[dsraven]

a/c systems like to recirc the air because the way the system works best is with dry air, thats why there is a condensate drain on the evaporator. if you bring in fresh, outside air then the evap can get frozen with ice due to the warm air having a higher humidity level. if you want fresh air for the heater then you will likely need to find a system that also heats and has provisions for the fresh air intake. or, make your own provisions with a seperate heater. an a/c system with a heater core needs to have a door that controls the a/c side because, even though you might think it is weird, the a/c system is used to contro moisture in the air when the heater is on defrost.