Ceiling fan integration with HVAC system

How to design, select, install and operate ceiling fans when integrated with a HVAC system?

This technical guide focuses on the integration of ceiling fans with air-conditioning systems in buildings, due to higher attention on design, installation, and operation. Besides, incorporating ceiling fans in the early system design stage can achieve additional savings in reduced construction costs and downsized HVAC system components.

Ceiling fan system design

Considerations of ceiling fan design in space are mainly two folds: fan size and installation.

Fan size and number determination

Determining the appropriate ceiling fan size (and number) within space is critical to air speed distribution and effective cooling. Table T5 lists the key features for fan size (and number) determination.

Table T5. Key features in determining fan size (and number) in space.

Features	Considerations	Remark(s)
Fan mounted height and clearance from wall and other obstructions	Standard ceiling fans: Mounted at least 2.1m [7 ft] above floor for safety concern. Minimum blades to ceiling height of 20 cm [8 in] or 0.2 times of the fan diameter (Whichever is larger). Minimum blades to wall clearance of 45 cm [18 in] Large diameter fans (>2.1m [7ft]): Mounted at least 3m [10ft] above floor. Minimum blades to ceiling height of 0.2 times of the fan diameter. Minimum blades to wall clearance of 90 cm [36 in]	Ceiling fans size (i.e., diameter) is limited to the floor to ceiling height of the space, due to safety concern and to avoid “starving” of the fans (i.e., insufficient air feeding the fan). Refers to “Fan installation” section for more detailed on minimum clearances.
Space area, room ratio and layout	For small and roughly square room: Single fan with diameter of 0.2 – 0.4 times the room with shall be used. A single fan can effectively serve a rectangular room with aspect ratio (length : width) up to 1.5 : 1. For high aspect ratio (> 1.5 : 1) and unconventional shape room Multiple fans shall be considered to ensure uniform air speed distribution. If a single fan is used, the fan diameter shall be 0.2 – 0.4 times the characteristics room width (i.e., square root of the floor area). For larger size room: It should be sub-divided into multiple equal square-shaped “fan cells” (i.e., < aspect ratio of 1.5 : 1) One fan will be centred in each fan cell and operates similarly within a small room as describes above. Size and number of fan cells and ceiling fans are dependent to the function of the space (i.e., the design intent). In each fan cell, the fan diameter should be between 0.2 – 0.4 times of the fan cell’s characteristics width.	If uniformity of air speed is the design intent, the largest available fan size that fulfils the spatial concerns (i.e., height, clearance, area, and layout) shall be selected. For multiple fans design, space with higher uniformity may require larger size fans and closer fans spacing. See Figure T7 for recommended fan size and layout for single-fan case. See Figure T8 for recommended fan size and layout for multi-fans cases.
Design intents	Design intents of fan speed and distribution criteria based on category presents in Figure T5. For example, whether the desired air speed is designated for personal usage, a targeted area, variability of change, or maximizing uniformity	Larger fans or more fans will be required in spaces that are likely to be benefit from uniformity than variability.
Other ceiling installations	Fan size should compromise potential conflicts with other building system, such as fire sprinklers and lighting.	Refers to “Fan installation” section below for details.

Figure T7 demonstrates an example of recommended ceiling fan size for single-fan applications. Provided that the room area aspect ratio (length : width) is within 1.5 : 1, one fan operation is applicable. The room area is 24 m² [258 ft²], and the characteristic width is 4.9 m [16 ft] (i.e., square root of the room area). The recommended ceiling fan diameters are between 0.2 – 0.4 times of the room characteristics width, equivalent to 1 m [3.3 ft] – 2 m [6.6 ft], assuming all other features in Table T5 have been fulfilled.

Figure T7. Recommended ceiling fan size and layout for single-fan applications.

