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Originally Posted by 15289577
The recirculate mode is only intended to be used for short periods of time. When you get in the car and it's very hot in the car then you might use the recirculate to try and cool the inside air of the car quickly for example. After that the A/C system would be working to try and cool already fairly cold air. This is no good. It's better for the A/C system to use the normal air mode as opposed to the recircuate.
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I disagree. Imagine inside car temp is 120F, if on recirc, the A/C must try and cool 120F air down. If not on recirc, the hot air will be expelled thru the vents, and the A/C will be cooling incoming cooler air at, say 90F (or whatever ambient is). Further more, A/C systems are equipped with a thermostat that reads the evaporator temp and adjusts A/C accordingly. Therefore, after the car has cooled down, it is easier for the A/C system to keep the car cool by recirc already cooled air. Naturally, some fresh air is bled in in most designs, regardless of recirc on or off.
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It's also better not to use the recirculating mode when heating the car in the winter either, but for different reasons.
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Same logic applies here as does to A/C, but supplying heat is not as parasitical drain on engine power like the A/C compressor. The only caveat to using recirc all the time is if the system is soooo tight, that infiltration of "bad" air, like an exhaust leak, could pose a danger. Recirc of heat in the winter can allow a build-up of moisture from passengers' breath, etc but should not present a problem except in extreme cases (back seat at the drive-in?)...