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Using Heat Burn up Gas Like A/C..??

10-07-2004, 07:38 PM
I believe i'm right on this one.. A buddy of mine said that using heat in your car burns up gas like the ac does-not as much, but it does..??

Is that true.? I told him he was full of sh**..

10-07-2004, 08:08 PM
The heat uses no extra gas. It has basically a small radiator under the dash and the fan blows air through to heat up the car.
The Defroster on most new cars uses the a/c to dehumidify and that uses extra gas. So maybe that is what he is talking about.

10-08-2004, 12:18 AM
In a lot of older cars, the heat is a seperate system, and has no effect.
It does act as a radiator, as mentioned by public, and could actually improve mileage ever so slightly by extracting more hot air from the engine cooling system.

New cars, unfortunately, seem to have integrated the heater into the a/c system, and yes, the a/c drain is apparent when you turn on the heat. Along with the resultant slight loss in mpg.

Here is a trick that works on some cars: (GM, specifically pontiac GP, and Buick LeSabre, and possibly most others)
turn the heat all the way to max, let it run for a minute, and turn the a/c off. The air passageways will stay set to the hot path, and you will get some residual hot air through the vents.

This works even better in the opposite direction. Turn the a/c to full cold, for a minute, and shut off. the heat pathways are blocked, and you get no residual heat through the vents. Great for mild summertime.

The amount of air moving through is not a lot, but the heat is noticable in the summer, when you leave the a/c at an intermediate setting. Having the windows open seems to accentuate this.

10-08-2004, 11:53 AM
The heat in a car is just a heat exchanger using coolant as the heater, it doesn't take any power from the engine. The AC run a compressor which takes up to around 10 hp from the engine, this results in increased fuel consumption.

NOTE that the AC will only consume power while the AC compressor is running, modern cars typically have variable displacement AC compressors which can adjust the cooling after what is needed.

Both the AC system and the heat system has one heat exchanger each and they are typically called vaporiser (AC) and heater. The system works quite simply. In the vaporiser the fluid in the AC system is vaporised which cools it down to a temperature around 0 degC, the fluid flow controls the cooling effect. In the heater the coolant flow is controlled. Then there is a fan with motor which speed can be adjusted and typically some flaps which can adjust how the air will flow.

As the vaporiser also can condense water in the intake air it can remove water vapor from the intake air. This means that the AC can be used not only to remove heat but also to reduce the possiblity of misted over glass. There is usually a button which can shut off the AC compressor so it won't take any power.

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