Cooling systems play an important role in maintaining a comfortable and healthy indoor environment during warm weather. Many types of cooling systems are often referred to as “air conditioners” (even though heating systems also “condition” air to specified comfort levels). Cooling systems condition air by reducing its temperature and, in many cases, reducing its relative humidity.

Relative humidity measures how much moisture is in the air compared to how much moisture the air could hold at its current temperature. For example, if the relative humidity is 40%, that means the air is holding approximately 40% of the water vapor that it can hold before that vapor condenses to liquid water. Humans are generally most comfortable in relative humidity levels between 40-60%.

Because warmer air can hold more water vapor than colder air, simply cooling air without removing any moisture would cause its relative humidity to increase. Many cooling systems therefore cool and dehumidify the air at the same time.

However, maintaining comfortable indoor temperature and humidity levels requires energy. Cooling can represent a significant portion of household energy consumption, particularly in hot climates. A cooling system's energy use depends not only on the cooling technology itself, but also on factors such as outdoor climate, home insulation and air leakage, thermostat settings, and heat generated inside the home.

Understanding how your cooling system maintains comfortable temperature and humidity levels can help you identify opportunities to reduce cooling costs while maintaining a comfortable and healthy indoor environment. It can also help reduce the risk of heat-related illness during periods of extreme heat, an increasingly important consideration as summers become hotter.