WHEELER ON HVAC: Controlling Humidity
Because many of my articles in SUPPLY HOUSE TIMES can be found by Internet search engines, scarcely a week goes by when I don't receive a question from a consumer asking an HVACR question that he or she has taken the time to research on the Web. One such inquiry came from a man who asked what he could do to lower the humidity in his home. I thought you might also be interested in my reply.
Of course, the air-conditioner evaporator is where all dehumidification takes place, so this is the logical place to start. The simplest and least expensive way to improve humidity removal is to simply reduce the amount of air flowing through the evaporator. A dirty filter or coil can do that. However, if the air flowing through the coil drops much below 350-cfm/ton, it will likely freeze over and cause system damage. Now I'm not recommending dirty filters or coils, but I mentioned that because many systems are already doing more dehumidifying for this reason.
What I am recommending is that you have a QUALIFIED service technician slow the fan speed to about 350-cfm/ton using the motor's internal speed taps.
Remember, the larger the air conditioner, the less it tends to dehumidify. This is because dehumidification only takes place when the compressor is running, and one that has to run all the time to keep up with the load also removes the most moisture. I've seen large commercial jobs where equipment and products were ruined by oversized air conditioners - and I've seen homes where the wallpaper and furniture was covered with mold for the same reason.
The first solution that most people think of is to install a portable dehumidifier in a room, which helps some, but that method uses the most energy. Improving the operation of your air conditioner is a better idea.
The type of evaporator a person selects also has an effect on dehumidification. The more fins per inch (fpm), and the deeper the coil, the more moisture it will remove. On commercial jobs, special evaporator coils are available for greater dehumidification.
If the air conditioner isn't oversized and the evaporator fan is operating at about 350-cfm/ton, and you still have a moisture problem, you need to look at the source of the excess moisture. If it is from a lot of cooking or someone taking too many hot showers, better vent fans may be the solution. However, if the source is coming from introduction of outside air, there are some excellent heat/moisture transfer devices that can be installed at the entering air source which will greatly reduce the problem.
As for the man who originally contacted me about this problem, his idea was to reduce the evaporator coil size. Yes, that would work, because the smaller coil would create less heat transfer while reducing the airflow. However, he would also reduce his cooling efficiency. As I told him, you can reduce the size of the coil on most residential evaporators to about one square foot of face area per ton. But my suggestion was to buy a better coil, add a Thermostatic Expansion Valve (and possibly a compressor hard-start kit) and reduce the fan speed.