Run Rooftop A/C While driving?

Proud new owners of a lightly used 2017 FR3 32DS.  We completed our first trip driving it home 275mi from the prior owner's town in Phoenix.  I figured I would try and pick the brain of the willing experienced and knowledgeable members of this forum:

Problem:
Inside of RV is not comfortably cool while driving through the desert 90*-105* outside.  The dash A/C doesn't blow very cold, and the owner said it has been that way since new (09/2017).  I will get that checked out, but assuming it is operating as it should due to age.  Even if the dash did blow cold to keep me and my wife comfortable up front, the rest of the RV is not comfortable for the kids.

Solution?: 
Find a way to run the A/C units while driving.  My understanding is that the RV only has a charge controller/inverter (Inteli-power 9200) to charge the coach batteries and run 12v appliances.  The dual  A/C units are 115v Dometic Atwood 15k BTU.  The Q.A. performance test sheets for each show cooling power consumption at 1015w and 897w.
- Run generator while driving?  Is this possible/safe/acceptable? 
- Install a larger battery bank with a DC/AC inverter to run off of while driving?  I have been researching adding solar and a Li batter bank, but this would be down the road.

If you have an on-board generator with sufficient rating, that would be your best way to run the roof a/c while underway. Our dash air is adequate for us, but we have no issue running the generator while underway, and would have no issue using it to run roof a/c.

One caveat is that your generator, depending on size, will use 0.5-1 GPH. Also, when the fuel tank runs low, the generator will be starved of fuel, because the generator fuel pickup will be at approx 1/4 tank. This 1/4 tank is very approximate, and forum members have reported a wide range on this number.

CruzinCarlz said:

Proud new owners of a lightly used 2017 FR3 32DS.  We completed our first trip driving it home 275mi from the prior owner's town in Phoenix.  I figured I would try and pick the brain of the willing experienced and knowledgeable members of this forum:

Problem:
Inside of RV is not comfortably cool while driving through the desert 90*-105* outside.  The dash A/C doesn't blow very cold, and the owner said it has been that way since new (09/2017).  I will get that checked out, but assuming it is operating as it should due to age.  Even if the dash did blow cold to keep me and my wife comfortable up front, the rest of the RV is not comfortable for the kids.

Solution?: 
Find a way to run the A/C units while driving.  My understanding is that the RV only has a charge controller/inverter (Inteli-power 9200) to charge the coach batteries and run 12v appliances.  The dual  A/C units are 115v Dometic Atwood 15k BTU.  The Q.A. performance test sheets for each show cooling power consumption at 1015w and 897w.
- Run generator while driving?  Is this possible/safe/acceptable?
- Install a larger battery bank with a DC/AC inverter to run off of while driving?  I have been researching adding solar and a Li batter bank, but this would be down the road.

Click to expand...

Solution: Run generator AND roof A/C while driving.

All our coaches have been too big for the dash air to adequately cool them while on the road, when it's hot out, so that's what we do, running the generator to power the A/C. Many, many folks do just that.

Larry N. said:

All our coaches have been too big for the dash air to adequately cool them while on the road, when it's hot out, so that's what we do, running the generator to power the A/C. Many, many folks do just that.

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Thanks guys!  I guess that's the logical solution.  I just wanted to make sure I wasn't missing anything important that would prevent that.

First trip with the family will be this Thursday, going to be staying at Lake Mead campground for a few nights!

Your generator needs to be exercised under load monthly. This is an excellent way to get it done.

It also helps to pre-cool the coach interior by starting up the A/C and running it at least an hour before you leave on a trip. And, close blinds/curtains (at least most of the way while still allowing for some scenery viewing) while driving. Incoming sunlight and the resulting "greenhouse effect" is probably where most of the heat inside the coach is coming from. At least among the sources that can be controlled. If you feel a significant amount of engine heat transmitting through the doghouse cover between the driver/passenger front seats, that can be insulated better from the bottom too. I had that problem on my Class A, but it was a 1994 so hopefully they improved that feature by 2017!

[quote author=scottydl]If you feel a significant amount of engine heat transmitting through the doghouse cover between the driver/passenger front seats, that can be insulated better from the bottom too.[/quote]

Good point Scott. Of course, it's not an issue with a diesel pusher.

Way to rub it in, Tom... Oh wait, I have a trailer now! ;D

(I did always envy the DP owners when we had our gas motorhome)

I also run the generator + A/C on hot days. It takes about 1/2 mpg on average for my 50 amp generator.

Before you spend any time or $$ on getting the chassis/dash a/c looked at get a good thermometer and check the temps coming out the vents. If in the high 30's low 40's it's probably fine.

We also have "trucker" fans above the driver/passenger seats that help but when it gets above 90 the roof unit is probably on.

I've heard some people say you use less gas running the generator verses the front AC. Obviously, its less work on the primary engine. I remember turning off the front AC once going up a hill when is was 107 degrees outside. It wasn't overheating but I figured it wouldn't hurt to lighten the load.

Tom55555 said:

I've heard some people say you use less gas running the generator verses the front AC. Obviously, its less work on the primary engine. I remember turning off the front AC once going up a hill when is was 107 degrees outside. It wasn't overheating but I figured it wouldn't hurt to lighten the load.

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I have heard that too but do not believe it at all.. I won't go into why beyond saying that turning rotary motion into electricity and turnining electricity back into rotary motion is NOT 100% efficient. I think 90% is the target if you can hit it.

BUT.. This I know for a fact. The dash air is designed for a Truck or car. very small "Cabin".. No way can the dash air cool a 300 square foot house (roughly my RV going down the road) 50 square feet perhaps.  But not with that big greenhouse window.

