Q: How does “latent heat” (used by installers of air-conditioning systems) differ from sensible heat?
A: Clayton DeKorne, editor of JLC, responds: Air conditioners are designed to cool the air, but air temperature is influenced by humidity, and that needs to be part of the evaluation when you are sizing an air-conditioning system. HVAC designers need to track two types of loads: sensible load and latent load.
The sensible cooling load refers to the air temperature in the building. Factors that influence the sensible load include sunlight striking windows, skylights, and glass doors; the insulation value in exterior walls, in ceilings under attics, and in floors over open crawlspaces; air infiltration through cracks in the building; and (primarily in commercial work) the heat output of lights, appliances, and other equipment. All of these factors are included in the energy load calculation.
Latent cooling load refers to the energy carried by humid air. Water vapor holds heat and requires energy to remove it.
Conduction, convection, and radiation are the heat transfer mechanisms of sensible heat (see “Heat Transfer Through Buildings,” Training the Trades, Jul/19), while evaporation and condensation are the transfer mechanisms for latent heat. When water evaporates or condenses, it changes phase. In evaporation, latent heat is the energy required to overcome the molecular forces of attraction between water particles. This energy allows the molecules to separate; the liquid becomes a vapor where the attractions between molecules are minimal. When water changes from a liquid to a vapor (evaporation), or from a vapor to a liquid (condensation), there is no change in temperature between the liquid state and the vapor state. But while there is no sensible change in temperature, the air gains latent heat by gaining moisture (evaporation), or loses latent heat by losing moisture (condensation).
HVAC system designers must account for the latent heat carried in the volumes of air in a home, because it directly impacts the amount of heat that the system must add or subtract to maintain comfort. From a system designer’s point of view, latent heat can be removed by condensation (which occurs when an AC unit runs and humid air passes over the cold evaporator coil). The longer an AC unit runs, the more water condenses out of the air. Difficulty arises during shoulder seasons in a hot, humid climate, for example, when the air temperature is not high, but humidity is. Then, the AC unit doesn’t need to run very long to reduce the sensible heat, but it does need to run to remove the latent heat. The result: The system overcools the air. This becomes even more of a problem when codes require continuous ventilation (and hence, a continuous stream of humid air entering the house). In both situations, dedicated dehumidification (which pulls water out of the air by condensation but does not disperse cool air throughout the space) is needed to maintain comfort.