Ceiling Fans are an Important Tool
Ceiling fans can save energy, increase comfort and even mitigate airborne disease transmission
Ceiling fans are surprisingly a very valuable tool for indoor air quality, thermal comfort and energy savings. Although they do not provide any clean air like ventilation or filtration, they can still have an important impact on a space from an indoor air quality perspective. Ceiling fans are currently not required when designing a space, but should be given much more consideration in the future. Here are five advantages that they provide.
1. Mitigating Short-Range Transmission
Transmission at short-range is high risk because the initial exhaled air contains a higher concentration of infectious particles before it mixes into the space.
Increasing the air speed in a space is an important tool to mitigate short- range transmission. This will cause the exhaled plume to get diluted quickly and mix with the air in the room. This can be compared to being near a smoker on a windy day, where the wind will blow the smoke away before it is inhaled.
For more details on mitigating short range transmission, see this post.
2. Air Distribution
Air distribution refers to how air moves throughout a space and how well clean air is supplied to the occupants. In poorly designed systems, a lot of the air that is supplied to the space goes directly from the supply to the return without mixing into the space. For a detailed explanation of air distribution, see this post.
Most systems are well-mixed — the goal is to have the dirty air fully mixed with the clean air supplied to the space. Ceiling fans are very effective at mixing the air in the space and improving air distribution.
Furthermore, one concern about using fans is creating direct air currents to carry the exhaled air from one person to another. This is only a concern with fans that blow air horizontally. Because ceiling fans push the air downwards, this isn’t a concern.
3. Destratification in the Winter
In the HVAC world, ceiling fans are often called destratification fans.
Air stratification occurs because hot air rises. It is why the upper floors of a house are often too warm and the basement is usually cold.
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Stratification can cause significant energy waste in the winter. In a space with a high ceiling, the hot air will rise to the ceiling leaving the lower occupied space feeling much colder, requiring additional heating. If the heating is coming from air supplied at the ceiling, the air will often not make it down to the occupants and remain buoyant, further reducing the heating effectiveness and wasting energy.
Using a destratification fan ensures the air in the occupied zone is mixed with the warm air near the ceiling, warming the space and reducing heating requirements.
4. Cooling in the Summer
People cool themselves down through moisture evaporating from the skin. This is why people sweat when they overheat. Even when people are not sweating, moisture on the skin evaporates to regulate body temperature.
The rate of evaporation is related to the temperature, humidity and air speed. As air speed increases, evaporation rate increases and people feel cooler. This is helpful in the summer where cooling loads can be reduced and replaced with ceiling fans, so people would still be comfortable in warmer spaces which can save energy.
5. Enhanced Upper Room UV
Upper room UV systems shine UV light above occupants’ heads. Pathogens exhaled by people travel into the upper room, become inactivated through the UV exposure and then travel back to the lower room where people can inhale them, but not get infected.
Upper room UV is a very effective tool with high equivalent clean air delivery rates. The effectiveness can be limited by the mixing rate between the upper room and lower room.
Evidence shows that use of ceiling fans can significantly improve the effectiveness of upper room UV systems.
There is also evidence that it is not necessary to run the fans at high speeds. Once there is good mixing in the room, additional airflow is not needed.
More information on upper room UV can be found here.
Practical Use
From my experience, practical use of ceiling fans can be a problem. Because of the increased air speeds, occupant control is required. Occupants generally are unaware of existing equipment and the fact that they should control it or how it should be controlled. This means as soon as anyone finds them a nuisance, they will be turned off and no one will turn them back on.
The way to solve this problem is to have proper labelling, signage and have people in the building responsible for ensuring that they are on whenever possible. This is easier said than done.
Given the advantages of ceiling fans, they are worth the effort so spaces can be more energy efficient, healthier and comfortable.
Disclosure: I have no conflicts of interest.
