied zone, as compared to ceiling-mounted
mixing systems. The main determinant of this improved performance is the transition between
the lower and upper zones. Improvements in IAQ are typically reported when the occupant’s
breathing zone falls within the lower zone. However, if there are substantial pollutant sources
located close to floor level, or if thermal plumes are not sufficient to remove pollutants to the
upper zone, these improvements can be undermined. In such cases, pollutants will remain in the
lower zone and can result in reduced IAQ in the occupant’s breathing zone.
One drawback of increasing the height of the lower zone is that it can lead to
uncomfortable vertical temperature differences. As the lower zone is characterised by an upward
flow, the difference in temperature between the occupant’s head and ankles could exceed that
required for local thermal comfort2
. Therefore, the operation of displacement systems requires a
careful balance between promoting IAQ, whilst restricting vertical temperature differences. The
use of chilled ceiling panels has been suggested to reduce the vertical temperature gradient, and
so provide a wider range of conditions in which both IAQ and thermal requirements can be
achieved (Novoselac & Srebic, 2002).
A final consideration of displacement systems is their potential for creating draughts. As
air is supplied at floor level, and at cooler temperatures than typically used in mixing systems,