We often get asked if it’s a good idea to run the fan in the ON position through the thermostat. My answer is always “it depends”. In most cases we do not recommend running the fan in the ON position, especially in commercial buildings. There are benefits to running the fan continually such as having constant air movement and constant air filtration. Everyone likes clean and fresh air, so it can be appealing to leave the fan running all the time.
The other side of the equation is what we focus on. Most commercial buildings have some sort of fresh air intake built into the duct work or equipment. This is requirement so that our buildings do not get stale and have the correct amount of air changes required by code. Fresh air is great when its conditioned before it enters the building. If the AC or heat turns off but the fan keeps running, the system will draw in outside air into the building without properly conditioning it. In the summer this air is hot and humid; in the winter the air is cold and dry. This can lead to uncomfortable work environments which is not appealing to anyone.
A 2nd side effect is reduced dehumidification. AC units remove humidity when running. Water vapor is turned into liquid to be drained out of the building. If the AC shuts off, and the fan is left to run, it will re-evaporate some of the moisture it tried to take out. This can lead to higher than desired humidity levels.
A 3rd side effect is building infiltration. This is caused by unbalanced air flow and many older buildings can suffer from this. This has to do with the duct work not being properly installed and/or sealed. I don’t recommend running the fan continuously if the duct work is known to be leaky as it could cause the building to be negatively pressurized which can bring in outside air and all that comes with it.
A 4th reason is energy usage. Running the fan all the time costs more with most systems. Newer systems will have higher efficiency fan motors but if its over 5 years old, its likely to have a basic motor.
We recommend leaving the fan in the auto position. This way you know the air is being conditioned before it gets to your co-workers which means they will be comfortable and able to work more productively.