Heating/Cooling System

You are on the right track. All you need to do is set the temperature to "LO" instead of 70º. I use this setting about 90% of the time around here. By setting it to 70º it will still try to maintain that temp by varying the outlet temp. If the outside temp was 45º and you set the temp at 70º my guess is the interior of the car was lower than 70º that and it was trying to bring the temp up by introducing warmed air. By setting the temp to "LO" with the a/c off you force it to provide outside air no matter what the temp is.

 

Your post reminds me of how frustrating I find the default that turns on the A/C compressor whenever the fan is turned on. Who thought that was a good default given there is already a Auto A/C button? If I wanted the A/C compressor with the fan, I would push that button. Annoying...

 

The concept is that it's a set-and-forget system -- you just pick the target cabin temperature, and the car takes care of the rest. Even if the A/C button is enabled, the compressor will only come on as needed.

 

It doesn't turn the compressor on just because the button is illuminated (yellow bar or whatever). That just means it will use the A/C compressor, i.e. COOLING if your requested temp is lower than the current inside temp. I think the exception is defrost, that should always have the compressor on to pull moisture out of the air.

Auto A/C does NOT mean Auto Air Conditioning the way we in the U.S. think of A/C - i.e. cooling. It means Automatically conditioning the air. The computers will set the vents and fan speed, heat or cooling depending on your requested temp and the outside temp.

I wish the settings were modifiable. For those of us in colder climates, it should be doing floor/defrost anytime the temp hits 40 or lower. It WILL get to that setting, but at a much lower outside temp.

 

Yes your explanation for how it works is accurate and irrelevant to my point. My point is that there is a button that sets the AC (and also heat) to come on as needed based on a set temp (called Auto/AC). So the default for the fan button should be to run just the fan at the set speed level REGARDLESS of the temp setting. That's not what it does. For fuel economy reasons (and other reasons) sometimes I don't want the compressor nor the heat (as both require the ICE to run more) but just to have some air circulating around the car and/or some white noise. Its just annoying that I have to explicitly set the AC system off after I set the fan speed to get just the fan.

 

Don't worry about short-cycling, Tony. For all the worrying and thinking they did, you think they'll let you hurt the compressor just by doing that?

Actually the problem with short cycling is the condition where you try to restart a compressor that's run in the last few minutes. It needs time for the pressures in the system to equalize before the compressor can start again. For residential/commercial AC systems, they usually prefer 3-5 minutes of rest before trying to restart. The risk is the "locked rotor" state, where the motor tries to start but encounters so much physical resistance that it can't- it gets hot very fast like that.

On the other hand, many automotive-type AC compressors do their capacity modulation by cycling the compressor on and off every 10-30 seconds anyway. The Prius doesn't do that as much as traditional systems, because it can vary the RPM of the compressor motor itself to adjust its output, but under very light load conditions the compressor motor can't go slow enough to reach the desired (small) level of output. Under those conditions, with ideal acoustics (sitting at a red light, with my hyper hearing) I can hear the compressor cycle on for maybe 1-2 seconds, then rest for maybe 20-30 seconds. I don't know if it's in print anywhere that this happens, but I've been paying attention to my car for long enough that I'm sure it's happening. It may only be to provide dehumidification, but it has a clear purpose in mind for running in light load conditions, even when it's not apparent. When it's cool AND dry, I don't hear the compressor cycling anymore.

There is such a thing as a "short cycling" problem in automotive A/C, but that's usually a sign that the system has lost most of its refrigerant charge. In those cases it will cycle on for about a second, then off for about a second, repeating endlessly.

So [back to the main thread topic now], after going to the effort for some time of manually turning the A/C off when I felt it was unnecessary, I've stopped making the effort. I use Auto almost exclusively. Toyota has already optimized the system for efficiency. In the conditions that I was turning it "off", it was usually already off in reality anyway. The important things to remember: the temperature you set on this system is your commanded cabin temperature, not a command of what temperature should blow out of the vents. And the A/C light on the Climate screen has to do with whether or not the computer is allowed to use the A/C if it thinks there's a reason to. The light being on doesn't necessarily mean that the computer is actually going to actually use the compressor, just that it has permission to do so.

 

I have a 2005 Prius. The first two winters ( MPLS, minnesota) I was very impressed with the heat coming out at cold start up. After about two miles, warm/hot air was warming the cabin. I now have 100,000 miles and had a coolant change by a local dealer. Now, It takes forever to warm up. My commute is about 30 miles at highway speeds. The cabin temp struggles to reach comfortable range at 20 miles into the commute. I am taliking about 5 to 10 degrees above zero at atart up.

 

It's possible that the dealer didn't purge the air properly. If there's air in the system, it may block coolant from circulating through the heater core.