Comfort Control

Comfort Control Heating and Air Conditioning, Inc.

The Relationship Between Your Thermostat’s Fan Setting and Humidity

When your heating and cooling system’s fan is set to ‘on’ it operates continuously. The compressor continues to periodically cycle on and off automatically to cool and dehumidify your home when the fan is set to ‘on’ or ‘auto’ on your thermostat. The difference between the two settings is that with the fan ‘on’ and continuously circulating the air, the temperature stays more even throughout the house by clearing out pockets or layers of static, warm air.

However, another factor to consider is how running the fan continuously (the ‘on’ setting) reduces dehumidification. Humidification is definitely helpful in many homes and businesses. Particularly during cold weather, insufficient moisture in the air often is responsible for such assorted problems as stuffy noses, sore throats, even more dust than usual, cracks and dried-out joints in wood furniture, wilted plants, and static electricity which jolts hair, clothes, and computer disks.

Indoor relative humidity may fall to around 7 percent in some cases — much drier than even the 25 percent relative humidity of the Sahara Desert — all because your system does such a good job of extracting moisture from the air. However, ideal indoor relative humidity is between 30 to 50 percent. On the other hand, if relative humidity rises and averages 70% or more, mold can begin to grow and reproduce alarmingly. A further problem that can develop is increased duct leakage.

If humidity rises in your home with the fan running continuously, the culprit is likely the indoor coil. Water vapor from the air passing over the coil would condense and then evaporate as the fan recirculates all that stored moisture throughout the house. Ultimately, the symptom of high relative humidity despite continuous fan operation points to your system being oversized for your home. At Comfort Control, we recommend getting an assessment of the size of your system relative to the size of your home, and a diagnosis of the average relative humidity within it.