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Thread: Running heater consumes more gas?

  1. #1
    Ridin Dirty dom's Avatar
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    Question Running heater consumes more gas?

    Does running your heater affect gas mileage?

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    Generally, no; you're just taking advantage of heat that would otherwise be rejected to the outside by the radiator. Maybe running the associated electric fan would affect consumption a bit, but it would be hard to measure.

    There are/were exceptions. Early Corvairs, some Volkswagens, and a few other cars had optional gasoline- fired auxiliary furnaces, that greatly affected fuel consumption, but if the alternative is being cold, you wouldn't care. I think similar furnaces are still available, and useful in instances when you wouldn't want to run the propulsion engine just for heat.

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    Vulture of The Western World Eric's Avatar
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    Quote Originally Posted by MikeHalloran View Post
    Generally, no; you're just taking advantage of heat that would otherwise be rejected to the outside by the radiator. Maybe running the associated electric fan would affect consumption a bit, but it would be hard to measure.

    There are/were exceptions. Early Corvairs, some Volkswagens, and a few other cars had optional gasoline- fired auxiliary furnaces, that greatly affected fuel consumption, but if the alternative is being cold, you wouldn't care. I think similar furnaces are still available, and useful in instances when you wouldn't want to run the propulsion engine just for heat.
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    I owned an old ('73) VW with the gas heater. Ha! Who needed that when you had the carbon monoxide-laden warmth coming from the leaky tinwork?

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    Senior Member grouch's Avatar
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    If you're just using the heat, it doesn't make enough difference to matter. Your fuel economy usually goes down in the winter anyway because of Uncle Sam. The EPA requires fuel to be more volatile in the winter so cars warm up faster and don't need to idle as long. The fact that nearly every car built in the last 20 years is fuel injected makes no difference. I rarely idle my cars to warm them up. I just drive them and try not to race the engine. By the time I scrape the windows, they're warm enough to drive and in a couple of miles, I'm getting good heat.


    However, if you use your defroster, that's different. Fuel consumption does go up with the defrost on since it uses your a/c system to dry the air hitting the windshield. Personally, I'd rather burn a little extra fuel than plow into that tree that jumps out in the road.
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    Senior Member DonTom's Avatar
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    Quote Originally Posted by grouch View Post
    However, if you use your defroster, that's different. Fuel consumption does go up with the defrost on since it uses your a/c system to dry the air hitting the windshield. Personally, I'd rather burn a little extra fuel than plow into that tree that jumps out in the road.
    But if you don't have an A/C, then you can use your defroster with no extra fuel consumption because then there is no A/C compressor to turn on.

    But if the car has no A/C, is there really a noticeable difference when the defroster is on? Do they attempt to dry the air at all?

    -Don- SSF, CA

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    I run the ac in winter sometimes to help clear the windows. It's maddening trying to find the right combination of not too hot or cold and keeping my windows clear. I have noticed a slight decrease in mpg but at least no accidents as of yet.

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    Vulture of The Western World Eric's Avatar
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    Quote Originally Posted by Adam View Post
    I run the ac in winter sometimes to help clear the windows. It's maddening trying to find the right combination of not too hot or cold and keeping my windows clear. I have noticed a slight decrease in mpg but at least no accidents as of yet.
    That's because the AC compressor is is cycling in "defrost" mode, which draws some power, which affects your gas mileage some.

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    Senior Member grouch's Avatar
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    Quote Originally Posted by DonTom View Post
    But if you don't have an A/C, then you can use your defroster with no extra fuel consumption because then there is no A/C compressor to turn on.

    But if the car has no A/C, is there really a noticeable difference when the defroster is on? Do they attempt to dry the air at all?

    -Don- SSF, CA

    Now you're getting into relative humidity and thermodynamics. The warmer the air is, the more moisture it can hold. That's why it rains when the temperature drops to the dew point.

    When you have the windows closed, the moisture from your breath raises the interior dew point. Since the glass is usually the same or close to the outside temperature, it will have moisture condense on it from yuor breath. You can lessen this by cracking a window open a bit. When you run the defrost setting on a car without a/c, the defroster is raising the air temperature at the glass. Since it is warmer, it's relative humidity is dropping as it can hold more moisture. It will absorb the moisture from the glass. If you have a/c, it will be more effective since not only are you raising the temperature to increase it's moisture capacity and the a/c coils are removing moisture from it by condensation. That's why a/c equiped cars clear windows faster.
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