Design goals and fan selection

Which are the relevant air movement goals for your design and how to select fans that meets those goals?

Fans are effective for comfort cooling and air circulation. However, fan applications are highly dependent on the design intent for desire air speed and distribution, and any physical environment limitations. Understanding the elevated air movement design intent is the prerequisite in selecting an adequate fan type for a particular space.

Design intents for air speed and distribution

Figure 32 outlines the key considerations for defining the fan design intents. These design intents are divided into four types, namely personal control, targeted, variability, and uniformity. Table 4 outlines the description of fan applications in each design intent category. The table also provides example application types, example target air speeds, and additional considerations for each design intent approach. For any of the described design intent, the ideal condition should include the automation of fan with the possibility of user override. This would allow, ideally, for fans to be activated automatically according to room occupancy and indoor temperature conditions, but maintain occupants control, allowing them to make temporary changes to fit their needs.

Even under identical environmental conditions, occupant comfort needs vary significantly based on individual preference and expectation, clothing levels, and metabolic rates. “Personal control” design emphasizes the goal of fan system to provide thermal comfort for a single occupant, while the adjustment of fans is unlikely affecting other occupants. In such a case, the design intent of air speed and distribution are not particularly relevant, as the subject has full control of the fan speed, airflow direction (i.e., relocate a desk fan position), and operation schedule according to their own needs. One example would be a private office where the single occupant has control over the fan and another one would be an open plan office where every occupant has a small desk fan (see classification of control levels in the “ASHRAE Standard 55 (2020)” section).

In general, design intent of “Variability” has its advantage at multi-occupant space where occupants have flexibility to adjust fans operation based on their desire thermal comfort needs, or they are free to move around and choose their preferrable locations or thermal conditions. In many cases the variability in air speeds created by a fan can help improve overall occupant comfort. For example, a manually adjustable ceiling fan that serves multiple persons within the space for different thermal needs (e.g., adjusts higher fan speed for transient period, but lower speed at steady condition). Another example is in spaces where occupants can easily move around, such as a lobby, gymnasium or cafeteria, variability is likely beneficial as the comfort needs of different occupants can be met by choosing their location in the space.

In spaces where there are variable or transient occupancies, non-uniform thermal conditions, or spaces with specific thermal requirements due to architectural features or activity levels, “Targeted” air movement may provide more comfort. For example, the thermal comfort impact of increased solar radiation near a poorly shaded, highly glazed façade could be offset by locating ceiling fans near the façade, but the areas further from the façade might not need any or fewer ceiling fans. Other examples could be to locate ceiling fans over a dance floor area instead of the audience or directing air circulators to the exercise area in a gym and not directed to the front desk.

Design intent of “Uniformity” (i.e., more regular control) emphasizes uniform air speeds and consistent thermal comfort experience applied in multi-occupant spaces where occupants do not have flexibility to control fan or change their location, especially when occupants will be staying in those areas for extended periods. Uniform air speed and distribution are applied because there is no way of guaranteeing that the person who feels the warmest happens to be the one who is located where the air speeds are highest in the room. Examples here include a shared office with fixed assigned seats, classrooms, and allocated seating dining areas.

Figure 32. Flow chart of design intent for air speed and distribution.

Table 4. Recommended guidelines classified by design intent for fan applications.

Design Intent	Application Examples	Target Maximum Air Speed	Comments
Personal control: Provide cooling directly to the occupant, taking into consideration furniture configuration / space layout, locate fan directly close to occupant and above occupant in case of ceiling fan, where possible.	Office – Private Residential	High to very high	Provide local manual control. Ensure fan can turn down to a low fan air speed. If Ceiling fan is chosen, the locations should consider potential/planned furniture locations.
Variability: For spaces where occupants have the flexibility to move around the space and choose their preferred conditions, allow variable air speed conditions within the space, up to medium air speeds. Uniformity is a lower priority.	Assembly / Event Spaces – Flexible Cafeterias Gymnasium / Exercise Areas Offices – Shared, Hoteling / Unassigned	Medium	For office spaces, ensure maximum design air speed will not blow papers around.
Targeted: For spaces with inconsistent or transient occupancies, non-uniform thermal conditions, and/or spaces with specific thermal requirements, target fan to where they are most needed. Consider providing different air speed conditions for different occupant types. For example, permanent occupants in a space (e.g., receptionists in a lobby) may have different thermal comfort requirements than temporary occupants who are passing through the space. Similarly, fans can be used to address inconsistent thermal conditions in a space, such as areas of high solar heat gain near windows, or areas where occupants can be expected to have notably different metabolic rates.	Assembly / Event Spaces – Transitional Lobbies Agricultural / Livestock Industrial Production / Manufacturing Kitchens (commercial) Outdoor Gymnasium / Exercise Areas – directly over areas or equipment where occupants likely will have high metabolic rates	Low to high; depending on application and occupancy. Target may vary throughout the space	Allow flexibility or ensure lower air speeds for any permanent occupants in the space (receptionists, etc.). Maximum target air speeds to high metabolic rate occupants or under higher heat gain or other source of higher cooling demand Maximum target air speeds in agricultural, industrial, and warehouse applications will depend on whether the space has other mechanical cooling (unconditioned spaces may require higher maximum air speeds).
Uniformity: For multi-occupant spaces where locations are fixed or inflexible, maximize uniformity and coverage at relatively low air speeds throughout the occupied subzones of the space to ensure consistent experience for all occupants.	Assembly / Event Spaces – Fixed Classrooms and conference rooms Dining Areas Offices – Shared, Assigned Seating Retail	Low to medium	Prioritize uniform coverage to provide consistent conditions throughout the occupied subzones of the space. For offices and classroom spaces, ensure maximum design air speed will not blow paper off desks (approx. 0.8 m/s [160 fpm], can be as low as 0.4 m/s [80 fpm] for lightweight paper). For dining and retail, separate air speeds for subzones containing diners versus servers, or clerks versus shoppers, may be created to account for their different activity levels and cooling needs (see targeted category above).

