I have heard advice that one should turn off the air conditioning before the end of the trip so that the moisture in the system gets a chance to be dried out. Apparently, the collecting moisture can cause mold growth, leading to a stinking A/C system.

Is this a myth or is there any sense in this advice?

If there is any sense in this, why haven't car manufacturers built a system that operates the fan for minute or two after turning off the car? My inverter microwave does that: if the components are hot, the fan is operated even after the microwave is turned off.

If the advice is useful, how many minutes before the expected end of the trip should one turn off the air conditioning?


I could not find any sources, like AC shops, that recommend this. All do recommend having the system serviced annually, including the fresh air filter (if equipped).

The one thing I did find was this guy who explains what you are talking about in this video.

Considering the AC system removes moisture from the air as part of its cooling process, it makes sense to avoid condensation buildup in the ducts. It is easy enough to turn off the AC and put the selector to fresh air for a couple minutes before you get to your destination. It certainly won't hurt anything.


It's useful, because keeping an airflow over an inactive evaporator effectively warms it up fast (it's meant to happen, because evaporators are designed to exchange heat with the air in the most efficient way so that they can cool your car fast) so that water soon ceases condensating over it, and with it the ducting, keeping in mind that most car AC systems, when in good working order, have, on average, a 40F-41F air temperature at the vents (and no water condensation means that any leftover humidity immediately evaporates and get pushed out of the vents, and that's why you can feel the air coming from the vents get progressively humid as soon as you turn the AC system off but keeping the blower on). Bacteria effectively thrives in an humid environment.

The time the airflow should be kept on is some minutes. There's not a precise time (it varies depending upon the heater unit, evaporator and ducting dimensions and arrangement). As a side note, keeping recirculation on during this period lets the evaporator cool down slower than with ambient air blowing over it, so that even if no refrigerant is circulating, you can still get cool air at the vents for some minutes, especially if you have an efficient AC system (one that can easily get, and maintain, the evaporator near the freezing point) and not one struggling to do its job. If you have a fixed displacement compressor, disengage the AC and keep the blower on when you hear/feel its clutch disengage: right after the clutch disengages, the evaporator is at its coldest; if you have a variable displacement internally controlled one, this doesn't apply because the evaporator is always at its coldest temperature; if you have a variable displacement externally controlled one, temporarily setting the temperature to a much higher value causes to evaporator to warm up due to less refrigerant reaching it.

However, if you have an habit of periodically cleaning the ducting and evaporator by spraying air-con cleaners, along with fitting new cabin air filters yourself when required, you don't have to worry too much about it.

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