Figure T8 presents two examples of recommended ceiling fan sizes and layouts for multi-fan applications with the same site area (240 m² [2580 ft²]) but different room heights and room functions. Figure T8a shows a warehouse divided into two identical fan cells, and one larger-diameter ceiling fan (4.4 m [14 ft]) is centered in each fan cell. The design and installation are valid with sufficient room height (8 m [26 ft]). Figure T8b shows an office with the same area of the warehouse but a much lower room height (2.6 m [9 ft]). It also demonstrates two layout settings: fewer fan cells with larger fans (on the left) and more fan cells with smaller fans (on the right). Settings on the left have four 30 m² [322 ft²] fan cells and a 2.2 m [7 ft] diameter fan centered in each cell (calculated by 0.4 times of room characteristics width). A 2.2 m [7 ft] diameter fan is considered a larger-diameter ceiling fan, which requires at least 3 m [10 ft] mounting height above floor level (see Table T5), and it is not suitable to be installed in office space with relatively shorter room height. In addition, considering this is an office space, too large fan size would block the lighting fixture on the ceiling, initiating unwanted visual flicker. Using a smaller fan size may fulfill the installation requirement but result in a smaller airflow rate and reduced uniformity. The layout settings on the right, with more but smaller fan cells (13 m² [140 ft²]) and a 1.45 m [4.7 ft] diameter fan centered in each cell, can provide more uniform air speed within space, and it is a more suitable design for office settings.

Figure T8. Recommended ceiling fan size and layout for multi-fan applications. (a) Warehouse with higher room height, (b) office with lower room height.

Fan installation

In general, all ceiling fan installation procedures shall follow the manufacturer’s recommendations. The ceiling fan should be fixed on a structural surface (slab or beam) to guarantee sturdiness and stability. With the appropriate type of mounting bracket, ceiling fans can also be installed in sloped ceilings. There is no reinforcement requirement if the diameter of the fan is smaller than 2.1 m [7 ft].

The fans mounting heights from ceiling / floor and clearance from walls / obstructions are important regarding both safety and performance consideration (See Table T5 in details for both standard and large diameter ceiling fans). In rule of thumb, ceiling fans require a minimum distance from the ceiling of 0.2 times of the fan diameter to avoid “starving”.

Installation of ceiling fans should avoid conflicts with the lighting fixtures to minimize changes of visual flicker and strobing effect, as well visual discomfort. Figure T9 illustrates the potential problems when ceiling fans interact with lighting fixtures. It suggests that visual flicker effect is dependent to view angle of the occupants. Thus, the position of ceiling fans should not only be installed away from the recess lights, but also considering occupants’ position in space. Alternatively, designer may consider the possibility of using dropdown lightings (see Figure T9c) with minimum glare to the occupants. If the above limitations with respect to lightings cannot be resolved, designers may consider using other non-ceiling fan alternatives.

Figure T9. Sectional illustrations on the interactions between ceiling fans and lighting fixtures across different configurations: (a) strobing and flicker, (b) flicker, and (c) ceiling strobing.

Operation of ceiling fans near windows/doors opening would impact the room air changes per hour (ACH) or ventilation rate. Figure T10 illustrates the airflow patterns for normal window and door-like opening settings. Designers should consider the impact of room air changes per hour via window in natural ventilation conditions by the fan airflow patterns. The use of door-like openings may induce more outdoor airflow.

Figure T10. Illustration of airflow patterns induced by ceiling fan for (a) normal window opening, (b) door-like opening settings.

Air-conditioning system design

The conventional air-conditioning system requires diffusers and extended air ducts to distribute air evenly within the space. However, ceiling fans integrated with conventional air-conditioning systems can effectively mix and re-distribute the room air without the need for extra diffusers and extended supply air ducts. In principle, ceiling fans integrate well with most ventilation settings that require air mixing, including radiant cooling systems. However, they are unfavorable for systems that rely on stratification, such as displacement ventilation and underfloor air distribution system or systems utilizing active or passive chilled beams.

Figure T11 compares the design layouts between a conventional air-conditioning system and a recommended ceiling fan integrated air-conditioning system. The ceiling fan integrated air-conditioning system requires only the main supply air duct to throw cool air from a high-sidewall vent into the occupied space. Then the ceiling fan will mix and distribute the cool air in the space. The cool supply air would be best fed above the fan blades to enhance air mixing and avoid cold drafts. Immediate benefits of such design are reduced capital and maintenance costs for unnecessary ducting, diffusers, and variable air volume (VAV) boxes. In addition, the ceiling fans could work more efficiently with larger blades to ceiling height (assuming no false ceiling and without an extra supply air duct).

