It’s easy to identify those hot, sticky days when a home would be unbearable without a central air conditioner. There are many times, however, when the average dry bulb temperature is lower than 75°F, but the average dew point temperature is greater than 60°. This condition equates to rh values greater than 65 percent; which makes most occupants of a home uncomfortable in conditions when an air conditioner will not run, as its primary control is based upon dry bulb temperature.

Combining cooling and dehumidification in the same device creates an inherent conflict of interest for the controls. During many hours, the home may not need cooling but might be in great need of dehumidification.

TheASHRAE 1997 Fundamentals Handbookexpanded outdoor design conditions to include “peak dew point.” Unlike the traditionally used “peak dry bulb” condition, which represents hot, mostly sunny days; peak dew point conditions occur when the outdoor air is cooler but loaded with moisture. In these conditions, a dedicated dehumidification system should be used to maintain comfortable humidity within a home.

Graph 1: Miami’s opportunity for additional dehumidification is prevalent most of the year. (Courtesy of Aprilaire.)


These critical times, when moisture removal is required but temperatures are not high enough to air condition, vary during the year. For most of the South and coastal regions, these months are nonsummer and run from October through April. For the mid-states and North, these times are during the warmer, summer months, between May and October.

For example, Miami, historically would fall within these conditions 31 percent of the time between October and March, which averages 224 affected hours each month. (See Graph 1.) Meanwhile, in Pittsburgh, Pa., these conditions would be 28 percent, June through September. (See Graph 2.)

Graph 2: Pittsburgh’s opportunity for additional dehumidification is during the summer. (Courtesy of Aprilaire.)

Analyzing further, when looking within a 24-hour period, late evening through early morning periods typically lend to the low temperature-high moisture situation. In Atlanta, during a typical day in September, for example, the dehumidifier would run almost 14 hours of the day and the air conditioner would run the other 10. (See Graph 3.) Note that the times when the dehumidifier would run are when a homeowner is most often at home.

In addition, the need for central dehumidification is further supported when analyzing partial load conditions and cases when air conditioners are oversized. ASHRAE 62.2 states: “Under partial load conditions, the required outdoor air and occupant activities often will produce a larger latent load that cannot be handled by the air conditioning system. Under such circumstances, independent dehumidification control should be used.” Further data from field studies show that the moisture removal capacity of a cooling coil degrades at part load conditions.

Air conditioners that are “peak-sized” or are too large for a given area will cool the area too quickly, causing the air conditioner to frequently turn itself on and off. This often results in fewer opportunities to reduce moisture levels.

Figure 1: Optimum relative humidity range for human comfort and health. Note: A decrease in bar height indicates a decrease in effect for each of the items. (Courtesy of Aprilaire.)


Central dehumidifiers are a great solution to reducing moisture levels and, therefore, discomfort, within living areas. Central dehumidifiers are a mechanical refrigeration system designed specifically for the removal of moisture from the air, controlled by a humidistat, rather than a thermostat.

By utilizing a central dehumidifier in conjunction with the central air conditioner, humidity levels present throughout the home should be continuously monitored and automatically controlled. This should be considered an optimal comfort strategy by way of efficiently improving complete, whole-home living space comfort and IAQ.

The optimal comfort zone, as published by ASHRAE, is between 30 percent and 60 percent rh. (See Figure 1.) Additional benefits from maintaining humidity levels between 30 and 60 percent include, but are not limited to, reduced allergy, mold, mildew, and bacteria issues. Dust mites, for example, continue to be a major contributor to allergy rates, but can be controlled and reduced if humidity levels are maintained at or below 50 percent.

The advantages of central dehumidifiers include flexible coverage options for the home, remote monitoring and adjustment capabilities, typically lower noise levels, and elimination of the consumer emptying drain-reservoir trays.

Figure 2: A typical whole-house installation of a central dehumidifier, in a basement or lower-level HVAC location. (Courtesy of Aprilaire.)


Understanding the numerous advantages of integrating central dehumidification with an air conditioning HVAC system typically leads to the question of installation complexity. For most applications, installation can be completed in less than three hours, during a complete HVAC system change out or install. Adding a central dehumidification to an existing HVAC system will add a few more hours to the install time.

Installation locations vary. However, most are in close proximity to the HVAC in the basement or attic space. (See Figure 2.) There are three basic steps to remember to complete the installation: duct it, drain it, and power it.

1.Duct it - For most installations, the inlet side of the central dehumidifier is ducted to the return duct, typically at least 6-inches prior to the HVAC and any in-line advanced air filtration system. The outlet connection side is then ducted to the supply duct, after the HVAC.

2.Drain it - A majority of central dehumidifiers include a p-trap to allow for removal of the excess moisture to an agreed-upon drain location. Small pumps can also be installed to provide additional assistance in moving the moisture farther away from the home. In contrast to most portable versions, central dehumidifiers have no reservoir tanks that need to be dumped on a regular basis.

3.Power it - Residential central dehumidifiers typically have 120-V connections, so powering them up requires only a standard 15-A, 120-V outlet during a majority of installations. (Twenty amp outlets are recommended if the powering circuit has many other large loads on it during the day, or when a larger-capacity, light-commercial unit is installed.)

Graph 3: A typical 24-hour period in the month of September for the city of Atlanta, Ga. (Courtesy of Aprilaire.)


In regard to ducting of the central dehumidifier to the HVAC system, it is highly recommended that there is some type of back-flow damper installed after the outlet of the central dehumidifier. This is done to prevent cooled air from being pulled back through the loop when the central dehumidifier is not running. A back-flow damper also prevents dry air from flowing back into the central dehumidifier and re-absorbing moisture that has been trapped.

Most central dehumidifiers also come with an additional ventilation option, a feature designed to facilitate fresh outside air to be brought into the home and dehumidified, prior to being put into the HVAC system. This feature is a great option for tighter homes that don’t already have a ventilation system installed.

Application flexibility is another important factor when considering a central dehumidifier. Meaning, beyond the basic installation mentioned previously, other installation versions can be adapted to the system to allow for addressing high moisture issues in localized areas of the home, in addition to managing the whole home. This application is called a convertible installation.

Publication Date:08/13/2007