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Following the guidance from this Q&A, I measured the output temperature of my 2012 Town and Country A/C. Under the test conditions proscribed, with outside temperature of 87°F and 55% relative humidity, the following observations were made:

  • When temperature set to maximum cold ("LO") and blower set to minimum speed ("1" bar), the output air flow is 42°F.
  • When temperature set to maximum cold ("LO") and blower set to maximum speed ("6" bars), the output air flow is 60°F.

I am not expecting a nearly 20°F rise in temperature with the blower going. This explains why the A/C feels "cool" to me under normal operating conditions (which is some where in the middle of my experimental range).

What's going on here?

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Welcome to the wonderful and wacky world of thermodynamics.

A volume of air has heat. For example a gallon of air needs a certain amount of cooling to drop from 87 degrees to 42 degrees.

An AC evaporator can absorb heat at a constant maximum rate. When the blower is on low and lets say flowing one gallon of air a second, the evaporator is able to drop its temperature from 87 to 42 degrees. Now you turn the blower to high and the flow rate goes up to six gallons a second. The cooling rate of the evaporator has not changed, it is still pulling in a fixed amount of heat. So when the evaporator could cool one gallon to 42 degrees, it now cools six gallons to 60 degrees in the same amount of time.

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    I knew this ... and yet I didn't know it, if you know what I mean. At any rate, I sure wish I had had this answer to read and had read it two cars ago ... – davidbak Jun 20 '16 at 17:11
  • @davidbak Here are my rules of thumb. 20ishF degrees drop compared to ambient. This is with; all windows rolled down, both front doors open, blower on high, AC on (duh) and recirculate on (that one is important). The "ish" is in there because it's up you to judge the situation. 87F down to 60F sounds perfect. If it's 120F outside then 105F would be respectable. – vini_i Jun 20 '16 at 18:04
  • @vini_i Windows down, doors open, recirculation on? What kind of setup is this, what should it be good for? - Just turn on the A/C, fan to lowest, increase rpm to 1500 to 2000 or so, and measure inside the vent. This should give something between 5 and 10°C if your A/C is ok. Due to the low airflow, that value is largely independent of outside temperatures, open doors, or things like that. – JimmyB Jun 21 '16 at 14:45
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    @JimmyB The idea is to bench mark the AC with the worst conditions possible. If it preforms then it will preform in any conditions. The bench mark comes from inducing a constant input temperature and measuring the output temperature drop. The worst conditions come from outside air at the maximum flow possible. Doors open windows open bring in outside air. Recirculate on ensures the maximum air flow because many cars bypass the heater core in recirculate. Blower to maximum brings in maximum air flow. This setup guarantees consistent results by limiting the variables. 2000 rpm not a bad idea. – vini_i Jun 21 '16 at 15:26
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    @JimmyB What i was taught in school is over 20F delta below 90F ambient. Above 90F ambient play it by ear, 15F delta is probably sufficient. vehicleservicepros.com/product/10100692/… I also have this tester which does roughly what i described but it appears that it has been discontinued. – vini_i Jun 22 '16 at 12:09

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