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Air Conditioner in Vehicle

My husband and I disagree on the cooling function of our Expedition’s a/c. It has always been my understanding that if you wanted the temperature to be colder, you simply reduced the digital temp setting (say from 72 degrees to 68 degrees) and colder air will result. My huband believes you should merely adjust the fan setting to a higher speed (while remaining at 72 degrees) to reduce the temperature. I need an arbitrator. Thanks

Most of us here don’t like to act as marriage counselors since half of the couple posing the question is going to be mad at someone.
The end result here would be six of one, half dozen of the other.

JMHO, but I favor lowering the temperature instead of raising the fan speed. The reason for this is simply to hold the wear and tear down on the cabin fan motor.
With some Fords (do not know if the Expedition is one of them) the fan motor current (amount of electricity used) is routed through the ignition switch. Over time, and especially with a worn blower motor, it’s possible the ignition switch could fail due to long term use of a worn and dragging blower motor, which uses more current than a good low-use blower.

The point here is why put unnecessary use on the blower motor which could then burn up the ignition switch over time and lead to the vehicle quitting. In most cases this will not occur but it can and has happened.

An addendum here; I should have added that the faster the fan motor is running the more current is being consumed by the motor which means more current being pulled through the ign. switch.

Some systems will use the blend door(heat) to achieve a higher temperature using AC. Basically the coldest setting digitally means the heat door is off while the hottest setting means the AC compressor is off.

I had it explained to me in a AC course (maybe those of you better at physics can offer opinions) Here it is the slower the air travels across the evaporator the colder it will become.The idea is that the slower moving air will have more time to give up its heat. Point lower fans speeds will deliver colder air. am I full of hot air?

You seem to have climate controls that work like a household thermostat. If it is set at 72 degrees and the temperature is above 72 degrees, there is no need to adjust the temperature setting. Increasing fan speed will allow the temperature to reach 72 faster. If the interior temperature is already below 72 degrees, increasing fan speed will not lower the interior temperature.

If the car’s interior temperature is 80 degrees, setting the climate controls at 68 won’t make the air coming out of the air conditioner any colder. Leave the climate controls at 72 until the temperature reaches 72 degrees. If you are still hot after the interior temp is 72, that is the time to set it lower.

You are correct, ideally the supply air temperature will be cooled down to the surface temperature of the evaporator, but that process takes some amount of time. As the air velocity increases the air spends less time in contact with the evaporator and my not reach it’s minimum temperature. However, the higher air velocity will increase the total heat transfer so you will be removing more total heat while supplying slightly higher temperature air. For example, of low speed you may supply 10 SCFM of 40F air to the car, but on high speed you may supply 20 SCFM of 45F air to the car (giving you more total cooling).

I once believed the same thing. However, using a meat-style thermometer in the air conditioning vent and an interior/exterior dash-mounted thermometer, I have determined that the air coming out of the vents is the same temperature regardless of the fan setting. Also, on a hot sunny day, a higher fan speed will bring the interior temperature down faster than a low fan speed.

I encourage you and the OP to get a couple thermometers and a stopwatch to test for yourself. You can find an air conditioner thermometer at an auto parts store and you can find an automotive interior/exterior thermometer at Wal-Mart or a truck stop. I am sure not every car’s climate controls behave the same way.

I would let the Ford engineers make the decision; use the temperature setting. While I don’t have a Ford, my Honda does have a temperature setting. The computer controller decides how to run the fan and the ventilation system to cool the car. When it’s really hot, the fan is maxed and the air recirculates. As the inside temperature nears the set temperature, the fan slows and the outside air vent opens. Whether it cools the cabin faster or not isn’t as important as how fast I cool off. That occurs quickest when the fan blows fast and directed at my torso.

Well you are both right and wrong.

First you need to define [i] if you wanted the temperature to be colder. [/I] Do you want the interior of the car colder or do you want the air coming out of the vents colder.

