A/C Engaged With Defrost

[Sterno]

OK, perhaps a dumb question but ... WHY? When I first noticed water dripping from beneath my car after using the defrost on the way to work I thought I'd just run through a puddle. But after I noticed it on dry days I recalled reading something about the A/C compressor being engaged when the temp knob is in defrost or defrost/floor mode (is this correct?) and was just wondering why this is the case. So, essentially, having your defrost on is robbing some power from the engine and lessening mpg?

Also, what exactly happens when it's turned to the 'off' position (straight up, 12:00 o'clock)?

[Sterno]

Sorry ... not "temp knob" but the large function knob.

[billnchristy]

I don't think I have ever owned a car that didn't run A/C when defrost was on cold.

I believe it is a humidity thing.

[BondiBlue]

Originally Posted by billnchristy

I believe it is a humidity thing.

^ This.

[Lonestar]

Window fog is caused by humidity (airborne H2O vapor). So, to eliminate this, you'd need/want the a/c which dries the air before directing it to the windows.

The loss of mpg and power is negligible, safety is always a priority. Today's a/c systems are designed with maximizing both, the compressor kicks in only when necessary, even when the a/c is on. You can maximize this further by setting the 'recirc' feature ON, so you're not sucking in more humid outside air which causes the compressor sensor to detect the need for engagement (and therefore lower power/mpg).

[Sterno]

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THIS is what I love about this forum! Thanks for all the info. I learn new stuff here every time I visit.

[FORDSVTPARTS]

Yup, completely normal. It makes the defrost system much more effective.

[Reddog99]

Originally Posted by Lonestar

..... You can maximize this further by setting the 'recirc' feature ON, so you're not sucking in more humid outside air which causes the compressor sensor to detect the need for engagement (and therefore lower power/mpg).

I believe that recirc is unavailable when using defrost, isn't it?

[Audrey]

Originally Posted by Reddog99

I believe that recirc is unavailable when using defrost, isn't it?

Yep.

[ibinubu12]

Basic, really. Cold air = dry air, since cold air can't hold much moisture. So 100% of the air passes through the evaporator core 100% of the time, and then a portion is reheated by being directed through the heater core. In defrost, the compressor is active, so the air is cooled, regardless of desired temperature in order to dehumidify it. Believe me, hot, humid air doesn't work to defrost anything.

Source: Automotive HVAC engineer.