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2015 Forester Premium CVT
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Discussion Starter · #1 ·
Recently, I noticed an odd behavior with the cabin air.

On a hot day, if I keep the air directed towards the windshield with air temperature turned all the way to cool with AC OFF, I get cool air in the cabin. (Settings in the picture) If I change the air direction to anything else, then the warm air (almost same temp as the outside temp) will come into the cabin.

Anyone has any idea why such a behavior?

555456
 

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2014 2.5i Limited CVT
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It's not really odd behavior. The defroster (bottom of the windshield) uses the AC compressor to help dry out the air. So you're pulling in air and the AC system is cooling the air (and drying it out). The other positions (except foot and defrost) do not kick in the AC compressor and you get the outside air (at whatever temp) and it's not cooled.
 

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2012 Forester X Auto
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The setting you selected on the left above is defrost.
Defrost turns on the AC compressor to dry out the air and helps to eliminate windscreen fogging.
It will turn on with the AC set to off .
Once you change that, the air warms up because AC is off.
 

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2017 XT Limited + Tech
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395 Posts
The AC automatically comes on with defrost switch selected as others said above. If you then toggle the AC button to on, then off, the AC will turn off. I do this often during the winter when I don't need the AC with the defrost for fuel economy.
 

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Discussion Starter · #6 ·
Thank you, folks. It didn't occur to me that it is a defrost mode.
 

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2017 Forester XT Touring EyeSight
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the dehumidification effect of running a/c is a dramatic improvement when you're driving around in rainy weather, more and more so the higher the temperature is above 40deg F. As a boomer PNW sea-level resident, I owned and drove non-a/c cars for years and more than 100,000 miles, fighting against foggy windows and clammy interiors all fall-winter-spring, only rarely below freezing. Then I owned several VW and Honda products with completely manual air conditioning, where I could turn on a/c or not, without the car second-guessing me. It made a huge difference in comfort and visibility. My last several cars have all had automatic climate control, and all these cars have been correct and logical about when to run the a/c by their own automatic decision-making. (Except for the fact that I need to manually select a/c off in my convertible, when the top is down).
 

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With the Hondas, there used to be a hack to leave AC off as default when you press the defrost. With the Subaru, no such luck, but once the AC is off with defrost or defrost/cabin selected, it remains off even when the car is restarted.
 

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2014 2.5i Limited CVT
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I think that once many of the manufacturers started moving away from the manual "push button" AC switch (I remember many 80s and 90s cars having a button to push to engage the AC) - even without auto climate control - they've tied the AC compressor and dehumidification process... In some cases, I think that - again part of the process - you cannot have recirculated air on for defrost functionality - as it sort of defeats the purpose. Or is it only recirc...? As I'm in the desert southwest, I rarely (almost never) use the defrost function on the HVAC controls.

On my old PT Cruiser, it was no longer a button to push but the fan control had two directions - one for AC and one for non-AC and I don't think that the defrost setting ever engaged it - you still had to do it manually by turning the fan speed the other way.
 

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FozzieBalou "you cannot have recirculated air on for defrost functionality - as it sort of defeats the purpose" You are absolutely correct. Study up on the concepts of absolute versus relative humidity. Absolute humidity is the mass of water vapor per volume of air, and with normal ambient air it generally rises with rising temperature. Thus our natural habit of warming things up to dry them out - the increasing warmth allows the air to hold increasing amounts of gaseous water. Relative humidity is just the percentage of water vapor that the air does hold, divided by the maximum amount it can hold at the dew point, when fog would begin to appear. Then imagine you are driving along in cold weather, on recirculate, with the heater and defroster set to a comfortable temperature of 70F. All sorts of sources of liquid water inside your car -- your wet shoes and coat, your own breathing, and so forth, are all continuously evaporating and thereby slowly increasing both the absolute humidity and the relative humidity inside the car, as you drive along. No water vapor is leaving the car interior. The cold outside ambient air may have high relative humidity (even 100% if it's foggy), but it has lower absolute humidity than your car interior, so if you turn off recirculate and bring in that outside air to heat it up in the heater, you will end up with 70F air with lower relative humidity than you had with recirculate on.
 

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Yep. Very familiar with humidity... having grown up on the east coast (south eastern PA) and west cost (SF Bay Area) and having resided in coastal cities, desert cities and desert towns (Vegas = cities, Palm Springs = towns), humidity has often been a part of my life - good or bad... Right now, in the PS area, we're seeing higher than normal humidity and dew points (very important if you plan on using evaporative cooling) than normal. Usually we'll have maybe 5 or 6 weeks of higher humidity and dew points and usually August into September. But it's been much higher since (really) the beginning of July. It's been a really ugly summer this year....

But back to the OP - yes, it is normal behavior...
 
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