For optimum FE, a/c manual or a/c auto?

[JEFNLSA]

- AZ Prius -

Upon buying the car a few months ago, I set the a/c on 70* (manual) and it's been that way ever since...never shut it off, fan set @ LO and occasionally 1 or 2. Works great this way and the wife & I are always comfortable. What has me wondering is; seems to me, after perusing many threads on the subject of a/c, that a majority of you use the auto mode. Now my question is, from a fuel economy standpoint, would it be more beneficial for me to run the a/c in auto mode...and what is a safe temp to run it at before killing my MPG?

FWIW...my avg MPG currently is around 50. Could I up that by switching the a/c to auto?

TIA,
Jeff

[N3FOL]

You will notice that as soon as you turn on the AC and put it on Auto AC, the fan will blow almost to maximum. In order to prevent that, I usually run my AC at a low blower setting...after a few minutes of driving and the interior starts to get cooler that is the best time to switch your AC to Auto mode. My thermostat is also set at 70°. MPG is at 53 right now.

[qbee42]

There isn't one clear answer to your question. It's mostly a matter of the temperature setpoint in relation to the ambient conditions. You can get pretty much the same efficiency running in manual or auto. If you are very careful about settings when using manual, you should be able to save a little energy, at the expense of being slightly less comfortable. If you stay at the same level of comfort, auto and manual should work out the same. If you are careless with either it will cost you, but not a lot.

Tom

[JEFNLSA]

Originally Posted by qbee42

There isn't one clear answer to your question. It's mostly a matter of the temperature setpoint in relation to the ambient conditions. You can get pretty much the same efficiency running in manual or auto. If you are very careful about settings when using manual, you should be able to save a little energy, at the expense of being slightly less comfortable. If you stay at the same level of comfort, auto and manual should work out the same. If you are careless with either it will cost you, but not a lot.

Tom

That's what I was thinking. It's just that a vast majority of you seem to run in auto-mode and I guess I've just been wondering why...? I tried the auto-mode and actually found it annoying, as the fan is on high on startup and keeps kicking in to maintain the pre-set interior temp. In manual, you just set it on a particular temp & fan setting and leave it.

[qbee42]

Originally Posted by JEFNLSA

That's what I was thinking. It's just that a vast majority of you seem to run in auto-mode and I guess I've just been wondering why...? I tried the auto-mode and actually found it annoying, as the fan is on high on startup and keeps kicking in to maintain the pre-set interior temp. In manual, you just set it on a particular temp & fan setting and leave it.

Many of us run in auto mode because it's easy and does a good job controlling the cabin temperature and humidity. All I do is bump the temperature up and down as needed, which is one step less than your manual method, plus it adjusts for changing conditions automatically.

Tom

[JimboK]

Originally Posted by JEFNLSA

That's what I was thinking. It's just that a vast majority of you seem to run in auto-mode and I guess I've just been wondering why...? I tried the auto-mode and actually found it annoying, as the fan is on high on startup and keeps kicking in to maintain the pre-set interior temp. In manual, you just set it on a particular temp & fan setting and leave it.

Same as Tom. I can easily control it from the steering wheel and don't have to fiddle with the MFD.

[Tideland Prius]

I'm leaning towards AUTO. The reason is that it can toggle the A/C on and off when it's necessary. In manual, it's always on so you either have to toggle it yourself or keep moving the temp up/down to get a comfortable temperature.

[N3FOL]

Originally Posted by qbee42

Many of us run in auto mode because it's easy and does a good job controlling the cabin temperature and humidity. All I do is bump the temperature up and down as needed, which is one step less than your manual method, plus it adjusts for changing conditions automatically.

Tom

That is a good idea and never thought of that. The next time I turn on my AC, I will put it in AUTO and just increase the set temp. so that the fan does not blow to MAX. Then I'll incrementally lower the temp setting after a few minutes of driving. Hope this works for me.

[rak]

I wish there was a wheel-mounted control for the blower speed. I cannot tolerate the high blower speed on auto - and making the temp higher just feels both too fast and too warm.

[efusco]

I used to use manual settings, but have converted to auto over the past year or two. BUT....
1) When the cabin is hot (after being parked in the sun) I start with the AC off (set to vent (max cold, fan low, no auto, no AC) with the windows down.
2) I drive for a short time while the car works through S1 and S2 (initial warm-up stages) with the windows down allowing the air flow to reduce the cabin temp from the 'very hot' to ambient.
3) Once initial warm-up is done (~1 min), the cabin has equilibrated with the outside world (which I believe happens just as quickly this way as it would have with the A/C but at ZERO energy cost) I turn the AC to Auto at 78F, then roll up the windows.
4) ~80% of the time I'm comfortable enough with this 78F auto set temp, the fan tends to not run crazy, but sometimes it's a little fast for a short while until the cabin temp comes down. If necessary I'll bump the temp down a degree or two.

The closer you have the A/C set temp to the outside temp the less the AC will be used. While not "cold" the 76-78F temp range can be quite comfortable unless you happen to be directly in the sun for an extended time.