How does A/C work on the Prius?

I need to clarify A/C operations. This post says "Auto controls two things, which can each be auto while the other is not; fan speed and air outlets." "As far as I'm aware, it will only use A/C if the A/C button is lit, or if you are using the defogger switch. Otherwise, it will only use the fan."

I am new to the Prius and it seems the Auto mode is not like other cars, meaning cooling-A/C temperature is not controlled by Auto. I just want to confirm that in order to use the air conditioner, you MUST press the A/C button.

Seems like a simple question that is certainly not clear in the owner's manual.

Thanks.

 

See my answers in blue. Hope this helps.

 

Thanks for the clarification. My wife was driving the car in Auto mode with the temperature set to 70 degrees and said the car was not cooling off. She did not have the A/C button pushed. It sounds like what she needs to do is hit the A/C button, leave Auto mode on and turn the temperature up a bit. The car will then cool to the desired temperature.

 

Confusion comes from overloading the term "A/C". In the case of the Prius, Air Conditioning is used in the literal sense, meaning all heating, cooling, and dehumidification of the cabin air. Unfortunately Toyota also uses the term in the more conventional sense to mean the A/C compressor. The A/C button should be labeled "A/C Enable" or something of that nature. When it is off, the compressor is disabled and no cooling can occur. When it is on, the compressor will run as needed to cool and dehumidify. Note the "as needed"; the compressor does not run continuously when enabled. It runs only when needed, and only as fast as necessary.

Tom

 

In case it hasn't been pointed out, there is one thing that can be controlled while in full auto mode that won't change back to default the next time the system is powered on and that's the intake/recirculation function. Due to constant humidity most of the year or in traffic, we leave ours on recirculation. In the winter when it's less humid, I'll let it "decide".

 

Two things: the temperature setting is also maintained.

Tom

 

As I don't have a Scanguage, does anyone know if during hot weather, if I manually set the fan speed to maximum speed to create a windchill and and set the temp higher (say 82F to 84F) to supposedly reduce A/C compressor load, is this a good way to improve mpg in the current hot weather (100 F outside) in the mid-west. I would only do this set up when I'm alone in the car as I can point all the vent towards my body.

What I hope is not happening is that when I increase the fan speed that this tells the compressor to increase output and then with the high temp selection blends with heater air to give desired temperature. Hopefully only setting it to "HI" would be the only situation where the blending of heater air would occur.

I would rather on hot days that the "AUTO" setting would increase the fan speed instead of lowering it when switching to ECO mode while still running the compressor economically.

 

I just installed a Trane Heat Pump unit that has a two-stage variable speed compressor and variable speed fans. It only uses the second stage of the compressor when the first stage cannot maintain temp setpoint. It does a great job of maintaining temps, even in this 90-100F weather.

I have only had it for about three months now but am already enjoying considerable savings over the old unit that it replaced. Of course I paid a premium price for this highest SEER unit but expect it to pay for itself over time.

 

I run my car in ECO mode and leave the AC controls set on Auto and 74F. The system will run the fan at top speed until the cabin begins to cool down. It does an excellent job for me and lowers the fan speed about the same way that I would do it manually.

 

No conclusive test, but had a commute yesterday that was about 10mpg's lower than normal for the inbound commute according the MFD. Temp was 98 degrees, with humidity making it feel like 102 degrees. Fan was set for two bars and temp set for 83 degrees for the AC.

Other times I have tested the Auto AC mode with 5 to 10 degrees below actual air temp I noticed only a few mpgs less than without.

 

I don't believe the car's onboard computer has as much control of the A/C system if you take it out of AUTO mode. If I keep the A/C in AUTO mode, temp set to 75 degrees, and car in ECO mode, I only seem to take a 2-3 mpg hit.