Figure T11. Example design layouts for (a) conventional HVAC system with supply air ducts and diffusers, (b) Ceiling fan integrated HVAC system with limited supply air ducts.

Ceiling fans integrated AC system

One major advantage of the integrated air movement design is the additional convective effects on occupants. This means additional energy saving from space cooling is possible due to the optimization of the air-conditioning system.

Design room conditions

The presence of space air movement allows higher dry bulb room temperature (2-3 °C [4-5 °F]) and dewpoint temperature (1-2 °C [2-4 °F]) when compared with conventional air-conditioning system design. Alternatively, if the originally designated air temperature is 23-25 °C [73-77 °F], it can be increased to 23-27 °C [73-81 °F] with elevated air movement to achieve similar or even better thermal comfort. The room operating air speed can be up to 0.8 m/s [160 fpm] without personal control, while there is no air speed limit if occupants have full control of the fans (i.e., just ceiling fan or ceiling fan + desk fan) within the space.

Components selection

Regarding the relaxation of designated cooling demand (i.e., lower sensible and latent load), the chiller and the air handling unit (AHU) from the original HVAC system can be downsized in the system design stage. The design supply air temperature setpoint (SAT) and chilled water temperature (ChWT) setpoint relaxation should be 50% to 100% of the zone cooling setpoint temperature adjustment. For example, if compared to a conventional HVAC design, the HVAC design with elevated air speed could have a 2 °C higher cooling setpoint temperature, and the SAT and ChWT setpoints should be increased by 1-2 °C. In addition, a smaller size fan in the AHU can be used for the ceiling fan integrated design because it returns smaller static pressure along the critical supply air path (only require the main supply air duct).

HVAC and fans system control

Figure T12 demonstrates the control schematic for the heating, ventilation, and air conditioning (HVAC) system with and without the integration of ceiling fans. It shows that when ceiling fans are integrated with the building automation system (BAS), it can react at the first stage cooling setpoint (say 23 °C [73 °F]) to cool down the zone before the HVAC system beings to operate for cooling. The ceiling fan speed shall increase with the room temperature, which is determined by the cooling effect or a representative point in space. The HVAC cooling starts when the indoor temperature increases to the second stage of the cooling setpoint (say 26 °C [79 °F]). Operating the HVAC system at this higher cooling setpoint has significant energy savings potential.

Figure T12. Example HVAC control schematic with / without ceiling fans.

Some ceiling fans have onboard sensing and controls that allow fan speed and temperature automation without integration with the building automation systems (BAS). A lower-cost, simpler alternative to automatically control the ceiling fan based on temperature is to use a relay to turn fan(s) on and off. The fans then operate at a fixed pre-set speed. Fans' operation can also be tied to occupancy sensors in the zone, preventing unnecessary operation, energy use, and maintenance. In some cases, it may be beneficial to operate fans even when unoccupied, such as pre-cooling applications that benefit from increased convection from surfaces in the space due to the air movement generated by the fans.

Potential savings

Comparing the two designs in Figure T11, the ceiling fan integrated air-conditioning system could save up to 45% in capital construction cost compared to the conventional system. These savings are mainly obtained from reduced ductwork, diffusers, VAV boxes, sensors, and controls. Savings can also be obtained from the extra time and workmanship for additional ducting and fittings installation. More importantly, the majority of these ducting and fittings are not reusable and become construction waste when the building is demolished. The cost of purchasing and installing ceiling fans in the space is low compared to the above cost savings.

In addition, substantial energy for space cooling can be accrued when implementing the elevated air movement with a higher temperature cooling strategy. Approximately 17 % of cooling energy saving can be achieved by increasing the cooling temperature setpoint from 22 °C [72 °F] to 25 °C [77 °F]. In such room temperature setpoint adjustment, higher chilled water supply temperature (10 °C [50 °F]) can be used instead of the conventional temperature setpoint (7 °C [44.6 °F]), which it would account for approximately 12% of additional energy saving from the chiller. In a Singaporean zero-energy building, a 32% HVAC energy saving was obtained when the setpoint was increased from 24 [75 °F] to 26.5°C [80 °F]. These cooling energy savings are at least two orders of magnitude higher than the energy used for ceiling fans operation in space.