If you want to defrost your windows, you want hot, dry air. Air conditioning dries your air. If you combine A/C and heat, you get the best defrost performance.

If you want to defrost your windows, you want hot, dry air. Air conditioning dries your air. If you combine A/C and heat, you get the best defrost performance. A/C also consumes some amount of power and fuel efficiency- you may want the coolest air you could possibly have without A/C.