Fan type considerations

There is no absolute rule to be applied on how to select a suitable fan. In practice, fan selection criteria are mainly dependent on design intent of air speed and distribution, purpose of elevated air movement (i.e., direct cooling across human body or air circulation), and any limitations of fan usage in space.

In general, ceiling fans are effective for both comfort cooling and air circulation in nearly all scenarios and applications. Ceiling fans are recommended due to their effectiveness in air movement, provided that some limitations of implementation are considered. Firstly, “is there sufficient height clearance to install ceiling fans?”. To maintain ceiling fan efficiency and operational safety, the ceiling fan should neither be installed too close to the floor (i.e., prevents subjects accidentally contact with blades) nor the ceiling (i.e., avoids “starving” of the fan). For more details, please refers to the section of Fan mounting height and clearances). Secondly, “are there obstructions (permanently placed) at the ceiling or on the floor that interfere with the ceiling fan operation?”. Ceiling fans work more effectively in open space with less obstructions (see Figure 23 for air speed distribution from ceiling fan). Air distribution and speed within space could be significantly distorted by the presence of bulky furniture or partitions. In addition, the lighting fixtures installed on the ceiling could have interacted with the rotating blades of ceiling fans, causing strobing and visual flickering effects, if the location of lighting fixtures and ceiling fans are not arranged properly (See section ceiling fan interaction with lighting for details). Furthermore, higher renovation cost and time for ceiling fan integrated design, when compared with portable air movement devices, could be a factor of concern for building practitioners when they are deciding to implement elevated air speed strategy in an operation building space.

If the above limitations do not apply, ceiling fans are likely to be recommended. Figure 33 shows that ceiling fans work effectively in different design intents summarized in Figure 32. Ceiling fans can be arranged to provide uniform air speed and distribution within a multi-occupancy space when the occupants do not have control of the fans. Alternatively, in spaces where ceiling fan speed can be centrally or manually adjusted, or occupants can freely move around to satisfy their variable thermal comfort needs, uniform air distribution is less important, and variability is recommended. In single occupancy rooms, the occupant can be given full control of the ceiling for fulfilling different air speed demands at different times of the day or throughout the year, in this case ceiling fans can provide personal control. While in a particular area that has special thermal demands, ceiling fans can be pre-set/adjusted with stronger air movement or located only over the most demanding area so, by design, higher air speed target part of the space and lower air speed reaches its adjacencies.

If the limitations of ceiling fan usage cannot be overcome, designers may consider choosing other fan types described in the Other air movement devices section, such as desk fan, pedestal fan, tower fan, bladeless ceiling fan, wall-mounted fan and air circulator. Figure 33 demonstrates examples for these fans’ selection regarding different design intents. The bladeless ceiling fan and wall-mounted fan can provide relatively uniform air speed and distribution, but are only applicable to small size rooms due to installation limitations (i.e., wall-mounted fans can only be installed in a wall or column and will not affect occupants sitting too far from it) and airflow (i.e., bladeless ceiling fans require a boundary to deliver air towards the occupied zone, therefore are not efficient for big spaces). Most of the non-ceiling fan devices mentioned above are portable, operate near the occupants, and allow personal adjustment. This means that both variability and personal control design intents are achievable at the same time where occupants are free to move the fan and to adjust its speed based on their thermal preferences. In spaces where occupants are not supposed to have fan control, but they have special thermal demands, wall-mounted fan can be used to create variability or targeted air speed, by setting different air speeds in spaces where occupants can move around or by directing the wall-mounted fans to the most demanding occupants’ group. In this sense, the wall-mounted limited coverage area may constrain a targeted design intent. Nevertheless, facility management teams are encouraged to complement adjustability providing personal fan to occupants when their personal demands are not met.

Within a space, different types of fans can be integrated to maximize occupants’ comfort and to achieve multiple design intents. For example, ceiling fans can provide a uniformly low speed background air movement for all occupants in a large open space, while individual fan allow additional personal control to further improve occupants’ local thermal comfort at whenever it is necessary (e.g., at transient condition or near the window).

Figure 33. Fans selection examples based on different design intents.