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When I turn on the fan and press the button to direct the flow of air towards windshield, relatively cold air starts coming as compared to when direction of air flow is for example towards face. Why does it happen, does AC start running, though AC light is not on. thanks for all the replies.

 
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Yes. The AC compressor is used at that point to blow DRY air towards the windshield and defog. The temperature should stay relatively the same unless you have it full cold. Defog works much better when your air conditioning system is functional because of this. AC system dries the air along with cooling it.

 
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Right.
Job #1 of any AC system is to remove moisture from the air.
Cooling the air is secondary.

 
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So gas consumption is same whether we run regular AC or we direct the flow of air towards windshield? thanks

 
Rep Power: 518

Um...Pretty much.

The AC compressor requires the same amount of power to operate, regardless of what you do with it (where you direct the air).


Some figures I've read say it takes between 2 and 5 horsepower to run an AC system.

It is also said that it takes about the same a mount of engine power to run the AC at 60 MPH as it does to have all the windows open, due to aerodynamic drag of open windows.



IMO, about the only thing I can think of that would make any difference in gas mileage (when using AC or defog) is the "on time" of the normal compressor cycling during operation, and that mostly has to do with (heat) load and fan speeds.

HTH

 
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Are you saying that compressor keeps turning on and off depending upon the fan speed and how hot it is? I thought that compressor keeps on running as long as AC is on unlike the AC for homes whose compressor starts running when temperature goes above the already set point and turns off when required temperature is reached. thanks again for all the answers.

 
Rep Power: 518

Very much like a home/central AC, except the home AC thermostat is mounted on the wall and senses the room temperature.


Yes, the AC system used in these cars is designed to cycle on and off to maintain the evaporator core temps within a certain range, normally between (approx) 35 and 40 degrees F.
(Some other manufacturers use systems that leave the compressor on all the time, but Honda does not.)
How often the compressor cycles on/off is dependent on the heat load placed on the system, and there can be several factors that affect heat load.


Obviously if it is a cool 70*F outside, the system will not have to work as hard as would if it were 120*F in the shade outside.
With low heat load, the compressor might only need to operate (example) 40% of the time.
Conversely, if it is 120* outside the compressor might need to be operating 100% on, the entire time you are driving....because the evaporator can't be cooled down to its 35* cutoff temp right away.


Did that explain it good enough?