Calorific value or heating value is the amount of heat released during complete combustion of a specified amount of a substance.
Higher heating value (HHV) – It is assumed that all of the water in a combustion process is in a liquid state after the process.
Lower heating value (LHV) – Is determined by subtracting the heat of vaporization of the water vapor from the higher heating value. This treats any H2O formed as a vapor. The energy required to vaporize the water therefore is not assumed to be released as heat.