Heat of combustion can be with a higher or lower heating value. Let's look at the difference between them.
Higher Heating Value
You can determine the high heating value (HHV) by bringing all end-products to the pre-combustion temperature. You need to concentrate on condensing any vapor produced in this process.
This higher heating value considers the latent heat of water vaporization in the products of combustion. This is helpful in calculating heating values of fuels where condensation of the reaction substances is useful.
Examples are gas-fired boilers making space heat.
HHV assumes that all the water components remain in the liquid state after combustion. It also presumes that heat generated below 302 degrees Fahrenheit is usable.
Lower Heating Value
Lower Heating Value (LHV) is the result when you subtract the heat of vaporization of the water from the HHV.
This method treats any water formed as vapor. This approach finds that energy required to vaporize water does not generate heat.
LHV assumes that water elements of the combustion process end up as vapor. This contrasts with the HHV, which assumes all water in the process of combustion is in a liquid state after the combustion process ends.
LHV is useful for comparing fuels for which the condensation of fuels isn't practical or where heat less than 302 degrees Fahrenheit isn't of use.
Which one should I use?
In summary, we measure HHV with the product of water in liquid form while we measure LHV with the product of water in vapor form. The difference between the two can cause confusion when references don't state the convention in use.
LHV might be useful for benchmarking purposes, but overall efficiency calculations should use HHV.
Always state the value as LHV or HHV to avoid confusion.