Most heaters use engine coolant heat to heat the air. That's why you can slow down overheating by switching the climate control to full heat... pulls heat out of the coolant and into the blower.

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As for AC, it's a bigger drag on smaller engines. Even today, a strong AC system can sap some 15-20 hp out of a small motor. In recent testing on one car, AC use dropped economy from around 22 km/l down to 16-18 km/l.