Proper A/C Operation

Tom and Ray: On a recent show, you

provided some bad advice regarding running a Subaru’s A/C on “max” or “recirculate”, not surprising given that the average July temperature in Cambridge is probably about 53 Degrees F.

Here is the proper way to start up and

operate any vehicle’s air conditioning

system on hot days. This will provide both optimal cooling performance, and the best fuel economy.

1. Start the vehicle, open the rear

windows, and turn the A/C to the highest fan setting on “normal” A/C(not “max” or recirculate). This will use outside air (which is hot, but not as hot as the air inside the vehicle) to force the inside air out.

2. Close all windows, and turn the A/C

to “Max” or “Recirculate”. Leave it on Max for a few minutes only, until you have chilled the inside air, and the A/C system components.

3. Turn the A/C back to “normal” and adjust fan setting as required.

4. Enjoy

This isn’t really a major issue. But to contribute:

Yes using outside air with the back windows open forces out the hot air inside the vehicle, replacing it with air that has been cooled by the air conditioning. The thing is that when a vehicle is moving at any significant speed (let’s say, upwards of 25mph), the rush of air passing the vehicle creates a pressure differential between the air outside and the air inside. The outside air has an artificially higher pressure than the air inside the car because it’s moving very quickly relative to the car, and so it actually forces its way into the car. This can throw a wrench in the “flush out the hot air” plan.

When the A/C is set to MAX (recirculation) mode, it’s taking the air inside the car and passing it over the cooler evaporator core. With the recirculator door open, the blower fan on any small to mid-size car can probably circulate all the air within the car in little more than two minutes. Even if it’s 120? in the car, a properly operating A/C will be able to cool that air down to about 75? within just a few minutes.

So if you’re sitting still, it helps to open the windows and use outside air. But if you’re moving right away, use the recirculator.

It also helps to open your windows wide a few minutes before you actually get in the car to go somewhere.

Here in Florida where it can get really hot, I will sometimes start my car 5 or 10 minutes before leaving to run the A/C, the same way someone in Maine would start his car to get the heat going in January.


Or you can

  1. Turn the A/C back to “normal” and adjust fan setting as required.

    The difference is too little to worry about.

Here in Maine on the hottest day my Toyota AC cools the car within 3 minutes and heats in the winter about the same. Patience is a virtue.

Any attempt to maximize initial cooling rates without taking into account the humidity is doomed to failure. Removing moisture in the air can often absorb more energy than cooling it. There is no simple answer for all situations.

On very humid days without much sun load, cars will often cool faster with the windows closed and the AC on recirculate.

How about I just start my car and drive away, letting the climate control system (which I always leave on anyway) just do it’s thing and cool the car? It might take an extra 30 seconds to reach optimum cooling, but it’s easy.