Stupid A/C question... (any MPG effect of max. vs low setting?)

[PaleMelanesian]

The big answer is, "it depends".

- Some cars mix in warm engine coolant when you set the temperature warmer. Run these on max cool and cycle on and off.
- Some cycle the compressor on and off to get the desired temperature. Run this one at the highest temp you can tolerate.

Of course, the most efficient answer is, "None". Even in Texas.

[slowmover]

You can also "up" the performance of an A/C system by bypassing the heater core, even if for just the summer months. Where I live an A/C system in a car is not optional, and it runs 9-10 months full-time (at least at mid-day). High humidity is a bear to condition.

Proper -- legal -- window tint is of benefit.

Use a windshield cover (interior; keep the vehicle windshield pointed away from the sun when parked year round; OEM-quality); crack the windows if security allows when parked (finger-tip width), and run MAX A/C after opening windows a bit as you get rolling to exhaust hot air, then closing them.

It will take a while to cool more than just the air as the A/C needs time to cool the "furniture" of the vehicle. I say "get it to that point" before switching to FRESH air. (The drier the climate the sooner this will be; humidity takes time to overcome). One can back off the TEMP setting a click or two to reduce compressor cycling, but maintain MAX until nearly blue-knuckled (your clothing is long past dried). This really is the earliest point to change over. Not chilled clothing, but dried & chilled.

FRESH plus an increased temp will take care of it the rest of the day (or until shutoff). It is also when I switch over to combined foot & head airflow (what we used to call the "highway" position). One can -- from hereon -- experiment with further reductions in, first, temp, and second, airflow.

I point most all vents as high as possible, and towards the back of the cabin. This produces the most rapid cooldown, and maintains even sorting (layering) prior to exhaust.

I use a thermometer to measure outlet temps, and this also informs me of how the system is doing as a check against the way I feel (a bit more consistency in operation).

All of this assumes: clean condenser (rare), clean evaporator (look into this, it is vital), proper refrigerant charge (rare), and proper weather seal function (as well as cabin exhaust function). The best long-term operation of the HVAC system trumps a tenth or two. Do some investigating. There are enthusiast sites with the occasionally well-informed membership (rare) and a couple of sites dedicated to mobile HVAC with service techs and others online (better). Some systems vent refrigerant more often than they should, and some leaks are harder to diagnose/find than others. Obviously, living in TX, AZ, and FL one finds the better, more experienced shops.

Another DIY is a separate condenser fan to "up" performance and to keep the compressor from cycling as often. Head pressure can be quite high at startup (hard on whole system).

Today's compressors are nothing in power draw compared to what we used to see on late 1950's thru mid-1970's cars (and why small cars simply didn't have A/C available). On the other hand you could hang meat in those cabins . . none of this bull about 20F below ambient stuff! 119F full sun Las Vegas heat and 62F in the car, ha!

A garaged car, or one under cover, always has better HVAC operation than one exposed to all elements. I pay for covered parking (not just for this benefit).

.

[JRMichler]

I used to do a lot of hitchiking. I hated seeing a late model luxury car pull over on hot or cold days. The worst hot day ride was 97 degrees and humid outside. The driver had the thermostat set to 55 degrees, and the air conditioner was holding to that temperature. BRRRR!

That was balanced by the driver who set his thermostat at 85 degrees when it was 20 below outside.

[khafra]

Thanks for the collection of cooling tips, Slowmover. I'll be taking many of them, but there's one I do a little differently: The air coming out of the vents when I first get into the car in the afternoon is hot. So I roll the windows down and driver for about 90 seconds, after which the incoming air cools to ambient temperatures and the interior air is flushed (even with tinted windows and reflective windshield and rear window covers, it gets hot). Then I roll up the windows and proceed along your guidelines.

[slowmover]

You're welcome. My Dad started driving in 1939 and was one happy guy when he moved to Texas and could also afford a new car with a SEARS underdash A/C unit (a '54 Ford). From him I learned to "set" the vents . . and never move them again.

I agree with wanting to flush out the interior stale air, but have found that it can sometimes work and sometimes not depending on both local traffic/route, plus whether a dry or humid climate. What worked for me in 108F Dallas is a bit different than 94F Corpus Christi with high humidity (and, even among humid areas it can differ, ha!) I sometimes crack a rear window to accelerate airflow, but only if moving steadily.

