Auto A/C in Winter?

No, it will reduce fuel economy. It's your choice how uncomfortable you want to be while driving :_>

 

If you have auto A/C on and it's calling for heat, then the engine will run until the coolant gets above about 62C, even when stopped at a light. Once it warms up it seems to shut the engine off even when the coolant is down to 50C.
This will lower your mileage.

That is the only reason it's less fuel efficient. The heater has no noticeable effect on fuel economy other than forcing the engine to run during warmup.

If you select defrost then the air conditioning compressor will be used to remove moisture from the air in the cabin. That will probably lower fuel economy a bit, though I've never seen any effect, as there are so many -other- reasons you will use more fuel, such as the above warmup, stiffer tires, stiffer grease, more resistance due to running in water or snow, etc.

However, it would be less expensive than having an accident due to fogged or frosted windows!

 

If you haven't switched off the AC in auto mode, it will still run during the winter and reduce your mileage as well. I switch mine off in the winter and there are times when it might be 40F out and raining and the windows start fogging. Simply turning the AC back on usually clears it without going to full defrost mode. (and I'm *thinking* is more fuel efficient, but no guarantees...)

Levi

 

AC on a Prius means "Condition the Air". It will heat it if it below your set temp, and cool it if it is above.

I guarantee you it does not run much when outside air is below freezing temps, much less at -20F.

We have come to think of air conditioning as cooling the air, but think about those words in a broader sense

 

I'm referring to the AC "button" on the screen which allows the operation of the compressor for cooling and/or moisture removal.
I agree that it should run less the colder it gets...

 

Yes, and if it's already cool enough -inside- the car, the compressor will not run. It -ONLY- runs for defrost (a switch selection) or if the inside temp. is higher than the set temp. You cannot force it to run any other way.

"Air Conditioning" or A/C means heating and/or cooling the cabin air. Most "climate control" systems work this way.

 

I would have to disagree with that. I have repeatedly had the system set to auto 65F degrees with the ac off, outside temperature about 40 degrees, typically when it's raining and the windows will start to fog. If I simply turn on the AC the windows will defog. And it's repeatable on a single drive. If I get the windows clear and turn off the ac and the windows refog, turning the ac back on clears them again. That's a pretty sure sign the compressor IS running when the car is HEATING the incoming air and NOT specifically in defrost.

 

No, when you heat air it's dew point rises. The fog on the windshield will disappear just from the heater. The A/C compressor will only run when the conditions I posted above occur.

Did you turn on defrost? THAT will run the compressor (as I posted above).

I see by your info. that you are claiming a 2003 Prius. Is this correct? It's not a gen II? The rest of us are talking about a gen II Prius, as this part of the forum is gen II.

 

Sorry, I hadn't realize I had signed up when I had the 2003 Prius and had that in my profile. I traded up to an 04' a little while ago which is what I was referring to. Though it works the same in either model. Which is to say that I have had both in situations where inside temp is set to 65-70F and it is perhaps 40F outside. heating is definitely occurring. Windows normally stay clear. But typically if it's raining out the windows will start to fog. In EITHER vehicle, pressing the AC button defogs the windows without going into Defrost mode(though defrost does admittedly work a bit quicker since it is aiming the vents at the windshield). And I can also once the windows are clear, turn OFF the AC and the windows fog up again. Turning ON the AC re-clears them.