I know when you turn on the AC it is extra load on the engine

a friend of me told me, it is like you having 3 passengers in your car.

but on the other side the heater does not create load on the engine as it is just get the heat of the engine itself

my question is when I turn the AC with the heater, in order to avoid fogging the windshield.

does that create load on the engine?

  • Yes, running the blower but that is relatively small.
    – Solar Mike
    Jun 13, 2019 at 5:21

3 Answers 3


This depends on the car, however in general.

If you use the heat alone to fog the windows: Yes but only slight.

You will create a slight load on the engine due to increased electrical demand from the blower fans on the Alternator ~probably sub sub 100W and likely un-noticable.

If you use the heat with A/C activated: Yes but how much depends on the car design.

Depending on the Car and A/C Design, some may not activate the A/C and would be the same as above.

However as described by @sweber most will run the air first through the A/C to help remove moisture from the air, then through the heater to help evaporate the fog from the glass. In this case, it because the A/C is in use, there will be an increased load on the engine by the A/C Compressor.

This can be quite significant (up to 10HP+ in some cars).

So yes in your case, you will likely see an increase in parasitic load on the engine, causing slight fuel economy decrease and slight loss of power.

  • 1
    Heater will increase the compressor cooling load only when hot air is being recirculated through the evaporator. Otherwise, evaporator will get cold as usual (or will even get warmer if the compressor has an electronic control valve that controls its displacement, and the climate control system/ECU is able to reduce compressor displacement according to the heater request, in order to provide enough cooling so that moisture condensation happens on the evaporator fins and nothing more).
    – Al_
    Jun 13, 2019 at 15:41

The heater alone does not put load on the motor, except a few watts for the blower. But the AC (AirCondition can be misunderstood today, I mean Air Cooler...) draws several horsepowers.

For defogging, the air is first cooled as much as possible, and then heated. The cooling removes humidity from the air, the heating makes it very dry after that. Thus, it takes more easily water from the wind shield.

And since air is cooled for defogging, this also draws several horsepowers.


On the defogger setting, your HVAC system typically engages the AC. When you turn on the AC it cools the air, which lowers its moisture capacity. Dryer air pulls moisture from the windows. If you turn on the AC (not defog) in the winter, and turn your blend setting to all the way hot, you'll notice that with AC on, the air coming out of the vents is a bit cooler than with the AC off. That's because the HVAC is cooling the air first, then the (now much dryer) air is being heated by the heater core.

The load on the car is created by the AC compressor. When you use the AC it engages the compressor pulley clutch, and drives the compressor. With the AC off, the compressor wheel basically free-wheels, acting basically like a dumb pulley.

So when you activate AC or defog, you're engaging the compressor, adding load to the engine. If you just use the vent system on hot, without AC or defog, you aren't adding load to the engine.

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