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Old 08-06-2015, 02:17 PM   #1
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Coach a/c vs cab a/c

Just curious about this. I have read the thread on this stating that the cab a/c uses only .17 gallons more per hour at 2000 rpms, and that the Coach a/s users over .5. The test data in that thread is great, but I am not sure how accurate revving at 2000 rpms in park is. I think if you are driving at 50-55 at 2000 rpms, and are running the cab a/c, that number is going to be much higher due to the strain the a/c puts on the engine while it is trying to maintain coach speed.

I would have replied in there, but that thread is over a year old, and would not allow it. Has anyone else done any testing with this?
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Old 08-06-2015, 05:03 PM   #2
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If I had to worry about the difference between .17 and .5 gallons used per hour I would definately have to rethink owning an rv that gets 6 to 8 mpg on average.
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Old 08-06-2015, 06:23 PM   #3
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If I had to worry about the difference between .17 and .5 gallons used per hour I would definately have to rethink owning an rv that gets 6 to 8 mpg on average.
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I agree. However, I some of the wealthy got wealthy by being smart and saving here and there. If something as simple as running one vs the other makes a difference, I would like to know about it. I personally just go for comfort and run the Coach and cabin a/c, but the thread I read got me interested.
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Old 08-06-2015, 07:11 PM   #4
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Difference at, lets say $3.00 per gallon, would be around .99 cents. Heck, you'd save more than that, over an hours driving time at 65 mph, by slowing to 64 mph.
Not even worth thinking about it.
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Old 08-06-2015, 07:32 PM   #5
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For me it is comfort and ease. At moderate temperatures the dash a/c cools enough for DW and myself. If passengers involved or temperatures get up to +95 then I go to generator for comfort.
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Old 08-06-2015, 07:42 PM   #6
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My cab ac will freeze you out of the cab area and I live where its hot but I'm not trying to cool the rest of the coach until about 30 minutes before I arrive so my wife can stay cool while setting up inside at the rv site or making lunch at rest stop never consider fuel


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Old 08-06-2015, 08:40 PM   #7
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It's not about the money for me. Just curious. I will have 5 people in the coach next trip so have no choice but to run the generator. I ran it last time, and that was my first trip. I do notice the difference in power when running the cab a/c though. I tried to avoid it, but even with the bunkhoise center folded up, I was still a little warm with just the coach a/c on.
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Old 08-06-2015, 08:56 PM   #8
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Can AC

I think I would run house ac if I was carrying more than a couple of people but my house ac never gets as cold as my cab does that's on s 3050s even when I run the house ac I run the cab ac to unless the rv is turned off
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Old 08-06-2015, 09:10 PM   #9
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I think I would run house ac if I was carrying more than a couple of people but my house ac never gets as cold as my cab does that's on s 3050s even when I run the house ac I run the cab ac to unless the rv is turned off
On my first trip, I have to agree. One is the same. Cab a/c feels really nice. Even with coach a/c blowing in the cab from bunkhouse, it's just not cool enough in the cab.
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Old 08-06-2015, 09:20 PM   #10
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Another thing to remember is do you really want the additional wear/tear of running the main engine that'll cost more to rebuild later down the road. I'd rather put those extra hours on the Onan and be confident the main engine will still get me on down the road. Easier to rebuild the Onan than say, the V10 in our case.
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Old 08-06-2015, 09:46 PM   #11
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I don't have a MH but have as much invested if not more. My diesel engine doesn't feel the a/c and I'll run the Onan in the 5er to keep it cool for my little woman if not traveling far. As much money as I have rolling down the road and getting 9 mpg in truck and Onan uses 3/4 gal/hr is little price I pay to have her say ; sure feels good in here babe. Puts a smile on my face. LOL if we are going 6 hrs away I'll stop an hour before we get there and turn on the Onan so it nice for her. LOL She's spoiled!!!! My fault.
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Old 08-07-2015, 10:46 AM   #12
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Living here in FL, we run both the coach and the cab a/c almost all the time. I keep the unit plugged in with the air on in the driveway and I want it cool when I park for the night - not an hour or two later. On the road, I set the cruise at 63 mph and that brings us to 1700 rpm - ideal cruising with the DP. As for what fuel the generator uses, don't you just hate it when you check into a nice hotel and they've had the air off all day so it takes three hours for the room to cool down? Me, too!
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Old 08-07-2015, 04:15 PM   #13
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The number for the coach A/C sounds low, I would say they should be about the same, the chassis maybe even a little higher.

I'm guessing the chassis A/C is around 20,000 BTU, This will require about 7hp. If we use a fuel burn of .5 lbs per hp, thats about 3.5 lbs per hour. Gas is about 8 lbs per gallon so it would be just under 1/2 gallon per hour to run the dash A/C
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Old 08-07-2015, 08:46 PM   #14
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Read my post of last year that the originator of this thread referenced. Its title is: Running coach A/C vs cab A/C while driving.

Efficiency is closer to 0.4 pounds of gas per horsepower hour. The cab A/C does not have to run 100% of the time to keep it cool. Gasoline weighs 6 pounds per gallon.

The two or three horsepower required to run the cab A/C is a small percentage of the 125 hp more or less that the engine is developing at cruise. Even if you ran at full throttle you probably wouldn't notice the difference with the A/C on or off. The differences in engine cooling load, fuel consumption and engine wear are insignificant.

In short, if the cab A/C keeps you cool, just use only it. Use the generator and coach A/C if you need it.
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Old 08-08-2015, 12:12 AM   #15
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In theory, it seems simple that the cab a/c is more efficient. I just don't think revving the engine is a valid test when comparing. You can easily feel the lag when accelerating when the cab a/c is on. I'm positive that when driving, that cab fuel consumption is much higher at 2k that when in Park running at 2k. I still think the generator is worse, but the comparison I believe is closer than the test results.
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Old 08-08-2015, 05:35 AM   #16
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Originally Posted by mud yapster View Post
I don't have a MH but have as much invested if not more. My diesel engine doesn't feel the a/c and I'll run the Onan in the 5er to keep it cool for my little woman if not traveling far. As much money as I have rolling down the road and getting 9 mpg in truck and Onan uses 3/4 gal/hr is little price I pay to have her say ; sure feels good in here babe. Puts a smile on my face. LOL if we are going 6 hrs away I'll stop an hour before we get there and turn on the Onan so it nice for her. LOL She's spoiled!!!! My fault.
So am I reading this right, your wife is in 5er, going down the road? Or are you just keeping the rv cool on the short trips so it's nice when you arrive?
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Old 08-08-2015, 07:04 AM   #17
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Coach a/c vs cab a/c

We experienced our first trip by motorhome this summer to Florida.

We eventually stopped at CW and bought a DC / AC / battery (rechargeable) fan and set it on the floor just aft of the cockpit seats.

This allowed for the recirculating of the cold dash air back in the coach. To us this made scientific sense and gave us the feeling of improved efficiency.
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