AC/ Heat problem,Anyone else having this problem?

[2011sienna]

2011 Sienna 4cyn
AC/ Heat problem

Anyone else having this problem?
When I use my AC everything is cold but when I turn down or off the AC I get hot air coming through the vents, hotter than the outside air at night it fogs up the windshield .
I told the dealer about it but they said they could not get it to happen and to just leave the AC on and to bring it back if it happens again but it happens every time.

[akphs]

Originally Posted by 2011sienna

2011 Sienna 4cyn
AC/ Heat problem

Anyone else having this problem?
When I use my AC everything is cold but when I turn down or off the AC I get hot air coming through the vents, hotter than the outside air at night it fogs up the windshield .
I told the dealer about it but they said they could not get it to happen and to just leave the AC on and to bring it back if it happens again but it happens every time.

NOT picking a fight, just asking:
1) Why do you think it's necessarily hotter than the outside air?
2) If it's fogging the windshield, isn't that because it's humid and the windshield is cold from the A/C?

My suspicion is that (1) is either illusory or is because it's coming through the engine compartment, and (2) is just because it's very humid as it's picking up some moisture from the damp A/C core. But I are not a mekkanik.

[Biggoldfish]

Same principle as when you breathe on a window. Does your breath fog up the window? The incoming air IS warmer than the a/ced air, that's why the fogged windshield. Do you always use the windshield setting when you have the a/c on? Try using the dashboard setting and you'll see when you turn down the a/c, your windshield will not fog up again. The cold air has been hitting it and any change in temp will cause it to fog up due to condensation.

[akphs]

Originally Posted by Biggoldfish

Same principle as when you breathe on a window. Does your breath fog up the window? The incoming air IS warmer than the a/ced air, that's why the fogged windshield. Do you always use the windshield setting when you have the a/c on? Try using the dashboard setting and you'll see when you turn down the a/c, your windshield will not fog up again. The cold air has been hitting it and any change in temp will cause it to fog up due to condensation.

Well, no, it's not strictly because the air is *warmer*, nor because the interior *air* is cool -- it's because the incoming air is *moister* and the *glass* is cool. Now, warmer air can carry more moisture, so warmer air will tend to be moister, but it's possible that the glass is (say) 65F and the interior air and the incoming are are both 70F, but the incoming air has more moisture in it. Then it won't condense until it hits the glass.

In fact, if the air in the car is enough colder and the incoming air is damp enough, you'll get "steam" coming out of the vents as the moisture in the incoming air condenses upon hitting the colder air of the car's interior.

None of which obviates your suggestion, just being picky about the science!

[2011sienna]

Originally Posted by akphs

NOT picking a fight, just asking:
1) Why do you think it's necessarily hotter than the outside air?

Because I used a digital thermometer

2) If it's fogging the windshield, isn't that because it's humid and the windshield is cold from the A/C?
Yes, but the AC has been off and the moisture is on the outside and the window is very cold to the touch.


My suspicion is that (1) is either illusory or is because it's coming through the engine compartment,

B I N G O ... this is what I think it is, Some how cool incoming outside air is being heated up by the engine BUT How and Were?

(2) is just because it's very humid as it's picking up some moisture from the damp A/C core.
Yes I think so too and Iam an Mex mekkanik and I don't think an closet hanger wire will do the trick this time.

But I are not a mekkanik.

Also I hear that other years are having this same problem
Anyone else have an solution?

[Biggoldfish]

Originally Posted by akphs

But I are not a mekkanik.

LOL!!! But you certainly sound like a physicist!

[akphs]

>I used a digital thermometer

OK. Was the car moving?

>the AC has been off and the moisture is on the outside and the window is very cold to the touch.

Uh. If the moisture is on the outside and the window is cold, then that has nothing to do with the new air coming through. The window is cold 'cause the AC was on before, and the outside air is moisture-laden enough that when it gets cooled by contact with the glass, some of the water vapor condenses on the glass.

>this is what I think it is, Some how cool incoming outside air is being heated up by the engine BUT How and Were?

Well, the air comes in through the engine compartment, so this might be normal.

Alternative theories:
- an air duct door is sticking
- there's some "cleverness" in the system such that when the AC is turned off, it forces some warm air over the AC core to dry it out (I don't really believe this one, though I'd actually like it to be true, to reduce the chances of "sweat sock AC")

But I'm Easter-Egging here.

[MatrixPC]

I'm not sure about the Sienna, but this is how coolant and AC in my 2 Supras, Matrix operate.

If you HVAC temperature is NOT at the coldest, the VSV valve will open to allow coolant to flow thru the heater core inside the carbin. Let said, coldest temp for the system is 65, and your current setting is 66. The VSV valve is opened and hot coolant is flowing thru the heater core.

The "flapper" inside the HVAC exchanger will adjust the open/close position between the AC core and Heater core. The position of the "flapper" give you an option whether you can set your temperature to 65, 75, or 85.

Try to leave the temp at coldest to see what happen.

[Biggoldfish]

Okay, lets not beat a dead horse here... 2011Sienna, it's normal. Don't worry about it. And if you want to keep the winshield "fog" free, don't use the windshield defrost mode all the time. That's what it is, it's used for defrosting the windshield during cold weather when condensation appears on the inside of the glass. I see you live in LA, maybe that's why you're not familiar with the defrost feature but us east coasters know alot about defrosting windows. LOL!

[MatrixPC]

LA may not be that cold but around this time, in the morning, all windows are fog up if park outdoor.

[2toyotaman]

I too have a 2011 Sienna 4 cyl that is the bottom line model, about manual everything; no thermostat-in-cab temperature control. MatrixPC is hitting on an influential factor relating it to the thermostat setting and what is happening with hot engine coolant in the AC/heater core. Even with my manual system, if it is getting too cold with the AC on, we can keep the AC on to keep humidity low and move the temp control off the very bottom setting allowing hot engine coolant into the heater core taking some chill off the air coming from the AC evaporator, albeit not thermostatically controlled but still cool if we don’t move the temperature setting too high. With such settings, if we turn off the AC, the air will come through noticeably warm after it warms up the duct system. If there is a leak in the valve that cuts off the hot water through the heater core, air passing through will be warmed even though the temp control setting is at the lowest.


I have had a similar but different problem when outside air is rather warm and humid, and the air flow is set over there where part goes to the feet and part goes to the windshield. With the AC on moisture is condensed out of the air and the windshield is not fogged up even with a low temperature setting and even with air being drawn in from outside. After getting the cab refreshed with the outside air, I change to recirculate the inside air which has been cooled and dried, and turn the AC off for a while. The windshield started fogging up on the inside way too soon and I noticed that the recirculate control by itself had switched over to bringing in outside air. This happened repeatedly and I had to keep my eye on that control and frequently reset it to recirculate. At my first service I complained to the service foreman about this and got a good answer. He said when the recirculating air temperature gets to some point that control is programmed to switch to outside air. That auto switchover can be turned off by holding the control down for at least 6 seconds when switching to recirculate. That seems to work.