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HVAC cold weather operation

 
  #1  
Old 02-05-2019, 03:53 PM
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Default HVAC cold weather operation

I have noticed that in cool/cold weather the HVAC system emits some air from all the vents as soon as the car is started, or if the ignition is turned on. When the car heats up (3-5 minutes), warm air comes from the heat vents, the fan speed adjusts, and only a faint trickle of air (also warm) comes from the AC vents (like a slight leak past the vent doors). I always thought that on starting, the system would send some air through the defrost vents, and keep the heat/AC vents closed until the coolant warmed up. The car is garaged, so it is probably in 45-55 degree air when it starts; maybe that confuses the system.
I tried to explain this to the Service Advisor, and he didn't understand the symptom, but said they could do a diagnostic check. Wasn't interested so don't remember the charge.
This is not a problem worth spending money on, but I'm curious about how it should work.
Our Outback, on the other hand, doesn't turn on the "auto" heat until the engine is really warm - if you want heat in a hurry you've got to turn off the "auto" and adjust the mode/fan to beat the system's brain. That IS a pain.
I must not have enough problems to worry about...
 
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Old 02-06-2019, 11:38 AM
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Ummm... maybe is something that I don't know of, but, as far as I know, there is no such thing as dedicated Heat/AC vents...

The car will sent the air (cold or warm) based on what you have setup on the screen - the only exception will be if you hit the "High" front-window defrost - in that case it will "close" all the vents, except the windshield ones.
As soon as you turn off that, the air flow and speed will go back to your original settings.

In the configuration you also have the option to turn off the AC use.
 
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Old 02-06-2019, 01:58 PM
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Originally Posted by mrNewt View Post
Ummm... maybe is something that I don't know of, but, as far as I know, there is no such thing as dedicated Heat/AC vents...

The car will sent the air (cold or warm) based on what you have setup on the screen - the only exception will be if you hit the "High" front-window defrost - in that case it will "close" all the vents, except the windshield ones.
As soon as you turn off that, the air flow and speed will go back to your original settings.

In the configuration you also have the option to turn off the AC use.
You are correct that in modern cars you can send outside air, heated air, or cooled air to any of the various vents. I suspect Dick is "old school" enough to be using '60s era terms. At that time heated air was only sent to floor or windshield vents, and cooled air only to face vents. If you substitute "floor vents" where he says "heat vents" and "face vents" where he says "A/C vents" I think you will understand him.
 
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Old 02-06-2019, 02:25 PM
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Swimref - in that case it does make more sense. I wasn't around in the 60s so I wasn't aware of that feature - thanks for letting me know.

Then in that case, maybe the explanation lies in the fact that the windshield in the XF is heated and maybe that's why is not sending as much air on it?

Dick - is the windshield defrost light on, on the button?
 
  #5  
Old 02-06-2019, 05:15 PM
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Hey guys, sorry to make this such a confusing issue - I'm talking only about "AUTO" mode (the Subaru calls it "FULL AUTO")
The last several cars I've owned (Jaguars, Subarus, Toyotas), had this vent arrangement (controlled by blend doors):
defroster (front/side)
dash (2 near the center)
dash (2 near the sides)
rear seat (back of center console)
floor (front and rear).
I've driven for years with a center and side vent each blowing on my hands, and occasionally checking the rear seat vent, so I know this is the way it has worked in my cars:
- when the system is trying to raise the temperature in the car, heated air comes through the floor vents and the two dash side vents..
- when the system is trying to lower the temperature in the car, cooled air comes through all the dash vents and the rear seat vent.
The system decides when to send air through the defrost vents through some space-age technology.
The Subaru has a display that constantly shows which vents are open (even in FULL AUTO mode), so easy to verify - raise the temp, the center vents close; lower the temp, they open.
In the Jaguar, you have to crank the temperature way up or way down, and see where the air comes out. (A technician once told me, regarding my '09, that there is also an outside temperature sensor input, further complicating the system's decision-making).

My only issue is that my Jaguar, when it's first starting, doesn't seem to know whether it's supposed to be heating or cooling (maybe set temperature is too close to cabin temperature), so some air comes through all the dash vents.
Whereas, on the Subaru (in cool weather), the dash vents are closed tight until the system decides it's good and ready.
Anyway, thanks for your interest, but certainly not worth all this commotion.
You may return to your regularly scheduled activities...
 
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Old 02-06-2019, 09:04 PM
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Thanks for the clarification Dick... sorry I was beating around the bush .
Like you have mentioned, most probably (like any computer) when in full auto and if the temperatures are close enough (inside/outside), maybe the computer spins around not knowing for sure what to do .
 

Last edited by mrNewt; 02-06-2019 at 09:10 PM.
  #7  
Old 02-07-2019, 04:41 PM
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Yeah, I came to the same conclusion. Today the garage temp was 65 and AC set on 71 when I started the car; air (slow speed) came from all vents. Cranked the temp up to 82, center vents closed and air (warmish already) started coming from the side vents. I need to find something else to worry about...
 
 
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