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Old 08-11-2014, 08:37 PM   #1
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Coach A/C vs. Cab A/C, Better fuel economy... fact or fiction?

My dash A/C is out in my class C. It has the Ford 460 chassis and I got two quotes for an A/C kit ($1700.00 and $1100.00). I have heard that you get better gas mileage by just running the generator and roof A/C than you do when you run the dash A/C alone.

Does anyone know if that is true? I have an Onan 4000 with a 1350 BTU roof unit....

V/R

Mark Spears
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Old 08-11-2014, 08:43 PM   #2
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Average gen set uses 1/2 gal per hour, engine mounted compressor takes 11>13 HP when cycling. You would need to run two identical units at the same speeds on the same roads, same outside temps ( interior too) to get any meaning full data.
My take : It's $2 an hour, on gen set , what's your comfort worth.
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Old 08-11-2014, 08:53 PM   #3
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On your C the other problem you may have is getting the cold air where you want it without dash air. You will probably want a fan or two.
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Old 08-12-2014, 08:29 AM   #4
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My experience over several years of using both units alternately is the roof a/c with gen gave me the best all around results. It also has a side bare effect of exercising your gen.
It sure is nice when you stop at a roadside rest to have lunch to have a nice cool coach to relax in. The dash a/c just cannot do that for you. Plus when you shut the engine off the heat build up inside the engine compartment is horrendous. This shortens the life of every thing made of plastic or rubber.
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Old 08-12-2014, 01:35 PM   #5
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Yeah the other thing is that we have 2 small kids and three dogs who have to ride back there.
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Old 08-13-2014, 06:52 AM   #6
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The A/C in your dash is far more powerful than the one on your roof, however, the rooftop unit has a far more powerful fan to move it's lower amount of cooling around. It's also more centrally located.

If you open the rearmost roof vent though, the vacuum created draws the cold dash air down the back and sucks out the hot air that's risen to the ceiling.

I can keep the back of mine nearly as cool as the front at highway speeds doing this.
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Old 08-13-2014, 08:11 AM   #7
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I always run with my rear roof vent cracked open about a half inch. I have covers over all of them. My dash a/c would not cool much beyond the cab area, plus it is a power hog. We went so far as to install a curtain behind our cockpit seats to retain cool enough air to be comfortable. With the roof a/c running off generator I use a small electric fan to push air up to the cabin area, works great. I have lugged the generator around for years. It now works for its keep.
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Old 08-13-2014, 01:13 PM   #8
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That sounds like a lot of cash and I have to assume/hope they are quoting for more than just a new compressor? With the kids and dog, I run the generator and coach ac. I figure I need all the engine horsepower I can free up
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Old 08-13-2014, 06:38 PM   #9
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Originally Posted by Izzyblueye View Post
I always run with my rear roof vent cracked open about a half inch. I have covers over all of them. My dash a/c would not cool much beyond the cab area, plus it is a power hog. We went so far as to install a curtain behind our cockpit seats to retain cool enough air to be comfortable. With the roof a/c running off generator I use a small electric fan to push air up to the cabin area, works great. I have lugged the generator around for years. It now works for its keep.

Try winding that vent wide open, more air out equals more cool air down the back.

What do you mean "power hog" with regards to your dash A/C?
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Old 08-14-2014, 08:22 AM   #10
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It takes horsepower to run the engine a/c. Horsepower translates into higher fuel consumption and less power available to pull hills. To cool two seats in the cockpit when the roof a/c will cool the whole coach and does not rob the engine of power. It's a no brainer.
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Old 08-14-2014, 09:13 PM   #11
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It takes horsepower to run the engine a/c. Horsepower translates into higher fuel consumption and less power available to pull hills. To cool two seats in the cockpit when the roof a/c will cool the whole coach and does not rob the engine of power. It's a no brainer.

It takes horsepower. Yup, it sure does. A typical Ford A/C unit used in a pickup or van, not the aftermarket units used on the F53 chassis under a class A m/h, draws about 5 hp while the compressor is cycling which is FAR less than all the time. If you look at the ratio of that load to what pushing the coach down the highway takes, I'd be surprised if you could even measure the loss. If for discussion purposes the compressor was running even half the time, that would be 2.5 hp net.

Just as a 'back of envelope' calculation. If a typical Ford chassis class C is making even only 150 hp at highway speed, 2.5 hp would be an increase of 1.67% in horsepower and therefore fuel consumption.

If that same typical class C gets about 8.5 mpg an increase of 1.67% would bump that up to about 8.64 mpg.

On the other hand, 60 mph and 8.5 mpg is 7.06 gal. / hour and an increase of 0.5 gal. / hour on that would be about a 7% increase.


Yup, you're right, 1.67% increase to run the dash A/C versus 7% for the generator running the rooftop unit is a no brainier.


The generator would cost more than 4 times that of the dash unit.


BTW, my dash unit cools the whole coach no problem since it's nearly 4 times the theoretical capacity of the rooftop unit.
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Old 08-14-2014, 09:29 PM   #12
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Wow, good info
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Old 08-14-2014, 09:49 PM   #13
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Quote:
Originally Posted by Murf2u View Post
It takes horsepower. Yup, it sure does. A typical Ford A/C unit used in a pickup or van, not the aftermarket units used on the F53 chassis under a class A m/h, draws about 5 hp while the compressor is cycling which is FAR less than all the time. If you look at the ratio of that load to what pushing the coach down the highway takes, I'd be surprised if you could even measure the loss. If for discussion purposes the compressor was running even half the time, that would be 2.5 hp net.

Just as a 'back of envelope' calculation. If a typical Ford chassis class C is making even only 150 hp at highway speed, 2.5 hp would be an increase of 1.67% in horsepower and therefore fuel consumption.

If that same typical class C gets about 8.5 mpg an increase of 1.67% would bump that up to about 8.64 mpg.

On the other hand, 60 mph and 8.5 mpg is 7.06 gal. / hour and an increase of 0.5 gal. / hour on that would be about a 7% increase.


Yup, you're right, 1.67% increase to run the dash A/C versus 7% for the generator running the rooftop unit is a no brainier.


The generator would cost more than 4 times that of the dash unit.


BTW, my dash unit cools the whole coach no problem since it's nearly 4 times the theoretical capacity of the rooftop unit.
I imagine is does in Canada bring that Baby down to Texas for a while.If it cost $2.00 an hour that is about.033 cents a mile,I'll pay that to stay cool
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Old 08-15-2014, 05:43 AM   #14
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Ok, so if the 15k BTU rooftop unit keeps your coach cool, why can't the 60k dash unit keep you cool?

Simple, the dash unit doesn't have anywhere near as big a fan and it's at one end not in the middle.

Open the rearmost roof vent and you'll be amazed at how much cooling power you'll have from the dash.
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