What's the difference between A/C and defroster on cold?

[Crunch]

Since the compressor runs when using defrost, and the A/C isn't a heat pump what is the difference between running your A/C and the defroster set to cold? Sometimes when driving at night, regardless of temperature but related to humidity the inside of the front windshield will fog up. When it's too warm to run the defroster on warmer settings I set it all the way to cold. The dryer air clears the fog and the only difference I can think of is that the air is being directed out of the top of the dashboard instead of out the front vents. Is this the only difference or is something more going on?

 

[E7mack96]

thats about it, direction of flow. if it is too cold outside, the defroster should not cycle the ac compressor. oh and on defrost, it does not recirculate cabin air, ever. Always air from outside.

 

[rbattelle]

and the A/C isn't a heat pump what is the difference between running your A/C and the defroster set to cold?

Actually, the system is a heat pump, if you want to call it that. It's pumping heat from the inside of your truck to the atmosphere. Semantics, really. If we want to really sound official, we'd simply call it a vapor compression refrigeration cycle.

We sometimes call this a "heat pump" because it's making energy appear to flow backward without violating the 2nd law of thermodynamics. The term "heat pump" in the vernacular is used to describe a vapor compression refrigeration system with the condenser on the environmentally-controlled side of the system (inside the house or truck), but it need not have such a narrow definition.

The windshield will fog up at the right outside conditions for the same reason a can or bottle of cold liquid gets covered in condensation when left out in the heat. Quite simply, cold air has less capacity for water vapor than warm air. The cold windshield (or bottle of liquid) cools the air in its immediate vicinity to the saturation point (100% relative humidity), at which point the water begins to condense on the surface.

But that's more than you asked. Like Mack says, the only difference between cold defrost and other A/C positions is the vent location.

Ryan

 

[NavyDood]

if it is too cold outside, the defroster should not cycle the ac compressor.

Mine cycles even in the teens when defrost is selected. Has done it for the past 100,000 miles of owning the truck.

 

[rbattelle]

Mine cycles even in the teens when defrost is selected. Has done it for the past 100,000 miles of owning the truck.

That's interesting. Notice Mack and I have 2003 trucks, and both don't cycle the compressor if it's below something like 30°F outside.



I think I remember hearing something awhile back about the programming being changed for later models.



Ryan

 

[E7mack96]

yep, at lower temps (sub-freezing), there is a possibility of getting liquid refrigerant and oil slugging back to the compressor which can cause a lockup condition. Bad, indeed.