Contrary to popular belief, indoor moisture control is an issue in almost all geographic locations, not just in areas where hot, humid conditions prevail. Whenever a high relative humidity exists at or near a cold, porous surface, moisture absorption increases and moisture-related problems (increased maintenance, premature replacement of equipment and furnishings, and increased health risks) become likely.

If properly designed and controlled, the HVAC system can significantly reduce the moisture content of indoor air. Ironically, the most widely used means of ventilation—the single-zone, constant-volume (CV) system—is also the most problematic when it comes to dehumidification.

A basic CV system consists of an air handler that serves a single thermal zone. The air handler supplies the zone with a constant volume of air, usually a mixture of outdoor air and recirculated return air, at a variable temperature.

A thermostat senses the zone dry-bulb temperature and compares it to the set point. The thermostat then modulates the capacity of the cooling coil, adjusting the supply-air temperature until the sensible capacity of the cooling coil matches the sensible load and the zone temperature matches the set point.

Designers typically (and appropriately) size cooling coils based on the peak sensible load, that is, when it is hottest outdoors. In many climates, however, the latent load on the cooling coil—and often the total load (sensible plus latent)—peaks when outdoor dew point, not dry bulb, is highest.

Consequently, in some air-handler arrangements, coils selected for the highest sensible load may not provide sufficient cooling capacity when the highest latent load occurs. More importantly, however, coils controlled to maintain the dry-bulb temperature in the space often operate without adequate latent capacity at part-load conditions. Here's why...

At peak sensible load, the cooling coil removes both sensible and latent heat, directly controlling zone temperature and indirectly affecting the relative humidity. (A colder coil increases the rate at which moisture condenses from the air.) At partial sensible load, however, the control system reduces the capacity of the cooling coil by allowing the coil temperature to rise. Although this action successfully maintains the zone dry bulb, it also slows the rate of condensation: relative humidity in the zone rises.

Sizing the cooling coil for the highest total load will not prevent this shortfall in latent capacity if system control is based solely on sensible conditions. Whenever a part-load condition exists, the thermostat throttles the coil, latent capacity drops, and zone relative humidity increases.

An Example
Let’s consider a 10,000-cubic-foot classroom in Jacksonville, Florida, that accommodates 30 people. The thermal comfort target is 74°F DB and 50%RH, with nine air changes per hour (9 ACH). To provide adequate ventilation, 450 cfm (15 cfm per person) of the 1,500 cfm of supply air must be introduced from outdoors.

Note: Some codes require a specific air-change rate for classrooms, so we assumed an airflow and calculated the supply-air temperature for this example. Alternatively (and perhaps more commonly), we could assume a supply-air temperature and calculate the supply airflow.

Basic system at sensible design and full load
Chapter 26 in the 1997 ASHRAE Handbook—Fundamentals indicates that Jacksonville’s outdoor dry bulb equals or exceeds 96°F during 0.4 percent (35 hours) of an average year; the coincident wet bulb averages 76°F.

At this design condition, the sensible and latent loads calculated for the space—29,750 Btu/h and 5,250 Btu/h, respectively—yield a sensible heat ratio of 0.85. Given the supply airflow of 1,500 cfm, a supply-air temperature of 55.7°F is required to meet the sensible load and cool the space to 74°F.

But does this supply-air condition achieve the target relative humidity of 50 percent? The psychrometric analysis summarized in Figure 1 illustrates the answer. Simply controlling the zone temperature to 74°F results in a comfortable 52.4%RH and requires 4.78 tons of cooling to satisfy both the sensible and latent loads on the coil.

Basic system at latent design and part load
The 1997 ASHRAE handbook also shows that, for 0.4 percent of the time, the outdoor dew point equals or exceeds 76°F while the coincident dry bulb averages 84°F. Let’s see what happens in the zone if the sensible load drops to 60 percent of sensible design (17,850 Btu/h) as a result of a lower outdoor-air temperature and the correspondingly lower solar and conducted heat gains.

If we also assume that the latent load due to occupants remains unchanged (5,250 Btu/h), the sensible heat ratio drops to 0.77. Now 1,500 cfm of supply air at 63°F satisfies the sensible load.

As Figure 1 illustrates, the warmer, moister supply air raises the relative humidity in the classroom from 52.4%RH to 66.9%RH—well above the 60%RH maximum that ASHRAE recommends. Although the coil could provide additional cooling (up to 4.78 tons, if sized for the sensible design load), the thermostat reduces coil capacity to 3.68 tons. This action maintains the dry-bulb temperature in the classroom at set point, but at the expense of the system’s ability to dehumidify.

Packaged Air Conditioning Compounds the Problem
A chilled water coil can be selected to deliver 4.78 tons of cooling at 1,500 cfm, a flow-to-capacity ratio of 314 cfm per ton. Most packaged air conditioners, however, must operate within a narrow range of flow-to-capacity ratios, usually between 350 and 450 cfm per ton.

The classroom in our example requires a five-ton unit that delivers no less than 1,750 cfm (350 cfm per ton). To assure adequate cooling capacity, the designer must accept an air-change rate of 10.5 ACH instead of the desired 9 ACH.

The higher-than-required supply airflow (1,750 cfm) increases the supply-air temperature to 58.3°F; see Figure 2. As the total coil load drops from 4.78 to 4.66 tons, the humidity in the classroom increases from 52.4%RH to 56.2%RH at sensible load.

Not surprisingly, the classroom becomes even more humid when the sensible load drops to 60 percent (the latent design condition). With the thermostat throttling the coil capacity to 3.62 tons, the 64.6°F supply air removes even less of the latent load and the relative humidity climbs to 68.7%RH.

Note: An overly conservative estimate of the sensible load in the zone also results in too much supply airflow, along with the attendant increase in relative humidity.

Continue on to Enhancing Indirect Dehumidification