AC vs. Defrost question

[Crunch]

HVAC would have been a good person to direct this quesiton to, but hopefully other here can answer it.



What's the difference between running the AC vs. running the defroster set to the coldest setting? These days, when you run the defrost, the compressor is active. I started doing this when I noticed that running the AC on muggy days and the fresh air setting didn't prevent the windshield from fogging up. Also, do vehicle ACs function as a heat pump?

 

[B.G. Smith]

The AC compressor won't come on if the outside temperature is below a certain point. The defrost mode bypasses the ambient air switch and the compressor runs when in defrost setting. Sorry, no heat pump. bg