Humidity control is a critical part of any air conditioning system. Refrigerant-based systems remove humidity from the air by removing heat energy, causing moisture to condense onto the evaporator coils. Since your home's blower pushes air across the evaporator coils, the air delivered from your supply ducts should typically be less humid than the air entering the return vents.
While humidity control may seem like a secondary feature, it's crucial to maintaining a comfortable home. High humidity levels often make lower temperatures subjectively less comfortable, forcing you to turn down your thermostat to maintain acceptable indoor temperatures. If your system doesn't seem to be removing humidity, it can lead to discomfort for you and extra stress on your HVAC components.
Understanding Humidity Removal
If your home doesn't have a dedicated whole-house dehumidifier, then your air conditioner is your first and last line of defense against humidity. While ventilation features such as bathroom fans and range hoods can help with interior moisture sources, they can't do much to fight the ambient humidity level without your air conditioner's help.
However, the way your air conditioner removes humidity can be counterintuitive. It's easy to assume that more power means better performance, but this isn't the case with humidity removal. Instead, the key is runtime. Your AC only removes humidity while running, so an oversized system that cools your home too quickly may be highly ineffective at removing humidity.
Likewise, the condenser must be running to remove humidity from the air. If your home's blower runs without the condenser, it will push humidity around at best and potentially add moisture to the air at worst. As a result, improper fan settings can cause high humidity levels and ultimately result in a far too uncomfortable home.
Tracking Down Humidity Issues
Always start by ensuring you set your thermostat's fan setting to "auto." If it's on anything else, this simple trick may fix your humidity. However, if your thermostat was already on "auto," it's time to call a professional to look for other causes. Most contractors will begin by doing a quick load calculation for your home and ruling out the possibility of an oversized system.
Assuming you have a properly-sized AC unit, there are a few potential problems that can cause high humidity levels. A low refrigerant level is one common cause. Low refrigerant can cause your evaporator coils to freeze, preventing them from cooling the air and creating a continuous moisture source. In these cases, you'll need an HVAC contractor to find and repair the leak before adding more refrigerant.
Other causes can be more complex and may include problems with your home's ductwork design, improper blower speed settings, and other subtle problems. An air conditioning contractor can help you determine the most likely culprit and recommend a cost-effective course of action.