Cooling down a car--thermostat temperature setting changes rate of cooling

Today 9/17 you said that it made no difference in cooling rate whether an A/C was set on the lowest setting or just set to the desired temperature.

That would be true if the thermostat were perfect, circulation complete, and the car an empty, insulated sphere.

Instead, the solid materials in the car are hot and have a much higher heat capacity than the air. The A/C injects cold air and the thermostat measures the air (not solid materials) temperature in one spot only. If you set the thermostat to 70F, it stops cooling when the air near the thermostat is 70F and resumes after the air heats again (usually several degrees). Poor circulation to the rear of the car complicates this even more.

If you turn the thermostat all the way down, it runs continuously and sucks the heat out at a higher rate than if it were cycling on and off. The A/C is a heat pump and clearly the heat is being removed faster from the interior of the car if the A/C is running continuously on maximum.

The other way to think of it is that the solid materials cool faster if the surrounding air is 60F than if it is 70F. (Of course, the comfort factor is different with a cold wind in your face while sitting on a blazing hot seat).

Of course, the trick is to reset the thermostat once the solid materials in the car approach the desired temperature so you don’t have 60F air blowing in your face any longer than necessary.

Thanks for a wonderful show!
Rich Howard (PhD Applied Physics)

In your discussion about cooling the car down quickly, no one mentioned that if you crack the windows (that is, open them a little bit, for literal thinkers) for a minute to let the hot air out, the car will cool much more quickly. Low tech solution. Love you guys!

You said the A/C is not proportional, it’s only on or off. But that ignores any ability to mix warmer air with the colder air. The old style was proportional for heat level, a lever that let more or less hot water. On my wife’s car, it appears to do that. When setting for 72 degrees and the air is only 76, the A/C is on (vs. off), but the system is adding some warmer air to that A/C output. So even though the A/C is not proportional, the air output from the vent is proportional.

cracking the windows open seems a bit conservative to me- I open my sunroof if its not raining and the hot air floats away. I can’t imagine I am working against the A/C too much as the car cools quickly and the A/C backs off. With the windows now closed, I recirculate the air in the car. Usually, even if the car has been sitting in the sun, it takes only a minute or less to get rid of the hot air. If only it was that easy to accomplish that with people!

Everyone in town has a different idea about this. About 90% don’t have a clue what they are talkin’ about.

Rehoward, I would argue that the air being passed through the AC coils will continue to be at a substantially higher temperature than the coil surfaces, meaning that heat will be continuously transferred from the air to the coils, whether the setting is at 70 degrees or at the lowest setting. I cannot see any reason whatsoever why it would not be. Since the materials in the car retain heat better than the air, heat energy in the materials of the vehicle will continue to be transferred continously to the car’s air, it being fed through the AC system, under both of the identified conditions. The air in the interior will not drop below the “set” temperature any faster with the knob set to its lowest temperature than with it set to 70 degrees. The setting has no effect on the transfer rate of the naterials, and no effect on the time to cool.

Opening the windows a bit to start does help. The interior of a vehicle sitting in the sun gets much hotter than the outside air, and letting some of that hotter air out alleviates some of the heat energy that the AC system must dissipate.

EllyEllis, would you care to enlighten us all?

"I would argue that the air being passed through the AC coils will continue to be at a substantially higher temperature than the coil surfaces, meaning that heat will be continuously transferred from the air to the coils, "

Which is where the “max” setting comes in. Letting the cooler air inside the car recirculate over the coils rather than the outside uncooled air, thereby cooling the air quicker.
I agree that it will not cool any faster just by turning down the thermostat.

The caller said something at the beginning of the call that caught my attention. He said something like I know she is wrong, but I feel like she’s right.

The AC is either on or off, that is true. But, we feel the exchange of heat with our environment. That determines how cool or warm we feel. We exchange heat with our environment via conduction, convection and radiation. The AC directly affects the convection. It has an affect on conduction and radiation, but to our bodies, it is a secondary affect.

If the AC is set at 70F, then when the sensor, where ever it is located senses that the inside air temperature is 70F, then it either cycles the compressor or adds warm air via the heater to maintain the 70F. But the materials in the cabin that our bodies are in contact with may still be significantly warmer and is keeping us uncomfortably warm through conduction. The materials that we are not in contact with us will still be radiating excess heat on us. The result is that we are still uncomfortably warm.

Eventually, everything in the cabin should get close to the 70F because of the convection from the AC. But if the AC is set to a lower temperature, the air will be cooler, offsetting the heat gain from the materials in the car as well as cooling down those materials faster. In other words. we will feel comfortable faster with the lower setting, but the AC does not work any faster by setting it to a lower temperature.

I guess the bottom line of my argument is that the air being fed into the AC coils won’t reach the 70 degree level as long as it’s continuing to absorb heat from the car’s interior while passing through. Once the materials in the cabin reach 70 degrees, then the air also can return to the coils at 70 degrees.

I think it is important to remember here that the question is about whether or not the car cools down faster if you set the temperature lower, not whether or not it will get colder.

I’m not sure how the climate control systems in other cars work, but I have a 1999 VW Passat that has a “Climatronic” system that controls both the fan speed and the compressor. When it senses large difference between the desired temperature and the current temperature, it turns the circulation fans on a faster speed and so the car cools off faster and then it turns the speed of the fans down once the desired temperature comes closer.

This means that as long as there is a sufficient difference between the desired temperature and the current temperature, the car will cool down quickly. If there isn’t much difference between the desired temperature and the current temperature, then it would speed up the process by turning down the thermostat. This, of course, depends on the car you are driving as I am sure that different companies have different proprietary systems.

Well, A/C systems do not all work the same way but in most cases I would agree with “The Same Mountainbike”.

It’s true, AC systems don’t all operate the same. Mine is simply off or on, regulated by the setting on the dial, which has no temperature indication but rather just a “setting”. Mine is sort of the “base” system. Others have temperature settings, although most probably operate the same as mine but using a set temperature rather than a set dial. Apparently, based on Russionrick’s post and other posts, some moderate other parts of the system such as the fan speed.

However, I would suspect that all stay on their “high” setting as long as the air in the cabin is continuing to absorb heat from the cabin materials, as long as the materials are above the reference used by the system, whether that be a temperature setting or a simple dial.

I still agree with you

i agree, recent a/c system like my car work not by on off but my gradually using more freon depending on the temperature setting, i have found that the air is cooler if i put it on max then if i pick my temperature, therefore it cools down faster since the air is cooler