Q: Does using heat burn extra fuel?

Hey there. The heater in the car only requires the use of the heater blower to blow hot air out of the heater case. The heat is generated by blowing hot air across the heater core inside the A/C blower heater case.

The only way you would use more fuel is if you use the defroster in the vehicle. The defroster will turn on the A/C compressor so the defroster can direct airflow across the A/C evaporator and heater core so you will have warm dry air blowing onto the inside of the windshield.

If the A/C is not working, then you may get a moisture buildup inside the windshield. The running of the defroster will run the A/C and you will notice the drop in fuel mileage if you leave it on all the time. Once you clear your window then change the control to normal heat out of floor or front vents and the A/C compressor will turn off and an increase in fuel mileage will be seen.