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Old 03-15-2014, 11:18 AM   #1
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Running Generator and House AC on road

I was talking a RV tech at the dealer the other day. He recommended running the MH generator and and house AC rather than the chassis AC while on the highway. The reasoning is that the chassis AC compressor will rob the engine of power. He said the gasoline generator will use between 1 pint and 1 quart of fuel per hour and the house AC unit(s) will cool the MH more efficiently than the chassis AC. By running the generator it will improve the overall fuel economy of the MH. The additional benefit is putting more hours on the generator which for most MH owners does not get used enough.

I agree that putting more hours on the generator is a good idea. I seldom use mine to power the MH and run it for about 40 minutes a month for maintenance only. I'm not so sure about improving the fuel economy by running the generator. I know it takes power to run the chassis AC compressor but I find it hard to believe it would make a significant difference in fuel consumption while driving on the highway. That 6.8L V10 is a pretty big compared to the AC compressor.

The issue of AC vs fuel economy was tested by the Mythbusters on cars. They got better fuel economy running the AC rather than driving with the windows down. But of course a MH is not a car.

Anybody tried the generator/house AC and have any definitive results?
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Old 03-15-2014, 11:25 AM   #2
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Quote:
Originally Posted by AKJohn View Post
I was talking a RV tech at the dealer the other day. He recommended running the MH generator and and house AC rather than the chassis AC while on the highway. The reasoning is that the chassis AC compressor will rob the engine of power. He said the gasoline generator will use between 1 pint and 1 quart of fuel per hour and the house AC unit(s) will cool the MH more efficiently than the chassis AC. By running the generator it will improve the overall fuel economy of the MH. The additional benefit is putting more hours on the generator which for most MH owners does not get used enough.

I agree that putting more hours on the generator is a good idea. I seldom use mine to power the MH and run it for about 40 minutes a month for maintenance only. I'm not so sure about improving the fuel economy by running the generator. I know it takes power to run the chassis AC compressor but I find it hard to believe it would make a significant difference in fuel consumption while driving on the highway. That 6.8L V10 is a pretty big compared to the AC compressor.

The issue of AC vs fuel economy was tested by the Mythbusters on cars. They got better fuel economy running the AC rather than driving with the windows down. But of course a MH is not a car.

Anybody tried the generator/house AC and have any definitive results?
I also don't buy it, I get about the same MPG while towing a car as I do when I am not. So I think the AC Compressor would give you VERY LITTLE difference in MPG if it was running or not.
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Old 03-15-2014, 11:47 AM   #3
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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 03-15-2014, 12:25 PM   #4
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Most run the genset and house ac while underway and also get the benefit of running the fridge on elect. and having 120vac available throughout. Fuel consumption is going to be closer to 1/2 - 1 gallon per hour in hot weather, with many older gas models, but the dash ac burns extra fuel as well. The main thing with the dash ac, is with the fact that running them is a waste of effort and efficiency and eventually they quit working anyway. Why anyone would bother with repairing them or why they were ever installed in the first place for the expense, is beyond my comprehension, except that it probably comes with the chassis and is costly to remove them. I mean there are much better ways to spend the money and they could start with a transmission temp gauge and a lot more cooling for some.
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Old 03-15-2014, 12:27 PM   #5
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If, memory serves, and sometimes it does, A/C compressor takes 11hp when operating. The compressor cycles on/off depending on the amount of cooling required in the vehicle interior. Most vehicles have a full throttle cut out on the compressor , so it won't draw power when you need it most.
The parameters for a fuel mileage comparison are endless, road, speed wind, sun shine, exterior temp, interior temp, coach color, you name it.
Exercising the gen set would be the true benefit, but as others have stated, do what you have to do to stay comfortable.
If your running the gen set to exercise it anyway; why not do it while your on the road and make use of the power.
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Old 03-15-2014, 05:33 PM   #6
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If the dash air is not cooling the RV enough I start the generator and use the house air and shut off the dash air. A secondary benefit (to me) is when we get where we are going the entire inside of the RV has been cooled. Probably when using the dash AC the living area and rear area of the RV may be quite hot and take a very long time to cool down after you get to your location especially if traveling in the south in the summer.
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Old 03-15-2014, 06:39 PM   #7
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With our previous 2 coaches, one a C the other an A, on summer trips through Calif's Central Valley, the dash air caused the clutch fan to kick in periodically in temps in excess of 95. That's when I would run the generator and roof air(s). It is a very, very common practice in hot climates. What your mechanic is telling you is in line with my experiences.
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Old 03-15-2014, 07:12 PM   #8
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I run dash air if it can keep up if not I run both which is most of the time I summer.
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Old 03-15-2014, 07:29 PM   #9
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I run both as needed for comfort. The chassis/dash AC does nice job keeping the front of the coach cool.
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Old 03-15-2014, 08:23 PM   #10
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One pint to one quart per hour, nope, not likely for an onboard genset. My 3000W Boliy portable genset uses a gallon in 4 hours. I've never seen an onboard genset that small.

