An air conditioner's compressor causes the engine to run at higher rpm. But the same may not be true in heating. In fact, I think it is merely the engine's exhaust heat that is diverted to heat the air inside the cabin. Is that true?

 

no the engine coolant is run through the heater core (small radiator) and the fan forses air through it for the heat. when i turn on the defrost the a/c pump comes on on mine so mpg is affected when the defrost is on.

 

Only those who want to commit suicide will route exhaust into the cabin. For most people that I know, I don't recommend that
Seriously though, the old air cooled VW's, Corvairs, and Porsche's routed radiant exhaust heat into the cabin (they used special heater box/head pipes. The heating system in them kinda sucked, at best. Water cooled cars use a heater core. The heating system isn't going to make the engine consume more fuel, unless you have defrost on which does run the A/C along with the heat (as craby said) to dehumidify the cabin.