[agambhandari]

God, such a confusion here.!!

In my cars, even if observe fogging on windscreen
-Turn the temperature dial to red
- switch on fresh air mode
- switch on the fan to 2 speed

The fog clears up within seconds.

I am posting the instructions about heating as given in my car's manual(Chevy Spark) for manual air conditioning.
Hot air will come out of the vents even if A/C is switched on , provided that temperature control is set to hot(Red)
HEATING
Maximum heating Use the maximum heating mode for quick heating of the passenger area. Do not use it for lengthy periods because the interior air may become stale and the windows may fog. To clear the windows, turn off the recirculation button to allow fresh air into the vehicle.
For maximum heating:
1.Turn off air conditioning (A/C). (Indicator goes off)
2.Slide the recirculation lever to recirculation mode.
3.Turn air distribution knob to BI-LEVEL ( ) or FLOOR ( ).
4.Turn temperature control knob all the way to the red area for heating
5.Turn fan control knob to maximum speed.
Normal heating
1.Turn off air conditioning (A/C). (Indicator goes off)
2.Slide the recirculation lever to outside air mode.
3.Turn air distribution knob to FLOOR ( ) or BI-LEVEL ( ).
4.Turn temperature control knob to red area for heating.
5.Turn fan control knob to desired speed

[LithiumSunset]

What does heating of air on recirculation have to do with air becoming staler?

Don't we all use the recirculation when cooling? I know I do! On longer drives we are all advised to use the refresh mode on the hour for a little while.

[phamilyman]

Originally Posted by amitpunjani

In case of mist just switch off the Re-circulation mode and allow some fresh air to come in, automatically with few mins the mist will clear off.

In jaggery village's golden road - that's a NOO!!!! (Sohna Road, gurgaon for the non-NCR folks)

The amount of dust on Sohna road means that "fresh air" (as even GTO refers in his articles) is nothing but the outside dust & pollution. I'd much rather keep my car full of as much of the fresher less-polluted air from my apartment complex as long as possible.

So yes, I use AC 24x7. Engine may have a lower life, but my lungs are irreplaceable. Thanks but no thanks to the recirc mode!

[NYMEX]

In the Toyota Altis, the instruction to de-fog is to set the air distribution knob to front windshield de-fogging. The moment the knob is set on this position, the air recirculation automatically switches off and the mist is cleared quickly. The practice I follow in winters in Delhi, is to set the air distribution to floor + windshield, switch off the air recirculation and start the blower at 2 speed after 5 minutes of runnning of car.

The only down side to keeping the air-recirculation off is that the desiel fumes enter when you are parked next to a desiel vehicle, esp the buses, while waiting at a stop signal.

Mitigation measure ......switch on the re-circualtion when you approach a stop signal & while waiting.

[dutta_d1]

Just my 2 cents-

A/C accomplishes 2 tasks when compressor is running-
1. Cools the air
2. Dehumidifies the air.

So,
1. If you are running just the heater - you are warming the air with same humidity.
2. If you are running heater + A/C - you are warming the air + reducing the humidity.

How you use the HVAC is now what you want to accomplish.

That being said, running the compressor affects the FE negatively. Whereas only heating is free and FE is not affected.

Fresh air or re-circulation is another debate actually. However, I prefer to use fresh air by default and switch to re-circulation when passing though a dusty/smoky section.

[fine69]

Originally Posted by tsk1979

So you turned the direction to heating and did not turn on the AC compressor and the front windscreen fog dissappeared! Never happened with with me. I keep know at full heating, but have to switch on AC compressor to get rid of the fogging inside the front windscreen

Originally Posted by manolin

Do you have the re-circulation mode on? I get the feeling that if you switched that off, the fogging would go away with just windscreen vents+heating (and no ac).

I always keep the re-circulation mode off, not sure whether that's the reason why it works for me.

However, there was this one particular instance when I was driving through the clouds (somewhere before Shimla) and the outer part of the windscreen fogged up, had to run the wipers. Never experienced it anytime before.

Honestly, I don't even understand this fogging up business, the logic doesn't work everytime.

[tsk1979]

Originally Posted by fine69

I always keep the re-circulation mode off, not sure whether that's the reason why it works for me.

However, there was this one particular instance when I was driving through the clouds (somewhere before Shimla) and the outer part of the windscreen fogged up, had to run the wipers. Never experienced it anytime before.

Honestly, I don't even understand this fogging up business, the logic doesn't work everytime.

When humid air hits a cold surface you get fogging.
In summer when you are driving through clouds with AC on inside, the windscreen glass is colder than the outside.
So you get fogging outside as warm humid air condences on the cold windshield
In winters outside is colder, so inside is warm humid air. As it hits the cold glass it condences

BTW I put in outside air mode, and then used heater without AC and there was no fogging up. But I also had to endure the lovely atmosphere of our country full of smoke, and some weird smells which come out of noxious sugar factories. So I switched back to recirculation and turned on the AC. Again no fogging up, and no smelling like a pig sty

[fine69]

Originally Posted by tsk1979

BTW I put in outside air mode, and then used heater without AC and there was no fogging up. But I also had to endure the lovely atmosphere of our country full of smoke, and some weird smells which come out of noxious sugar factories. So I switched back to recirculation and turned on the AC. Again no fogging up, and no smelling like a pig sty

My bad, I meant I always keep re-circulation ON (no air from outside) and am still able to get rid of the fog when I turn the windshield vents on.

And by the logic you gave, I think hot air here helps in getting rid of the humidity inside which is why fog gives away.

I also tried using it with re-circulation to off once but as you said, the smell was simply intolerable. There's too much dust/pollution in Delhi to ever use re-circulation in OFF mode.