Chrysler recommends to limit the use of “recirculate” in order to reduce odor.
Perspiration and respiration moisture condenses on the evaporator coil, when parked bacteria from this fluid will cook (interior temps are 140-160 F where I live) and smells terrible when restarting the car. If the HVAC unit is on fresh air when the car is turned off the evaporator has a chance to dry out.
Another odor problem is new vehicles (less than one year) the bacteria can react with the evaporator coating producing the “cat urine” type odor. That smell becomes less apparent with age.
From page 283 of the 2007 Chrysler owners manual:
“The frequent use of outside air will help
keep odors from building up within the air conditioner/heater
housing. It is recommended that the recirculation
mode be used as little as possible, especially in humid
climates.”
Nash famous Weather Eye system
Before WW II, the Nash engineer, Nils Erik Wahlberg did extensive wind tunnel tests. At that time, car heaters were boxes that contained a small radiator and fan that hung under the dashboard. Cars were always cold. Wahlberg found that inducting outside air over the heater coil pressurized the cabin and kept the cold air from infiltrating the cabin from other areas. Consumer Reports said that comparing the Weather Eye heating system with the heaters offered by other manufacturers was like comparing a modern forced air furnace to a coal stove. Other makes rapidly converted to a heating system that inducted fresh air from the outside.
With air conditioning, it seems to me that pressurizing the cabin might prevent the infiltration of warmer air. I think even in recirculate mode that many cars still bring in some outside air and direct it over the evaporator coil.
Chrysler has an answer to the concern that the HVAC system will stay on Recirc if the driver does not reset to Fresh. Whenever the car is started, the HVAC is on Fresh, even it was on Recirc when the engine was shut off.
My owner’s manual ('09 Caravan) says and I quote “use the recirculation mode to block out any outside oder, smoke or dust. and to cool the interior rapidly after initial start-up in very hot or humid wearher”. “Continous use of the recirculation mode may make the inside air stuffy and window fogging may occur. Extended use of this mode is not reccommended”. Then the last thing it says is “For maximum cooling use the A/C and recirculation buttons at the same time”. Well, here in Missouri, we need maximum cooling right now, maybe in September we won’t. SO use your own judgement. I am using Rec most of the time.