Running the AC with Windows Open


#1

In response to the caller who wondered if it was bad for her car’s AC to run it with the windows open. It seems to me that when a car AC is on, the compressor runs continuously and the cabin temperature is controlled by adding heat to the air. If this is the case, it wouldn’t make any difference if the windows are open or not, since the compressor runs continuously anyway.


#2

It probably won’t hurt the AC system but the driver who does this is a few bricks shy of a load in my opinion. There is no valid reason for doing it.


#3

“but the driver who does this is a few bricks shy of a load in my opinion” Maybe you are a few bricks over the load. If I like fresh air and cool I doubt it makes more than a gnats hair of difference for in town driving, and on the highway that is my choice, valid reason, as far as i know,it isstill a free country, kind of.


#4

The AC system of most vehicles brings in fresh air as it operates unless you use the recirculate option. Still not a “valid” reason. If you want to waste money just try throwing dollar bills out the window while they are down and the AC is on. Waterboy, I support your right to do what you want to do because this is a free country, Rights can make people do all sorts of things like keep 75 cats in a 2 room apartment or become hoarders that fill rooms to the ceiling. Your “rights” have to be tempered with a little common sense now and then.


#5

The compressor will work harder and continuously if the cabin air temperature cannot be brought down.


#6

In answer to the original question:

Often there’s a thermostat attached to the evaporator which will shut off compressor if the evaporator temp drops below the set point to prevent freezing.
If the system is working harder the evaporator tends to get warmer and keep the compressor on more of the time.


#7

Many vehicles run the compressor all the time the A/C is on. In that case it would make no difference windows closed or not.


#8

"Many vehicles run the compressor all the time the A/C is on. In that case it would make no difference windows closed or not. "

The compressor works harder and is a bigger load on the engine when it is trying to cool hot air.

As for running the compressor all the time, can you give us an example?


#9

The compressor has two modes, “on” and “off.” How could it be working harder than “on?”


#10

Some cars have temperature settings; Presumably the compressor turns off or works less when the desired temp. is reached, much like a home air conditioner. For cars without temperature settings, the driver may turn down the fan or adjust the cooling setting once a comfortable temperature is reached, instead running it non-stop at the cold setting. For example, I live in an area that is relatively hot during the summer, so I tend to run the AC at the coldest setting and then adjust the temperature setting or the fan later. So driver behavior may be a factor.


#11

"The compressor has two modes, “on” and “off.” How could it be working harder than “on?” "

Whitey - When the air going over the cooling coil is hot, it raises the temperature of the freon returning to the compressor. This raises the pressure in the system which the compressor has to work against. Raising the pressure makes the engine work harder to keep the compressor turning.


#12

" When the air going over the cooling coil is hot, it raises the temperature of the freon returning to the compressor. This raises the pressure in the system which the compressor has to work against." The outside air will be the same temp with the windows open or closed!! For once, I agree with Whitey, the compressor is either working or it iis not. I don’t think the compressor knows if the windows are open or closed!! There may be some compressors that work with a thermostat (off and on) but many don’t.


#13

There are many compressors that work when defrost is on.


#14

Every car I ever had with A/C, the compressor ran with the defrost on!! And I have had A/C since 1966.


#15

The compressor has to be turned by the engine to do the work necessary to remove heat from the cabin. The more heat that the compressor has to remove, the more work (horsepower) the engine has to provide.

That’s thermodynamics.


#16

Thermo-what? “The compressor has to be turned by the engine to do the work necessary to remove heat from the cabin. The more heat that the compressor has to remove, the more work (horsepower) the engine has to provide.” Are you sure??

The compressor does all it can do regardless of the temperature. (On most cars anyway).


#17

Everyone must be able to agree the energy loss is negligible if any.

I agree that the compressor is either on or off, and that even cars with what appear to be thermostats do not shut the compressor off if the climate control is cooling. Your cars A/C is not just like your home’s.

I hadn’t thought about the fact that the compressor will have to work less hard to recompress cool air instead of hot air and I think that is true, but unless you keep you A/C on recirculate you would still be pulling hot air from outside. Not to mention the heat loss in a vehicle is so dramatic I’m not convinced the interior air would really be that much cooler.


#18

This won’t cause any real damage aside from unnecessarily wasting fuel and wearing the a/c compressor out faster. With the a/c on and the windows open, you’re wasting more fuel than at any other time. Driving in town with the windows open will save you fuel compared to driving in town with the windows up and a/c on. When you drive on the highway, driving with the windows up and the a/c on will save you fuel as well. If you drive around with the windows down and the a/c on at any time, you’re just plain wasting fuel.


#19

I always run my home’s AC with windows open, the car heater during winter with windows open, and I always leave the drain slightly open when running my bath. The only real problem I have thus far is that people keep trying to get me committed.


#20

Running a car heater doesn’t require any extra power off of the engine… Running the A/c on your house with the windows open is also a waste of energy… You’re putting all that energy into putting cooler air into your house only to have it go out the window, or have hot air go in and negate all the cooling effort.