While there are different systems on some cars, I am going to work with the most common. When you turn the A/C on two things happen. The A/C turns on and starts cooling a radiator type think under the dash. It also blows air across that which is cooled as it passes by and then blows into the car.

When you first turn it on, chances are the car is already hot as are all the ducts moving the air so even the air that has been cooled will be partly re-heated by the time it gets into the cabin of the car. It will not feel as cool as you might like. After a few minutes after it has cooled down those vents it will fell cooler and do a better job of cooling down the car.

So let’s look at the controls you have and what they do.

Likely you have recycel switch. When on it pulls mostly air from inside the car cools it and returns it to the cabin. This generally cools the car the fastest as the outside air that it would otherwise be cooling will always be as hot or hotter than what is in the cabin.*

The next is the fan speed. Faster will always mean more cooling, but unless it is on recycle it also means bringing in hot air from the outside which has to be cooled so it may not cool down as fast on high as on medium or low.

Finally is that digital setting. It just turns the A/C on and off (some models also change the fan speed) to keep it at the temperature you set. Turning it to a lower temperature will not make it cool faster, only make it keep cooling until it is cooler than you may like.

Now you may have noticed that all that is a little confusing. You also may have noticed that it does not make much difference which way the settings are set, you get the same results. That’s because all the above and everything you and your husband are thinking are right, but they just don’t make much difference in real life. my suggestion is to make him feel good and let him set it anyway he likes. You can continue to know it is not important, but you did something to make him happy and it cost you nothing. Maybe he will return the favor some day.

  • There may be a few minutes that the reverse is true when you fist start up the car, and I suggest returning to recycle-OFF after the cabin temperature is comfortable to get the fresh air.

Thank you, Mr. Meehan, for your sage advice.

It depends on the design of the evaporator, if the evaporator has enough surface area to cool the air down to the evaporator surface temperature at maximum air flow the supply temperature will always be the same (usually around 40F). If the evaporator does not has sufficient area, the air will not reach it’s minimum temperature at the highest air flow. In my old cars (pretty marginal AC by current standards), I can feel the temperature difference between fan speeds; the supply air is noticeably cooler at lower fan speeds, but the total amount of heat removed will always be greater at higher fan speeds.

Lower fan speed biases AC to greater water removal and less heat removal. High speed biases to more heat removal and less water removal.

Someday a vehicle’s computer might be robust enough and sensors complete enough to determine which fan speed is best and whether inside, recycled air or outside fresh air is going to make the interior comfortable in the shortest time. Wouldn’t that take all the fun out of it? I can safely say that in dry Tucson (where I used to live) it was always best to use outside air and a fast fan speed to get the temp down. In New Orleans, it generally takes more energy to dehumidify the air than to cool it so I rarely go out of MAX (recirculate) when I am alone in the car. If I was at capacity in my sedan with six people transpiring, it might be a different story.

its funny. im just now recovering from the verbal assault my wife gave me last week on a trip to FLA.

she is of the turn the temp knob all the way to the coldest setting. my feet were getting frozen from the air on the floor vents, and we were sweating on our heads.

so i voted for higher fan setting, temp at 70 to circulate more cool air (the temp we really wanted in the car… NOT an igloo) any way i lost the argument. so i can vent to you, and share the exact same dilemma.

as others have said, marital harmony was more important than argument, so i let the boss have the controls.

no end to the discussion, huh?

Actually, the climate control in my '82 does a pretty good job. I set the temperature to about 22C and leave the fan in “auto” mode about 99% of the time. It uses high fan speed to maximize the initial cool-down and then drops down to a lower speed to maintain temperature/humidity. I’ve used it from about -10F to about 110F without having to play with the controls. This really isn’t that hard.

Lower fan speed biases AC to greater water removal and less heat removal. High speed biases to more heat removal and less water removal.

True, assuming the evaporator is sized as I described above.

I have a somewhat related question: Is it better to use recyled air or outside air with the A/C unit? I always set my fan to “recyled air” to get better gas milegae, but my coworker says using fresh air is better for your car–doesn’t damage the compressor. ??