Air conditioners and dehumidifiers remove water vapor from indoor air by refrigerating the air to below its dew point to condense water vapor. The air conditioner (AC) should be the primary moisture remover whenever possible. However, when the AC doesn’t activate because of a low outdoor temperature, the Rh may cause condensation and discomfort.

Dehumidifiers reduce high relative humidity.

Many buildings have large sources of moisture, particularly from the ground. Plugged or damaged rain gutters and downspouts deposit water near the foundation. Air leaks bring humid air into the building. Residents shower and cook. Try to minimize these moisture sources to reduce latent cooling load, before purchasing a dehumidifier.
In humid climates, the definition of comfort varies from person to person. Consider setting the thermostat for 78°F (26°C) or higher. Try for a relative humidity setting of 55% or less on the dehumidistat. The higher your thermostat setting and the lower your dehumidistat setting, the less electricity your dehumidifier consumes. For IAQ and building durability, keep the relative humidity below 60%.

Air Conditioners Dehumidify

If the AC dehumidifies enough, you won’t need a dehumidifier. Air-conditioning systems may remove enough moisture to provide comfort and prevent condensation, or they may not. An AC tech could adjust your AC system to remove more water vapor.
Air conditioners remove water vapor in the process of cooling a building. However, AC efficiency and moisture-removal oppose one another. AC efficiency works best with warmer evaporators and higher airflow. AC moisture removal works best with colder evaporators and lower airflow.
Here are four conditions when air conditioners fail to dehumidify enough.

Dehumidifiers can’t solve standing water in a crawl space or other large or other large moisture sources.
  • Oversized air conditioners don’t run long enough to remove sufficient moisture.
  • Very efficient air conditioners operate with high airflows and warm evaporator coils. This way of operation doesn’t condense much water vapor.
  • In mild and humid weather, the thermostat doesn’t activate the air conditioner because thermostats measure temperature, not Rh.
  • Airtight, well-insulated buildings with very small solar heat gains may not need AC, but persistently high indoor relative humidity remains a comfort and durability concern.

If high humidity is a severe comfort problem, you may need a correctly sized dehumidifier to improve the comfort. Remember to reduce all the sources of moisture before making this decision because if the moisture sources are too prolific, the dehumidifier may fail anyway. For example, if you have a crawl space with standing water, you need a sump pump, not a dehumidifier.

For more information on dehumidification see: Residential Energy, Chapter 11