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Speaking generally about automobile A/C systems, there are two main modes, normal and recirculate. Normal draws warm, humid outside over the evaporator coils and into the car. When using recirculate cooler, dryer interior air is drawn in from the vehicle cabin instead of outside.

The question is: During hot summer days, which mode is more likely to cause the evaporator to ice up?

Some say recirculate is more likely to cause evaporator icing because the cooler air in the cabin doesn't have enough heat to keep the condensate build-up from freezing, but the cabin air has already been dried and thus has much less moisture to condense in the first place.

Others say normal mode is more likely to cause icing because it is drawing in the much more humid outside air that will subsequently cause more condensate and thus more chance of icing. On the other hand outside air is much warmer and that heat is perhaps enough to keep the condensate above freezing.

Note that in a properly designed system there should be minimal, if any, icing. This question is more about exploring when the A/C is pushed to its limits or starting to malfunction, which mode will cause icing first.

Also note that this question mainly pertains to the use of A/C on warm summer days to cool the interior of the car, not the use of the A/C to dry the air on a cold, wet day.

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  • I question the idea that the air inside the car is always dryer than the outside air. Besides exhaled breath being moist, wet clothes contribute too. Even if it is raining outside, the car will mist up quickly if you don't bring fresh air into the cabin. Another problem with recirculated air is that it gets stale quite quickly, so it's usually best to bring in fresh air. I never close the vent except to prevent bad exhaust smells when the vehicle ahead is defective. Jul 4, 2023 at 12:12
  • Fair point, during cooler, wetter times it may actually be higher humidity in the car. My question came from a discussion about cars using A/C during hot summer months, so I was thinking mainly in that context.
    – Mike
    Jul 5, 2023 at 16:26
  • Every vehicle ac system condenses moist air whether in vent or recirc mode. The water dripping under the car when parked with ac running. The difference between constant air flow from outside or recirc mode is incoming air from outside contains more moisture than recirculated air since recirc air is continually recycled thru the evaporator coils. R-mode reduces humidity. As to icing, r134a systems are designed to chill down to around 35F (outlet vent temps) using a thermal expansion valve metering refrigerant constantly (from factory assembly).
    – F Dryer
    Jul 13, 2023 at 23:34

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