Heating/AC Question

thanks chap, i missed that. it's good to know the compressor is controlled by fan speed as well as temp. that makes me like my method even more.

 

Yes. There's no free lunch: running AC does impact mpg. And it's a gateway drug lol: next you'll be sitting in a parking lot with the AC running.

 

doesn't everyone do that? imagine a solar roof powerful enough to run the a/c.

 

For those who don't know, that's because an A/C system is designed to maintain a given line pressure for the evaporator coil. Slower air flow = slower rate of evaporation = slower speed at which the pressure drops on the high-pressure line.

I'm not sure about ALL A/C systems, but for many years we basically never used max fan speed because it could blow air across the evaporator coil faster than the air could possibly transfer the heat into the A/C system. You always seemed to get more effective cooling from a slower fan speed.

 

Fan speed also gives you a way to adjust the effect on humidity. A/C always has a dehumidifying effect (water from the air condenses out on the coil, then drains either under the car, or into the carpet if the drain hose is clogged), but for a given rate of cooling, slowing the airflow over the coil results in more dehumidification.

For a decade or two now, higher-end home central systems have had a variable speed blower and a 'thermostat' that also measures humidity, and will slow the fan if extra dehumidification is needed as well as cooling, or speed it up if the humidity is ok and only cooling is wanted.

Prii (starting with Gen 2 if I recall correctly, or maybe 3) do have sensors for cabin humidity as well as temperature, and I'm pretty sure they play the same game when in auto mode.

-Chap

 

When using AC I like to control the fan speed indirectly, leaving the system on auto and adjusting cabin temp. Typically at the outset I'll run the temp up quite high, till the fan's not racing. Then adjust temp down as things stabilize, aiming for a final temp around 24~25C with slowish fan.

 

Pretty much what I do also, with the exception that I don't know how to get my US control panel to show °C ....

-Chap

 

Then you'd lose some fine-tuning capability---unless, for all I know, the car rounds all °F settings to the nearest integer equivalent °C before deciding what to do.

 

With my answer/comment, I was trying to keep it as simple as possible...

 

"... but no simpler" is a traditional way for that phrase to wind up.

-Chap

 

i tried 'auto' yesterday, out of curiosity. it would only send air to the dash, and my feet were hot. every time i switched to face/feet, auto shut off. so i took my shoes and socks off, problem solved.

 

I think there might be a Techstream-changeable setting for whether auto takes over the outlet selection like that. At least, I remember looking through the available Techstream settings and seeing one that sounded like that might be what it does, but I'm not sure because I haven't tried changing it.

Maybe I will, because that annoys me too.

-Chap

 

I think you guys might be overthinking this a bit.

I only use Auto on long trips when the car is already cooled down to where I want it. Otherwise I have to control my own fan/temp/vents to get the air I want in the places I want it.

I don't notice a lot of mpg loss from AC, and I run it May Day through Halloween. Heat, however, keeps the ICE on boil so it eats up MPG some. One of the reasons I specifically love my Prius is that I can get full AC while sitting in the no-idling school pickup line. It's a perk of the system imo.

Then again, I stay cool in the summer and warm in the winter because I didn't spend twenty-some-odd grand on a new car so that I could sweat or freeze. But I'm happy with a 45mpg average so maybe that's just me.

 

That's the way it works.

 

thanks, that's why i'm back in manual. also, it kept it on recalculation, and i prefer fresh air when the car has cooled down.

 

I don't understand "auto shut off". Seems to me if you switch Mode to High/Low that's the only parameter that Auto will stop controlling; it'll continue to adjust fan speed as it sees fit, for example.

That's the most common thing I do btw, setting Auto and thereafter setting Mode to High/Low, to reduce air coming outa the top vents only.

 

the 'auto' light shut off. is it still in auto?

 

auto controls more than one thing - fan, temp, air flow etc.

If you change one thing, the other items will still be in auto

 

even if the 'auto light' shuts off?

 

YES, auto light is only lit if auto is controlling everything. If you override one of the settings, the light will go off.

A good example is this:

Auto mode ON, change fan speed manually (light goes off), but other items (temp, airflow, etc.) will still be in auto mode.

The light is only lit when all of the features are being controlled by "auto". Override any one of them, and the light will go off, however any item that has not been over ridden (changed manually) will still be controlled automatically, (hence auto mode)

I should rephrase though, temp is controlled by the setting you set it at. The system will apply either a/c or heat depending on what is being called for. In full auto the compressor will turn on and cycle as needed varying power as being called for. This has been explained more than once in this string.