Which statement defines latent heat?

• A. The change in enthalpy associated with a change in humidity ratio, caused by the addition or removal of moisture (correct)
• B. The amount of heat required to raise the temperature of dry air
• C. The heat transfer due to a temperature difference across a surface
• D. The energy stored in a refrigerant as it changes phase

Answer: (A)

Latent heat in this context is the energy tied to changing the moisture content of air. When water vapor is added or removed from air, the amount of energy stored in the air changes without a necessary rise in temperature. This energy change is part of the total enthalpy of moist air and is described as the change in enthalpy associated with a change in humidity ratio due to moisture being added or removed. In other words, latent heat is the energy involved in evaporating or condensing moisture in the air, not the energy required to heat the air itself.

So the statement that defines latent heat is about the enthalpy change that accompanies a change in humidity ratio from moisture addition or removal. This distinguishes it from sensible heat, which is the energy that raises the air temperature; from general heat transfer due to a temperature difference across a surface; and from the energy involved in phase changes of a refrigerant, which, while latent, is not the latent heat concept being applied to moist air in HVAC psychrometrics.