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During the winter months I often have to drive in a cold car and I want to warm up as fast as possible. What is the best setting to put the air on in order to warm up the entire cabin in the least amount of time?

To simplify, let's start with a few assumptions:

  1. Assume a typical small sedan (four door, four/five passenger).
  2. Assume the engine is already warm.
  3. Assume you have standard vent settings. "Defrost" is directed up from the dash. "Face" is forward and directed toward the driver's face. "Feet" is directed down.
  4. Assume each setting blows the same amount and same temperature of air into the cabin.
  5. You may also have a combination of "Feet" and "Defrost" or "Face" and "Feet". Assume this setting splits the air flow 50/50 between the two directions.

My guess is there is some kind of thermal effect caused by blowing the air that will make one setting better than the rest. If such an effect will occur, is there a critical temperature for it to appear? Does it matter if the external temperature is $0^{o} C$ or $-20^{o}C$?

As an additional clarification, I am not looking for just making the driver feel warmer (I'd assume that would be the "face" setting since it blows on the driver's face/torso so will make them feel warmer faster. Feel free to correct me if I'm mistaken). I am curious was to what will warm the entire cabin fastest.

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  • $\begingroup$ Since each of the settings pushes the same amount of heat into the cabin, then any of the settings would be identical. However, since it is important to see out the windshield, you will either need to use the defrost or the defrost/feet setting. $\endgroup$ – LDC3 Dec 31 '14 at 4:42
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    $\begingroup$ I disagree, I think the defrost will warm up the glass faster and the air inside the cabin slower. Face, which is the closest to the center of the car as possible, is the best option. $\endgroup$ – Wolphram jonny Dec 31 '14 at 4:48
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    $\begingroup$ Feet will allow heat to rise. To get the most heating done in the shortest time, you will want to evenly spread the energy across the entire cabin, or to focus it in the center of the cabin to maximize diffusion of the heat to other parts of the cabin and reduce diffusion out of the car. So, cabin + feet is probably more efficient. I think there is no well defined answer without some very specific details about the car morphology. $\endgroup$ – ZSG Dec 31 '14 at 4:51
  • $\begingroup$ @LDC3 Wouldn't defrost cause you to lose more heat to the environment outside the car? $\endgroup$ – jkeuhlen Dec 31 '14 at 4:58
  • $\begingroup$ That might be, but at least the windshield doesn't fog up every time you breath on it. It also makes clearing the ice off easier. $\endgroup$ – LDC3 Dec 31 '14 at 6:27
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Depends how far you are willing to modify your car. At a simple level, most cars have a "recirculate" switch that draws air from the cabin into the heat exchanger as opposed to heating fresh outside air.

If you want to open the hood, you could blanket the radiator, thus causing the engine coolant to remain rather hotter than planned, which will increase the heat dumped into the cabin air heat exchanger. Similarly, you could muck with the coolant pump's thermostats to force the engine to run hotter. Not recommended if you want your engine to live a long happy life.

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  • $\begingroup$ While I suppose this is a plausible solution I don't really think it answers the question as written $\endgroup$ – jkeuhlen Dec 31 '14 at 15:12
  • $\begingroup$ The key in this answer is the recirculate setting. This will cause the air to increase much faster than using the cold outside make-up air. Feet or feet and face are the most effective to heat and distribute the air, the warm air will rise from the bottom most effectively. $\endgroup$ – KalleMP Nov 11 '16 at 8:45
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I have found through experimentation that running 100% of the heated air through the "feet" ducts warms the cabin fastest, and turns out to be the most comfortable until the entire cabin is heated. The effect is the same for 0 - 20 C. After the cabin is heated, you will need to reduce the air flow to the feet, or they will become too hot.

Some cars have embedded electrical defrosters in the windshield and rear window, which should be turned on immediately.

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