Question About Climate Control

The term A/C is in certain contexts synonymous with auto climate control. Auto-A/C certainly is.

The system can still be in auto mode, even if you override the recirc or A/C compressor setting. I believe you can even change the vent setting too and maintain auto.

You are correct, if you turn off the A/C compressor, it obviously can't cool down the car any more than the outside temp. It can still heat however, as well as change the fan speed if the interior is hot and the outside air is sufficiently cool.

Note that the A/C compressor is variable speed, so it is best to just leave the climate control on full auto and set the temp setting to your desires and just leave it. It can run the A/C compressor and fan speed so there is dehumidification with minimal cooling if such a situation warrants, as it does have a humidity sensor in the same package as the cabin temp sensor. The overall system is very efficient. Sometimes, if outside temps are sufficiently cool, I may turn off the A/C compressor to get a bit more MPG.

 

The "AUTO A/C" light and button are misleading. They actually denote "automatic climate control" generally, rather than the air-conditioning function. "AUTO CLIMATE" would be a better label.

Normally, the AUTO mode will activate the air-conditioning when it deems it appropriate for cooling or dehumidifying.

However, on the touch-screen, you can specifically disable the air-conditioning function, while leaving the rest of the automatic climate control on. This restricts the system to attempt to maintain temperature using only heated or fresh air without air conditioning. In this state, the "AUTO A/C" light remains lit, because the automatic climate control is still active.

 

Personally, I despise any Auto-Climate control. Ideally, I would like to set it at 70 degrees and if the inside temp is anywhere near 70, it doesn't turn on. There should be a range of several degrees. If I set it to 70 and the car is 72, I really don't need the A/C to come on!

Also, for some reason, the default air source on the Prius is "recirc". Unless I'm driving through a dust cloud, I NEVER, want anything but fresh air. For example, I get into my hot car and it's about 90 degrees inside, while it's 75 degrees outside. I turn on the A/C and it switches to "recirc". It would be much more efficient to use outside air - it's 15 degrees cooler!


Grrrrrr.

DGStan

 

There are other features of the climate system to give a better 'feels like' temperature. For instance, did you notice the little button on the driver's side of the dash by the windshield about where the VIN label is? That senses solar radiation, and adjusts the climate control to compensate for the radiant heat you feel pouring through the windows. The less you play with the climate, the better, unless you are really sure you can do better.
sometimes I do turn off the AC, and for temperature alone, that would be fine, but we usually have high humidity so that might make it feel worse.

 

Every "automatic" system I've seen has "deadband" of around 2 degrees up to 8 degrees, whether automotive or home.

As an example, at my hobby farm I have a Bryant Evolution Plus 90i gas furnace, a Bryant Puron Plus 2-speed A/C, a Bryant HRV, and the Bryant Evolution home control network. Evolution uses an Outdoor Temperature sensor to fine-tune the PID control loop.

[Note: the Prius also has a sensor, it's on the dash next to the auto headlight sensor.]

The system is really efficient especially when heating, since it compares the indoor temp to outdoor temp and knows how often it has to fire to ramp the temp. It also knows when to use low fire (35,000 BTU) or high fire (60,000 BTU) based on that sensor input.

As an option, you can set fully automatic changeover from heating to cooling and back. The default deadband is 4 degrees, you can select 2 degrees (Not recommended by Bryant) or 8 degrees. To minimize system cycles, you should specify 8 degree deadband.

Now that would mean if I specify 77 degrees for cooling, heating wouldn't happen until 69 degrees or colder. If I specify 75 for cooling, 67 or colder before heating happens. Most folks wouldn't accept this due to the large temp swings.

A 2 degree deadband would help with the comfort level but you'd introduce a lot of cycles to the system, which at the very least increases your utility bills. Also wears out the system faster, which you can prove as the Evolution network tracks system cycles.

Since we worry a lot more about heating than cooling up here, I manually select the modes for my home. When I'm away I set it for 65 heating. When I'm there I engage the auto setback mode (73 daytime, 67 night, switchover times up to user selection).