It's worth a little trouble to find the cabin exhaust vents to be sure they're clean; no torn weatherstripping, etc.

If the system is a few years old, have a shop hold the system under vacuum (500-microns; or 28") ALL NIGHT. Sometimes a small leak will show up this way. A thirty minute check is only JUST adequate. There are more details to learn for a Florida boy on some of the mobile HVAC sites for best performance.

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[KillerBeeZ]

For those who are interested ... Mythbusters tested the windows down vs AC thing and found that running the AC with windows up is much better than no AC and windows down. While I don't agree with Mythbusters on many of their techniques (sometimes they don't use scientific method) I do agree on this one as their science was sound.

Another was with pickups, its best to have the tailgate closed and not use one of those nets or leave the gate open as the truck is designed to have better air flow when air cycles in the bed. I remember they used a bed cover as well and if memory serves even that hurt gas mileage. This does not include boat tails however.

just my 3 cents

[California98Civic]

Originally Posted by KillerBeeZ

For those who are interested ... Mythbusters tested the windows down vs AC thing and found that running the AC with windows up is much better than no AC and windows down. While I don't agree with Mythbusters on many of their techniques (sometimes they don't use scientific method) I do agree on this one as their science was sound.

Did their test run with the windows all the way down? Did they try the fan option with windows narrowly cracked? And most importantly did they remove the AC belt? Even turned-off, the AC's compressor belt is a parasitic draw on the engine. I run dark metalized heat reflective tint, sheep skin seat covers, the fan, and slightly cracked windows. It is July and humid and I'll be fine (but I have almost never used AC, even when I lived in Virginia's tidewater humid zone).

[brucepick]

On the '96-2000 Civic it seems the compressor stays on as long as the "A/C" button is activated and the fan is running (it lights up green). Adjusting the temperature slider only seems to add heat from the engine coolant and leaves the compressor running. It's interesting to know that some other cars still use a thermostat to switch the compressor on and off like all our other older cars did.

When I do need the a/c either for cooling or for keeping the glass clear, I run the system on recirculate and cycle the compressor on and off via the "A/C" button. It's very rare indeed that I run the compressor without the "recirculate" setting switched on.

If only keeping the glass clear, I find it doesn't need to run much of the time, maybe 10-20%. If it's truly hot out the percent on time can definitely go higher. Mostly I can run the fan on speed "1" but sometimes I want more.

Like others wrote, I try to have the compressor on for downhills and off for uphills but that doesn't always work out. In those situations, I remind myself that I'm still getting probably 2x the mpg of most passenger cars and even better when compared to all the pickups and SUVs.

I also use a collapsible sun shield. If I need to get hot air out of the car after it's been sitting, I'll drive a couple minutes with windows open and compressor off

[Joenavy85]

Originally Posted by mcrews

First rule of thumb (imho) NEVER SUFFER!!!

Agreed, the physiological stress on your body due to higher temperatures can effect your reaction time and alertness. I know that when I'm driving that I get sleepy when it warms up in the car, but when it's cooler I'm more alert. That is enough of a reason for me to use the A/C as much as I feel I need to.

[FSUspectra]

Originally Posted by KillerBeeZ

For those who are interested ... Mythbusters tested the windows down vs AC thing and found that running the AC with windows up is much better than no AC and windows down. While I don't agree with Mythbusters on many of their techniques (sometimes they don't use scientific method) I do agree on this one as their science was sound.

They found this using an Expedition I believe, probably with a V8. For an engine of this size, yes, it is probably more efficient to use the A/C... the engine barely notices it at speed.

In a small, 4-cyl, A/C use puts a larger ratio of load onto the engine, so this curve of more efficient for windows vs. A/C likely changes. A/C takes a HUGE chunk of FE away at ANY speed, and is very noticeable on the highway, whereas windows down doesn't have as big an effect.

I also don't use MAX A/C because it causes the compressor to run constantly. If I have it to the "upper vent" position, you know, the picture of the seat with the arrow, and turn on the A/C, the compressor cycles itself at some point. I still self cycle at a higher temp though. Temperature dial is always at cold, because the compressor still runs, but heat is added from the coolant, as mentioned before, and is a waste.