In some sources, heating value may be expressed in BTU or kcal/mass (not volume) (http://www.engineeringtoolbox.com/gross-net-heating-values-d_420.html). It makes a big difference, as the same volume of gases of different molecular weights weigh in proportion to the molecular weights. Thus, the heating value of methane and propane are similar when compared on the basis of mass/weight, but are greatly different when compared on the basis of equal volumes (propane being much more). It is important to understand the units being used.