Somewhere on irv2.com in a thread, someone posted a chart of fuel consumption for almost every brand built into an RV. A search should discover the post.
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Old 03-15-2014, 08:28 PM   #11
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Here's another idea to improve your dash air.

My brother has also had problems keeping the cockpit cool while driving in warm weather. They decided to put up a curtain rod directly behind the driver and passenger seats but forward of the slideouts. Then DW purchased two colorful Shower Curtains. When the curtains are closed the air-conditioned area that the dash air has to cool has been drastically reduced and therefore the dash air works much more efficiently.

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Old 03-15-2014, 08:43 PM   #12
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Quote:
Originally Posted by AKJohn View Post
I was talking a RV tech at the dealer the other day. He recommended running the MH generator and and house AC rather than the chassis AC while on the highway. The reasoning is that the chassis AC compressor will rob the engine of power. He said the gasoline generator will use between 1 pint and 1 quart of fuel per hour and the house AC unit(s) will cool the MH more efficiently than the chassis AC. By running the generator it will improve the overall fuel economy of the MH. The additional benefit is putting more hours on the generator which for most MH owners does not get used enough.

I agree that putting more hours on the generator is a good idea. I seldom use mine to power the MH and run it for about 40 minutes a month for maintenance only. I'm not so sure about improving the fuel economy by running the generator. I know it takes power to run the chassis AC compressor but I find it hard to believe it would make a significant difference in fuel consumption while driving on the highway. That 6.8L V10 is a pretty big compared to the AC compressor.

The issue of AC vs fuel economy was tested by the Mythbusters on cars. They got better fuel economy running the AC rather than driving with the windows down. But of course a MH is not a car.

Anybody tried the generator/house AC and have any definitive results?
I would suggest you first consider doing what it takes to stay comfortable! I would also add that in warm weather your cab A/C will be MUCH more effective if it's run on "recirculate" mode where it's cooling reasonably cool interior air vs. what may be smoking hot air coming up off the pavement. You can make a good case too, for doing the same thing in cool weather. Heater much more efficient warming interior air vs. that coming from the outside on a 10-20 degree day?

My recirculating door is locked open full time for these reasons. If I want fresh air, I open a window.

Re: leaving a broken cab A/C inop rather than fix it, you might consider the benefit of a built in dehumidifier blowing air on your suddenly fogged up windshield on warm rainy days?

All that said, I've not noticed a difference in mileage on the road with gen set running vs. not. Our DP not affected by much, other than wind.
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Old 03-15-2014, 11:18 PM   #13
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I did an experiment where I monitored the engine coolant temp with the engine A/C on and then with it off.

I allowed about five minutes between cycles and discovered that the engine coolant raised about 4 to 5 degrees with the Engine A/C on.

When it's 100 degrees outside anything I can do to keep the engine running cooler is in my best interest.

So I run the generator and house air with the engine A/C turned off.

I couldn't care less what the mileage does.
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Old 03-16-2014, 12:10 AM   #14
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Quote:
Originally Posted by AKJohn View Post
I was talking a RV tech at the dealer the other day. He recommended running the MH generator and and house AC rather than the chassis AC while on the highway. The reasoning is that the chassis AC compressor will rob the engine of power. He said the gasoline generator will use between 1 pint and 1 quart of fuel per hour and the house AC unit(s) will cool the MH more efficiently than the chassis AC. By running the generator it will improve the overall fuel economy of the MH. The additional benefit is putting more hours on the generator which for most MH owners does not get used enough.

I agree that putting more hours on the generator is a good idea. I seldom use mine to power the MH and run it for about 40 minutes a month for maintenance only. I'm not so sure about improving the fuel economy by running the generator. I know it takes power to run the chassis AC compressor but I find it hard to believe it would make a significant difference in fuel consumption while driving on the highway. That 6.8L V10 is a pretty big compared to the AC compressor.

The issue of AC vs fuel economy was tested by the Mythbusters on cars. They got better fuel economy running the AC rather than driving with the windows down. But of course a MH is not a car.

Anybody tried the generator/house AC and have any definitive results?

I can tell you that my Onan Diesel 7.5 kw genset uses 1/2 gal. per hour.

So, at today's price that's about $2 per hour. But, I can run other things like the reefer on that same $2 per hour.

Don't know how much the RV is affected by running the engine/dash A/C but my car loses about 2 mpg. in the summer. For sake of argument that's about 9%. My CAT diesel burns about 7.5 gal per hour at 60mph ... if I lost 9% (and I don't know if I do) running the dash A/C, that would be 2/3 gal. per hour. Just theory but nearly the same cost ... except I'd get the whole coach cool on the genset. It would appear you want to do one or the other but not both.
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