Volkswagon proved a diesel engine can have a heating system far more efficient than a gas car. Some examples of the jetta from 85 to about 90 will actually burn your skin off if you put you hand over one of the heat ducts before you have driven far.

My understanding was the head coolant is the only coolant circulated through the heater core on the early ones. . Quite a few people asked me if the car had an auxillery gas heater when they were in them from a cold start

. Volkswagon dealers explained the tdi engine was too efficient to generate much heat. That if true was an understatement. I suspect they got the design of their heater system wrong. The old jeta got about 50 mpg up here and the tdi about 60 mpg in Canada. Thats only a 20 percent spread in energy consumption. It alone could not count for at least an 80 percent reduction in heater performance in my opinion.