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Old 12-13-2008, 02:08 PM   #1 (permalink)
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Does the Heater Waste Gas?

This is a discussion that I had with a coworker a while back. He has a mechanical engineering degree so he should be quite knowledgeable and have a fair bit of common sense and intuition.
The question is weather or not having the heater on burns more fuel. His answer was flat out "no", that it is waste heat and would be lost anyway.
My assessment was that during engine warm up, having the heater turned on would prolong the amount of time that the engine is below operating temperature and therefore increase fuel consumption. Once operating temperature is reached I agree with him that there is no loss in efficiency.
Does anyone here have any insight into this? Thanks.

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Old 12-13-2008, 02:38 PM   #2 (permalink)
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You are exactly right.

And its not only the cooler temperature that reduces efficiency, it takes more of a hit. When below a certain temperature, the car's computer often runs in warmup mode. Which defaults to a richer fuel mixture, and the torque converter does not lock up on an automatic transmission.

I am a mechanical engineer too, and some of my coworkers who have more seniority still dont know jack about cars. Its a different field in a lot of ways.

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Old 12-13-2008, 02:50 PM   #3 (permalink)
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The heat has to come from somewhere. I see about 8 degrees different in the water temperature when running the Heater. One of the reasons to run a grill block, beside aero, is to help warm up quicker and run hotter. The hotter the engine, to a point, the better the FE.

The Old beaters I drive when it starts to overheat I can turn the Heater on Full blast and the temp will drop.
"Judge a person by their questions rather than their answers."

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Old 12-15-2008, 03:15 PM   #4 (permalink)
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if I don't turn my heater fan down when at a stop, I can move my engine temp needle down a fair amount, I tend to keep the fan on the lowest setting or my car will never register on the temp gauge before I get to work, and if it's not warming up it's using more gas.
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Old 12-15-2008, 04:45 PM   #5 (permalink)
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I sure wouldn't call is waste, I like to stay warm, but during a long car ride my girlfriend asked me the same question, does it use more gas? I had some time to think about it... there are two other very small power consumers to consider:

A stretch, but the fan you use to get that heat into the cabin takes electrical power, requiring the alternator to work a little harder.

A real stretch, the waterpump works harder to push the coolant through another heat exchanger, increasing the surface area in which the coolant must flow over.

Again, these are both big stretches, and small consumers of ultimately slightly more fuel when you run your car's heater.
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Old 12-15-2008, 05:35 PM   #6 (permalink)
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I think your engineer friend was refering to when the car was already up to operating temperature. in that case, its very unlikely that the heat itself will cost you power, the blower fan will.
if you wait untill your car is up to temperature then turn on the heat you can have your heat and drive it too, so to speak. once the car is up to operating temps (NOT warmup, NOT extending warmup, already warmed up) then the extra heat IS just waste heat that would only heat the radiator.

btw the radiator is not ALWAYS flowing engine coolant, there is only flow when the coolant is above the preset temperature control (usually a wax that melts at a specific temperature) valve and allows the flow to go through the radiator, its a paralell line on the coolant circuit.

though I belive many cars use the engine oil to heat the car, or used to at least
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Old 12-15-2008, 06:04 PM   #7 (permalink)
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Thanks for the replies, you all bring up some interesting points. I know from my ScanGauge II that my engine consumes over 2.0gph at idle when cold and around 0.7gph when hot, so there is a significant difference.
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Old 12-15-2008, 07:03 PM   #8 (permalink)
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I'm sure it wastes fuel because now everyone idles their car for a half hour before driving anywhere.

But that behavior aside, and warmup time aside, it is just waste heat once the engine is up to temperature.
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Old 12-15-2008, 07:26 PM   #9 (permalink)
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yeah i leave my heater off till it gets to 170ish, stops at 182ish

but with the heater blowing at full, it does take alot longer to warm up. thus useing more fuel.

but once warm nah.

2 gph at start up wow.....
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Old 12-15-2008, 08:02 PM   #10 (permalink)
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Originally Posted by dichotomous View Post

though I belive many cars use the engine oil to heat the car, or used to at least
None that I've ever heard of.


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