The combustion fuel half of the system could be propane, natural gas, oil, or even coal and wood. In comparison with a combustion fuel-fired furnace or standard heat pump alone, this type of system is also economical. Actual energy savings depend on the relative costs of the combustion fuel relative to electricity.bsorption heat pumps use heat as their energy source, and can be driven with a wide variety of heat sources. For more information on these specific types of heat pumps, go to: 1  Air-Source Heat Pumps. 2  Ductless Mini-Split Heat Pumps. 3  Geothermal Heat Pumps.