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Old 04-15-2014, 06:21 PM   #29
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I don't have anything to add as far as which method provides better mileage but I do know that my coach stays cooler running the house AC. Biggest issue I have is when everyone (passengers) are sleeping and I get too cold while driving and have no way to turn the temp up. Anyone know of anyway to control the house AC's remotely?
Heck yea! Power down the gen from cockpit will kill the house AC's quickly!
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Old 04-15-2014, 06:46 PM   #30
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If AC is needed I will run the Gennie and house AC while under way. I hate sweating. The mileage or Gas consumption really don't matter to me I got to go where we want to go and I am going comfortably. I prefer to having the horsepower while underway and to be com-fie. I think that is why they were built they way they were and that was to make it com-fie weren't they?
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Old 04-16-2014, 08:32 AM   #31
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Heck yea! Power down the gen from cockpit will kill the house AC's quickly!
Functionally yes, but your not doing any favors to some expensive electrical components, or the generator engine itself.

From a "best practices" and longevity standpoint, the A/C's should be shut off first, then allow the engine to cool itself down without a load on it for a few minutes, then shut it down.
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Old 04-16-2014, 03:39 PM   #32
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I have heard that many times. and it is true in part.. YES, using the dash A/C will "Rob" power from the engine and it WILL reduce your Miles per gallon... As computed by the dash computer.

Running the GENERATOR will also consume fuel (Just this fuel does not run through the dash computer) so if you figure MPG by the "Miles since last fillup / gallons to re-fill, method,,, My guess is you will find your savings are negative (That is it burns more gas than the Dash.

HOWEVER.. there is more...

NO way can the Dash A/C keep the RV cool when it's 90 in the shade and you are in full sun. That takes all 3 of 'em. Dash plus both Roof top units.
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Old 04-16-2014, 06:25 PM   #33
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Interesting conversation. My dash air went out and I attempted to use the roof unit to cool my 37 foot Class A Motorhome with no luck. I ended up putting the window down and using the fans.

Does anyone have a scientific study as to the actual fuel consumption of a 7000 watt Onan?

r0n…..
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Old 04-16-2014, 06:54 PM   #34
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Interesting conversation. My dash air went out and I attempted to use the roof unit to cool my 37 foot Class A Motorhome with no luck. I ended up putting the window down and using the fans.

Does anyone have a scientific study as to the actual fuel consumption of a 7000 watt Onan?

r0n…..
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Old 04-16-2014, 07:34 PM   #35
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I run an 120 volt floor fan a few feet aft of and pointed towards the front seats when running the generator and house air conditioning. Really helps circulate the air though the RV.
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Old 04-18-2014, 12:41 PM   #36
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Thanks Ray….I am thinking about 1 gallon per hour. I was calculating the cost of running the AC instead of paying for a campground. 8 hrs = 8 gal * $3.50 =about $28.00 a night. Think I will get my Passport America book out.

r0n…..
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Old 04-18-2014, 12:43 PM   #37
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I run an 120 volt floor fan a few feet aft of and pointed towards the front seats when running the generator and house air conditioning. Really helps circulate the air though the RV.
I will give that a try…dog…….

Thanks, r0n…..
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Old 04-18-2014, 01:46 PM   #38
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Lots of great info on these 3 pages of postings…….

but generator fuel consumption is very hard to predict as it will vary with the load on your generator……one or two AC units, hot water, refer.

I liked the comment that using the generator to power an AC unit while driving is a good opportunity to give it some good exercise while under a load.
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Old 04-18-2014, 03:22 PM   #39
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My guess is you will find your savings are negative (That is it burns more gas than the Dash.
Of course. It's easy to confuse efficiency with capacity. The dash air is good for maybe 15k BTU or so and running two rooftop A/C units are about twice that so obviously the latter will use more fuel. Even running a single rooftop A/C, you are comparing a compressor driven directly off of the engine vs. one driven by running a second engine that first has to generate electricity (creating losses) and the the air conditioner has to turn that electricity back into mechanical energy to drive the compressor (more losses.) It seems very unlikely that a separate generator and A/C unit would be as efficient as a direct engine-driven unit when BTU capacity is similar.

But that's all efficiency... and if the dash air doesn't have sufficient capacity then you don't have much option than to run the house generator and A/C units to create additional cooling. But it's highly unlikely that running another engine and (probably multiple) electric A/C units could possibly use the same amount or less energy (and thus fuel) as using the dash air, it just about has to use more.
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Old 04-19-2014, 09:30 AM   #40
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In connection with the efficiency discussion in this thread, I am the one who compiled the published fuel figures from all the RV generators I could find. The latest PDF with this data is attached. This version has some other facts beside fuel.

My Brisk Air units draw 1.91 KW under full load for a total of 3.82 KW while running. Adding a Dometic refrigerator to that load adds 0.38 KW. That’s a total of 4.2 KW, and of course none of these will run100% of the time, and some service factor should be applied to arrive at a reasonable amount of power required. I suspect that service factor will be in the range of 50%.

Below is a little table showing four generators operating at half load which appears to be enough to support most any service factor that actually occurs.

PowerTech CD-7 Constant Speed Diesel__0.32 gph at 3.5 KW
Onan 8.0 HDKAK Variable Speed Diesel____0.49 gph at 4.0 KW
Onan 7 HGJAB Constant Speed Gasoline____0.70 gph at 3.5 KW
Onan 6.5 HGJABConstant Speed Propane___0.80 gph at 3.25 KW

As indicated, there is a pretty large difference in fuel efficiency between different fuels and also the price in fuel for the variable speed diesel as compared to constant speed..

These figures indicate to me that running the generator for coach air conditioning while under way is very efficient for diesel operation, but less so for gasoline and propane.

Good Luck!
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Attached Files
File Type: pdf General Fuel Facts.pdf (245.4 KB, 55 views)
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Old 04-19-2014, 12:02 PM   #41
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These figures indicate to me that running the generator for coach air conditioning while under way is very efficient for diesel operation, but less so for gasoline and propane.
No, it will be the same, assuming that a diesel powered coach is carrying a diesel-powered generator and a gasoline-powered coach is carrying a gasoline-powered generator, as is almost always the case. For a given BTU cooling capacity the energy is coming from either the RV engine or the generator engine, and since these use the same fuels then the amount of additional fuel used for A/C will be about the same (or perhaps slightly more for the separate generator as explained above.)
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Old 04-19-2014, 12:07 PM   #42
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