In most cases, the reaction products are found in the gaseous state at the end of combustion, but it is possible at low temperature, that some of them are liquid or even solidify, releasing a heat of condensation or solidification.

The problem arises especially during the combustion of hydrocarbons, water appearing among the products. The maximum energy release is obtained when the water contained in the fume is liquefied.

The complete value of the heat of reaction is named Higher Heating Value or HHV.

Where all the water produced remains in the vapor state, it is given the name of Lower Heating Value or LHV.

If only a fraction of the water condenses, the heat of reaction is an intermediate value between the HHV and LHV.