A/C on or off?

<div class='quotetop'>QUOTE(Dano @ Nov 17 2007, 02:04 PM) [snapback]540976[/snapback]</div>

Well, if you do that, and theres a lot of moisture buildup and you need to defrost the windshield, ac may come in handy...


on the other hand, it makes "damn sure" it doesn't think about turning on the AC and eating energy.

 

<div class='quotetop'>QUOTE(Dano @ Nov 17 2007, 09:04 PM) [snapback]540976[/snapback]</div>

The system is still automatically regulating the temperature and deciding where to send air, it's just not using the compressor to cool air anymore. I often use this setting because not using the compressor gives 'cold parts' in there a chance to dry (no more condensation), preventing the typical nasty A/C smell.

To keep the compressor functioning, it's best to switch it on every week or so for a while.

<div class='quotetop'>QUOTE(Bob64 @ Nov 17 2007, 09:33 PM) [snapback]540987[/snapback]</div>

The compressor automatically switches on when you use the front windshield defroster. Not using the compressor actually reduces moisture buildup, once the condensation in there dries up.

 

I have started manually disabling the A/C like this at times during the winter here in Georgia. I assumed that since the ventilation in the Prius was designed for maximum energy efficiency, and the system is actually designed to measure and control cabin humidity (not just temperature) that during cool, dry conditions it wouldn't run the AC compressor at all- after all, no need for cooling and no need for dehumidification. But I have gotten to where I can actually hear the AC compressor every once in awhile when sitting at a stop with the ICE off. I know it doesn't take much power to run it under those conditions, but it's providing zero benefit, so it's kind of un-Prius that it behaves that way.

It seems like even though the electric compressor is variable speed, at very light demand conditions like this it cycles on and off. So anyway, once it gets down under about 40 F and it's already plenty dry, I manually disable the A/C if I think about it, and sometimes the subtle sound of it cycling reminds me to do so. Once the humidity starts to return, I let the car have full control again.

Afterthought: On most cars with automatic climate control, they don't monitor cabin humidity, so they run the AC compressors down to about 40 F ambient just in case you might need some dehumidification. So in lots of such cars there is some efficiency benefit to disallowing the climate control to run the AC compressor when it's not providing any benefit to you. The Prius does monitor humidity but doesn't seem to cut back the A/C quite as much as it could when it's cool AND dry.

 

So what is the approximate mpg cost of leaving the compressor on when ambient temperature is pleasant? Anyone have a clue?

 

Based on the Prius MPG Simulator, it can be almost 10% depending on speed. At 25 MPH with 75F outside air temp, MPG is 97 without climate control and 88 with (cabin temp set to 75). At 35 MPH it's 84 without and 78 with. At 55 MPH it's 60 without and 58 with.

These assume steady speeds, a fully warmed ICE, level terrain, and no winds.

 

Thanks Jimbok.

That's a significant mpg bite!

I'm still confused about the discussion of how to keep the compressor off. I understand setting it to OFF with the steering wheel button, but what happens if one then touches A/C and/or AUTO on the MFD?

My brief experiments last night were inconclusive - I have not been able to hear the compressor turn on/off; old ears? (but I did not take some factors into consideration, such as temp setting and ambient temp - obviously important). I did detect differences in the ICE running or not - shut off A/C and ICE stopped while parked, turn on and ICE starts again)

Charlie

 

I was driving home tonight (after reading this thread earlier) and noticed that the car was blowing cold air at me despite the cabin temp set to 68F with outside air well below that. As soon as I turned off the compressor via the MFD, warm air returned and the right temp was maintained. So if nothing else the advantage is that you won't wind up with cold hands!

Why the car does that is beyond me.

 

The A/C is the only thing that bugs me about the Prius. I have only had mine for 3 weeks and I am still in my learning curve.

1) I hate the fact that sometimes when I hit auto, the air goes straight to my feet only and the fan speed can start anywhere from the lowest to the highest setting. The setting also never changes unless I change it then the auto turns off!

2) I hate that the display shows A/C when I use auto.

Overtime, I am sure I will get used to how it works though!

 

Note that the compressor is driven by its own electric motor and does *not* run all time, even when "A/C" or "Auto" are highlighted; it runs only when needed.

The best thing is to park somewhere safe, leave the car ON and experiment with the controls until you see what does what.