When I first got my 2017 Cooper S (in January of 2017), the A/C was on and the dealer told me to leave it on all the time to dehumidify the air. I've been doing this now for 2 years, and I've started wondering if this is indeed useful or adversely affects mileage or performance. Thoughts?

My gas mileage is pretty good, and the car is always nice and dry and odor-free...

 

Im sure it does but more so will get you back in faster to get AC service.

Basically, he is right, it is taking the moisture out of the air but I just leave mine off and turn it on when I need

 

More use does equal more wear. But conversely more use means more time with oiled refrigerant moving through the system keeping seals in good condition.

Down here we are nearly always above comfortable temperatures and have high relative humidity so mine stays on nearly all the time. I just got a new car with climate control so now I don't have to worry about it.

James

 

First car I've left the a/c on all the time because of the stinky odor problem that many people were having. So far no odors so I am leaving it on. In the old days you turned the a/c on and you felt it take hp out of the car, doesn't seem to do this anymore with modern vehicles.

 

Thanks, I thought so. Will continue to keep it on as it's worked well for me so far.

 

Depends upon where you live. Some areas one needs the A/C in the summer to keep the cabin cool and the humidity at bay. Then one needs the A/C in the winter to help keep the cabin warm and the humidity at bay. (I bought a Boxster in Merriam KS when I lived east of KC Mo. There were 2 weeks in the spring when I could comfortably drive the car with the top down and 2 weeks in the fall when I could comfortably drive the car with the down. The rest of the time I needed the top up and the A/C on to deal with either the summer heat and horrible summer humidity or deal with the winter biting cold and while the humidity could be low there was still the window fogging to deal with.)

Now I live in CA and while the Boxster is gone I now drive a 2018 JCW. Where I live I don't need the A/C that much at least after the heat of summer/early fall is gone. Even in the warmer months often the temperature doesn't require I run the A/C. Similarly in the "winter" often I have to remember to run the A/C a while just to circulate the refrigerant and compressor oil to keep the compressor lubed and the o-rings in good condition. (I did this with my Boxster, too, and I drove my Boxster 16 years and 317K miles and the A/C was just fine at the end. In fact I have made it a point to do this with every car I have owned with A/C and I have never had any problems with the A/C system in any car. And it bears mentioning that when I do use the A/C I make it a habit of shutting off the A/C compressor a few minutes from my destination to give the system time to "dry" out. This reduces the tendency of the system to develop a foul/sour odor. If the car is shut off with the A/C active the evaporator will almost certainly be just dripping wet from condensation and this is a very good place for mold/mildew to take root, so to speak, and if it does a foul/sour odor is almost a certainty. Also, the cabin air filter needs to be changed every so often. If the A/C system is used often maybe as often as once a year.)

Modern cars, at least in my experience those with a variable displacement type of A/C compressor, have a pretty efficient A/C compressor. Both my Boxster and my Porsche 996 Turbo came with auto climate and a variable displacement A/C compressor. This is a fabulous device that allows for adjustment of how much the compressor compresses the refrigerant and thus how much heat can be removed from the incoming outside air before it is routed to the cabin.. As a result after initially bringing the cabin temperature down -- which took very little time -- the compressor displacement is decreased (under auto climate control) so only the required amount of cooling is produced. As a result the parasitic load is very low.

Monitoring OBD2 engine load via an OBD2 data viewer/logger with my Boxster the engine load reading only decreases by a few tenths of a percent when I switch off the A/C or increases by a few tenths of a percent when I switch it back on.The Boxster which had around 215hp, the JCW 228hp.. Thus just 2 tenths of a percent of engine load represents less than 1/2hp. Even when working "hard" the A/C load is low, way less than 1% and this is running flat out. In more normal usage while the A/C might work "hard" initially to deal with a hot cabin due do to the car having been in the sun it only takes a few minutes for the A/C fan speed to decrease and the A/C compressor load to fall as the auto climate controller adjusts the A/C compressor displacement to a smaller value so there is less compressing of the refrigerant and thus less cooling.

By way of comparison just idling -- this with the engine up to temperature and no electrical loads like lights on and the A/C off -- the engine load at idle is just a few percent. For the JCW that is less than 5hp. IOWs, the JCW engine is working only about as hard as a good size lawnmower engine at idle. At highway speed on a flat stretch of road it takes around 40hp to 45hp to move the car down the road. At 45hp 1/2HP is nothing.

Which lead me to gas mileage. I have driven a number of various cars for in total hundreds of thousands of miles and in a number of them have tracked gas mileage. I have not noticed any real change in gas mileage with the A/C on or off.

Far and away the biggest effect on gas mileage is how the car is driven. More than once I (re) discovered that just one spirited acceleration run up through the gears getting back on the freeway after a stop for fuel can hammer gas mileage to the point that I can spend the next couple of hundred miles driving like a high miler god and gas mileage doesn't fully recover even then.

 

I always keep the AC on.

After hearing about the odor from the air vents, I have not experienced that.

Seems to help keep the windows defrosted. Maybe it will freeze anything bad growing on the AC evaporator. After parking, I sometimes check to see that water drains from the evaporator drain tube under the car.

 

Florida owner and cannot remember the last time I had the AC "off" Averaging 36 MPG w/ the JCW tune normal driving.

 

Today's cars also cycle the A/C on a schedule as programmed by the BCM (Body Control Module) or whatever moniker MINI/BMW uses in order to prevent issues when the A/C is called on for use. Also consider the fact that the A/C system is in use any time you have the defrost air vent setting on, as that setting not only heats up cold windows, but is also used to dehumidify the air at the same time. I also have a feeling that our cars have a humidistat that actively monitors the cabin's humidity, as my A/C will somewhat randomly kick on (i.e.- dash button lights up) in the dead of winter when temps rise a bit and there is snow melt sitting in my floor mats. Unless you drive around with your windows closed and system on recirculate, I don't see how you should have any humidity issues. I have never had any humidity issues in my MINI over the past 2+ years of ownership, regardless of season of use...and we can see temps around here from -20 to +100 deg F, FWIW.