To me, Jim L is describing the 'heat transfer medium' and not the 'energy source'.
The 'energy source' would be the source of energy which runs the system.
For a gas furnace, with the old octopus gravity feed systems, the energy source would be 'gas', but as soon as you introduce a blower, then it would (technically) be gas/electric.
Or, just to include the entire scope of the 'energy source' for heat transfer medium and operating power, the energy source would be the 'Sun'.
