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Old 05-16-2013, 04:44 PM   #1
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Running Roof A/C while driving

I read this on the BetterRVing website. I found it hard to believe. I'd appreciate other opinions, please.

"It was interesting to learn that it is less costly and more fuel efficient to turn off the dash air conditioner and run the generator and roof air conditioners while driving."
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Old 05-16-2013, 04:58 PM   #2
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Probably a tough claim to prove, but I bet it would be close.
The roof air's are going to do a much better job of cooling the entire coach.
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Old 05-16-2013, 05:01 PM   #3
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It's possible the generator might use less fuel than the main engine's A/C compressor, but if would have to be a real efficient generator. Now if they're talking turning off the dash A/C when using the generator and roof A/C, then it might be correct. Using both definitely uses more energy, but the dash air cools around driver and the roof air doesn't do as well in the 'fishbowl' front of most Class A's.
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Old 05-16-2013, 05:29 PM   #4
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Using both definitely uses more energy, but the dash air cools around driver and the roof air doesn't do as well in the 'fishbowl' front of most Class A's.
Ain't that the truth. I've driven school and transit buses where the passengers are complaining about how cold the a/c is, while I'm roasting like an ant under a magnifying glass. Being behind all that glass with the sun beating down isn't an easy thing to overcome.

I had a friend who kept a cooler full of frozen towels, and she'd one draped over her lap while driving. Probably the best solution for the "fishbowl effect" I've yet encountered.
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Old 05-16-2013, 05:40 PM   #5
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We use our roof airs just about all the time when we're on the road. Don't know about the claim for better fuel efficiency though. We find it does a much better job then the dash air. We turn the fans over the windshield on and it circulates the air great.
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Old 05-16-2013, 05:43 PM   #6
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1. We don't hesitate to run our roof ac(s). It obviously keeps the coach cooler and gives the generator much needed run time which a lot of rvers forget about.
2. We set the dash ac on max cool only when in use as the other mode is basically worthless.
3. We often use the fans over the front windshield to assist in the air circulation also.
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Old 05-16-2013, 07:54 PM   #7
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Quote:
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We use our roof airs just about all the time when we're on the road. Don't know about the claim for better fuel efficiency though. We find it does a much better job then the dash air. We turn the fans over the windshield on and it circulates the air great.

This works well for us. I just run the front A/C unit with the bedroom door closed though.
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Old 05-16-2013, 07:57 PM   #8
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It's entirely possible that the roof air units are so much more efficient than the engine driven A/C that it makes up for the generator inefficiency. The roof units are getting full outside temperature airflow on the roof at a good high speed. The engine driven A/C has to compete with the engine radiator so it can't remove as much heat from the freon as it would like. The roof A/C compressor and coils are pretty well matched for size. Often times the chassis A/C units are a mismash of parts that aren't well suited for each other but do work. Automotive A/C systems are not designed for energy efficiency because they have almost unlimited input power (tens of horsepower at least) to the compressor. Your rooftop A/C units have to be able to deliver 15,000 btu cooling with under 2 horsepower input power each (~1500 watts).
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Old 05-16-2013, 08:08 PM   #9
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Our diesel gen takes .72 GPH at full load (8KW), running both heat pumps only loads it to about half that so, in our case, it probably is more efficient to run the gen. In fact our dash air quit working a couple years ago and I haven't bothered to get it fixed. When I turn the dash air on the compressor clutch never engages, probably out of fluid (I want to say Freon but it's not anymore).
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Old 05-16-2013, 09:46 PM   #10
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Quote:
Originally Posted by BFlinn181 View Post
It's possible the generator might use less fuel than the main engine's A/C compressor, but if would have to be a real efficient generator. Now if they're talking turning off the dash A/C when using the generator and roof A/C, then it might be correct. Using both definitely uses more energy, but the dash air cools around driver and the roof air doesn't do as well in the 'fishbowl' front of most Class A's.
Just made the trip back from Houston to Vegas doing a good bit of the driving into the sun. Tried running the generator with both A/C units and we were miserable. At our noon stop, I shut them down and went with the dash air.
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Old 05-17-2013, 10:19 AM   #11
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We run our roof air probably 9 month out of the year. Like the above poster said, our dash A/C quit several years ago and we now only use the defroster in cold damp winter. We turn a couple vents toward the front seats and usually get quite comfortable. Of course we are not driving in the desert, just the Southeast.
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Old 05-17-2013, 12:22 PM   #12
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Genny & roof air for me... been that way since I started when it was explained to me on my walk-thru. The Onan 5.5 Kw does about 1 gallon an hour on maximum load (the manual actually says 0.9). Well if you're driving 60 mph and running the genny, you're getting a minimum of 60 mpg, or more. If the dash air is costing you 1 mpg and using the same numbers, you do break even. The plus is by running the roof air, it's less work on the engine allowing it to do what it was really designed to do (especially in mountains), and when you arrive, you're already cooled down.
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Old 05-17-2013, 12:37 PM   #13
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We run the dash a/c, fans above the dash, and the front roof a/c and turn off the rear a/c with the bedroom door closed. It's a little warm in the bedroom when we shut down but the rear air does a nice job cooling the room as the engine cools off too.
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Old 05-17-2013, 01:14 PM   #14
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We turn a couple vents toward the front seats and usually get quite comfortable. Of course we are not driving in the desert, just the Southeast.
I'll be staring out in the Mohave Desert and headed up to the Sierras. I'll give it a try on just the roof but am pretty certain that won't cut it for the desert portion.

Thanks everyone. As always, this site rocks!
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