Enthalpy describes what?

• A. The rate of heat transfer
• B. The amount of heat a substance contains from some starting point
• C. The total energy in a system
• D. The amount of work a system can perform

Answer: (B)

Enthalpy is the total heat content of a substance at a given state, especially at constant pressure. In HVAC, it combines sensible heat (temperature) and latent heat (moisture) for moist air, so it directly represents the energy carried by the air as it is heated or cooled. The amount of energy transferred as heat per unit mass when moving from one state to another at constant pressure equals the change in enthalpy, which is why enthalpy is used to analyze energy balance in air handling and ducted systems. It’s not simply the rate of heat transfer, nor the total energy of a system including all forms of energy, and it isn’t the amount of work a system can perform (that relates to exergy or available work).