For cooling, I manually select cooling only when at the home. Usually 75 degrees. Since it usually cools off to 70 or cooler on even the hottest nights, just having the HRV run at night will generally keep the temps below 80 while I'm gone.

For the Prius, I also find the auto climate mode objectionable. After playing with it a few times, I've resigned myself to manual mode and temp selection.

 

Stan,

If it is 70 degrees in your car on a sunny day and the AC isn't on, it will likely be 80 in the car in 10 minutes due to solar heating unless you have the windows open. I know if I shut down the AC it heats up pretty fast in there.

Also, I've noticed that in my car it only starts out in recirc for a few minutes, then switches to outside air as soon as the AC gets the car close to target temperature. Cooling recirculated inside air is faster and quite a bit more efficient than cooling outside air. Way smarter than I expected.

 

I'm sure this is going to sound like the dumbest question in this thread... but how would one (assuming one wanted to do so) simply activate the vent and fan? Is it as esoteric as matching the temp inside with what's outside, tapping the vent button, and activating the fan?

Again, I apologize for my ignorance... just trying to learn about my new car.

(PS Thanks for the rest of the great tips already posted... very interesting stuff.)

 

If you want to guarantee the car won't add heat, set temp to max cold with A/C compressor turned off.

I had set my car to 'vent' yesterday evening by setting fan speed to next to slowest, and turning off A/C. Temp was set to 72-74. This morning, it was a bit cool out, and soon I was feeling heat come out of the vents. What in the world? Then I realized the problem, set temp to max cold and no more heat.

I do have to remember to set the temp correctly this evening. I will probably need the A/C as it is supposed to be in the 80's this afternoon.

 

A lot more complicated than the old vent wings!

 

I just can't get over how darn smart this car is!! I had wondered about the A/C too as it was warming up the car but I figured it was doing what I wanted and I wasn't going to get too concerned about whether it said A/C or not....

 

In a home air conditioner, the "dead band" operates lilke this: Set for 75: AC starts when temp reaches, say, 77 (assume +- 2 deg deadband) and runs till temp gets down to 73. The heat cycle comes on when temp gets down to 73 and runs till temp gets up to 77.

What does the Prius Auto Climate Control do, fully activated, both heating and cooling?

In particular, I'd like the "dead band" to have a low limit for the heater and a high limit for the cooling,, each with a "dead band". Is this possible?

 

Don't feel bad. I was just logging on to ask the same " dumb" questions. I've had my car for a week during lovely spring weather here in Delaware (USA). Yesterday was the first time I needed some heat. The manual doesn't even talk about cabin heat. Just something about climate control. The newest car I had before the Prius was 14 years old, so the concept is new to me. Where are the sliding levers with the red and blue colors? (rhet. question).

My brother, who has had climate control for years assured me that it is just like the thermostat in your house. Set the temp and forget it. That seems to work just fine, but I like to mess around with any adjustments that I can. When the novelty wears off, I can just set it on auto and enjoy the ride.

Thanks to all for the explanation. This car is fun!

304 miles, 49.2mpg. have not filled it up yet. Still on the dealer's tank

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The climate system uses fuzzy logic, and has several output variables (fan speed, compressor speed, heat amount, vent selection) as well as the multiple input variables (outside temp, inside temp, inside humidity, temp selection), much more than your typical home HVAC. I am sure there are times when A/C is running while heat is applied, just as you often do in a conventional HVAC with A/C on/off (one speed), with a manual control for heat. You did know that inthe conventional car, that blue to red dial was only controlling the heating core valve, didn't you?

 

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The lever with the blue and red markings, in the old obsolete cars is sort of like pulling your chair closer to the fire. Yesterday I put the system in auto, turned the air conditioner compressor off and set the temp for max cool. the outside air temp was around 61F and the cabin was a little hot from sitting in the sun. The fuzzy system opened up the fresh air. Is this car cool or what?

I gotta stop messing with things while I'm driving.