So Running the whole shooting match is the only option. Dash and roof.  When it's August ... In January .. Well dash is good enough .. Usually.

When I had my motorhome, the dash air never worked and it wasn't something I wanted to spend money on, so I only ran the roof A/C via generator when needed. It always seemed like a fairly efficient use of fuel, for the amount of cooling it provided. And a good way to exercise the generator under load, as mentioned above.

I would like feedback on this since it's just fuzzy math, lots of assumptions, variables and no maintenance.

Driving 60 MPH
10 MPG
$2.00 per gallon
.5 gallon per generator hour

A 3 MPG or 25% reduction in MPG is typically considered normal for a car. Since most RVs are as aerodynamic as a brick let's use 1.5 MPG

No AC (windows closed) = 6 gallons on primary = $12 per hour
Front AC = 7.06 gallons on primary = $14.12 per hour
Rear AC only = 6 gallons on primary + .5 generator = $13 per hour
Both front and rear AC = 7.06 gallons on primary + .5 generator = $15.12 per hour

I am in the process of transferring the extended warranty plan from the prior owner.  It is $0 deductible, so I might see if I can go that route.  Not sure if they will do a free diagnostic though.

I would be surprised if something failed, being that it has fewer than 5k miles.  Although, I have owned more than a few older vehicles, and those seemed to blow colder than this.

I definitely don't expect the dash a/c to cool the whole RV, but if it were blowing cold directly at me I think I could stay comfortable.

Tom55555 said:

I would like feedback on this since it's just fuzzy math, lots of assumptions, variables and no maintenance.

Driving 60 MPH
10 MPG
$2.00 per gallon
.5 gallon per generator hour

A 3 MPG or 25% reduction in MPG is typically considered normal for a car. Since most RVs are as aerodynamic as as a brick let's use 1.5 MPG

No AC (windows closed) = 6 gallons on primary = $12 per hour
Front AC = 7.06 gallons on primary = $14.12 per hour
Rear AC only = 6 gallons on primary + .5 generator = $13 per hour
Both front and rear AC = 7.06 gallons on primary + .5 generator = $15.12 per hour

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If you're getting 10 mpg you're doing quite well.
I think a/c compressors use 3hp, not 3mpg (atleast that was the common thought some years ago, my Kia Soul mpg drops less than 1mpg with a/c on, barely noticeable with a 12gal tank). Don't forget, most cars have 4 cyl or 6 cyl engines, engines in motorhomes are considerably bigger putting out more power & torque than most car engines. The power drain of an a/c compressor on a motorhome engine (to me) is negligible.
I don't think dash air conditioner would make anywhere near 3mpg difference in a motorhome. I don't notice any significant difference in mpg at all with or without dash a/c, if I'm not using it, it's because weather is nicer which also influences mileage. 

I don't notice any significant difference in mpg at all with or without dash a/c, if I'm not using it, it's because weather is nicer which also influences mileage. 

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I agree -- no noticeable difference in mpg with or without the dash A/C running on the MH, though there's a small difference ( < 0.5 mpg) with the genny running for the roof A/C. Even on car or pickup (both older V8s), the difference with/without dash A/C is very minor, though I could measure it if I wished, certainly less than 0.5 mpg.

CC,

I am also pretty new to the RV Carnival Adventure riding Game.  I got about a year of Rookie Mis-adventures eXperience, so still trying not to be Dangerous.  ;D 

Naaaw not really, as they have mainly been Adventures with a Big Smile! 

My Artificial / Borrowed Virtual Brain Cells as I plug into and create collective personal hybrid solutions to my specific problem I am faced with I find knowledgeable people here to assist me gathering ideas on how to proceed.

Self inflicted poor drives a lot of my decision making process, but that is half the fun of it.

I bought in August 2019 out of the Motor City USA.  By the Time we Got to Tulsa, I knew I did not want to camp in Hot Weather. 

Buy the time I got back to my Dusty Hot Desert I had etched that into the Top 5 What not to do in the future with my RV Strategy Book in my head.

I have had a few camping eXperiences this year that drove home the point again this past 13 months as I had forgotten my own rule top 5.  ;D

The more Slides the Hotter your Interior Reality will be. 

Tom / Seilerbird made a statement on the boards when somebody was talking about a Artic Package?If RV were insulated as well as a normal Stick and brick House the darn thing would be getting 0 mpg and be too Heavy to Roll it down the pot holed highways of our beautiful land we share.

The Gen set will cut off at ? tank which is built in safety for drawing out of the same shared fuel tank.

Bottom Line: Avoid Hot Weather and Direct Sun if you can.  The Kids, well they need to rough it a bit.

Employ the Poor Boy 250 AC.  Two (2)windows down at 50 and tell the kids to buckle in the dinette and put their hats on backwards!

Give them a Toothpick for the Bugs.

JD

I adjusted the numbers from 1.5 MPG to .25 MPG using the front AC based on feedback.

If that is accurate were talking 31 cents an hour to run the front AC and $1.31 per hour to run both the front and back AC. We drive from Cincinnati to Tampa often which is about 1000 miles so I'll use that as an example.

Driving 60 MPH
10 MPG
$2.00 per gallon
.5 gallon per generator hour

No AC (windows closed) = 6 gallons on primary = $12 per hour
Front AC = 6.15 gallons on primary = $12.31 per hour
Rear AC only = 6 gallons on primary + .5 generator = $13 per hour
Both front and rear AC = 6.15 gallons on primary + .5 generator = $13.31 per hour

One way trip: (1000 miles at 60 MPH)

Front AC adds $5.17
Back AC adds $16.67
Running both adds $21.83