 

W

Well I think I answered my own question. After careful listening I could make out when the compressor cuts in. It's a very quiet type (no clutch sounds). I'm almost certain that this is the way to both get max mpg and feel comfortable on a hot day. But I will try this again when our next heat wave comes next week. Both the wind chill effect and the lower delta T in the evaporator make for more efficient A/C. The downside may be higher noise, but you can go one bar below max fan speed and it's not so bad. I set it at 84 early on and I find 83 or 82 F pretty comfortable with the high fan speed. And my mpg when back up by 4-5 mpg from 50mpg in my early testing.

Could some other Prius Gen 3 owners especially in the the South West replicate these results?

 

Don’t forget, it’s not just about your comfort. The cooling air for your Traction Battery comes from your cabin air. I wouldn’t want to risk overheating it for a couple of MPGs.

 

Thanks for pointing this out.. I wonder if I put an ice pack in front of the battery air intake in the back seat for my 45 minute commute, would that work?

 

Condensation water may leak and short out something.

 

You would have to have 110F temps with the windows up to run that risk.

 

You need to monitor your battery temps with the SG

 

My SGII is outdated and will not allow buf for the GenIII. With Torque I have never observed anything higher than 106F battery temp even when it was 106F outside and 114F inside. With cross flow venting the inside temp drops and so does the battery temp. So in most cases when it is in the mid to upper 90s I can only get the interior temps down into the mid 80s with the AC on for 30miles. Thus the difference in battery temp with and without the AC for most drivers would only be about 10-15F. In extreme temps over 100F it could obviously be worse and that is where caution should be observed. I just don't buy the argument that you'll kill the battery in average summer temps in the 90s. If your interior is getting to 120F or higher while parked then you should look into methods to keep it cooler.

 

Feel free to swelter in your car if you wish, and let your Traction Battery suck in hot air. I’m not telling you that you can’t, I’m just giving you my opinion, the choice is yours. I have spent the majority of my life caring for electronic equipment, including batteries of all kinds, and if there is one thing that I can take away from all that experience, it is that heat is the enemy. It destroys electronics and shortens battery life. Too much heat isn’t all that good for human life either. How many miles per gallon will you lose as your Traction Battery loses capacity from being too hot for too long? I’m not saying I know the answer, I’m just saying that it is a valid question. I guess, what gets me; is I see so many posts here about how hot folks let their cars get, just because they don’t dare turn on the A/C because they will lose 2 or 3 MPG, without ever a thought to what that extra heat is doing to their Battery. If it’s 84 degrees for you with all the vents blowing on you, how hot is it at the back seat air intake for your Traction Battery? The first aftermarket purchase for FRED will be a Scangauge, mainly so I can keep an eye on the health of that Battery. The way I see it, the Traction Battery is really the heart of the entire hybrid system. Maybe I’m being overly cautious, but I would rather be overly cautious than not cautious enough. In my line of work I have seen so many pieces of equipment fail, not from over use, but from lack of adequate cooling, just because the company didn’t want to spend a few extra dollars for the cooling fans that were needed, and I guess I see a parallel between that and refusing to run the A/C because of the few percent drop in MPGs. Maybe 110 degree air is just fine for your Traction Battery, but from my experience, I doubt it. Please don't take offense, I don't intend my comments that way, it's just that overheating equipment is a pet peeve of mine. I’ll get off my soapbox now and return you to your regularly scheduled thread.

 

Have you ever monitored the battery temp while driving in heavy, start/stop traffic? That is where I see the battery temps going highest. The constant start/stops "heat pumps" the battery as Bob Wilson describes.

If I have the cabin cooled down with AC prior to getting to town and getting in to the start/stop driving the rate of rise of temps is not as severe as if I do not have the cabin cool. That is the biggest point that I see for having the cabin cool with the AC versus not.

Also, after the battery gets heat pumped and then I stop at the house for an hour or so and go back out to the car to go to dinner the temps of the battery have continued to rise. I have seen the temps around 100-105F when stopping and then have seen it around 115-120 after sitting for an hour.