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Old 08-08-2012, 07:30 PM   #1
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Dash A/C or run generator for house A/C

I recently had my dash A/C compressor replaced on my relatively new to me class A and am getting ready to take a late summer trip. I have read posts about it's inability to cool the driver and passenger due to the big greenhouse in front and the cool air getting lost to the space behind the front seats. I know one solution is to hang some type of curtain behind the front seats so that the dash A/C can better handle that now smaller controlled air space.

I also know a solution is to run both dash A/C and have the generator running the roof air (probably just the front unit) while driving to cool the entire house space, or at least the front half of the coach.

I have also read that it is more efficient in terms of fuel consumption to run JUST the generator for the roof A/C and leave the dash A/C off.

I'm not a penny pincher, but wonder if anyone has tracked this and figured it out. With todays price of gas, the difference can add up over many miles. My 8.1 Liter Vortec averages about 7 mpg without the dash A/C on - so that's 8.57 gal per hr at 62 mph. And the 7.0 kw Onan gas generator is rated to burn 1.22 gallons per hour at full load or .73 gallons per hour at half load. I don't know how much load one A/C unit amounts to, but probably at least half load.

So......according to my calculations, driving at my usual 62 mph my total fuel consumption with the generator running and no dash A/C will drop my consumption to 6.13 mpg (assuming generator full load at 1.22 gph). I don't know how much drop in mpg there would be just running the dash A/C vs no A/C.

Confused yet?
To restate the question - is it better for fuel economy to run just dash A/C with a curtain behind front seats OR forget the curtain and run JUST the roof air? In other words, does the generator burn less fuel than the dash A/C does in added engine burden? Its just the two of us, so I'm only concerned about cooling us in the front seats while driving.
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Old 08-08-2012, 08:03 PM   #2
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Its just the two of us, so I'm only concerned about cooling us in the front seats while driving.
What about Bailey? We could not even consider running in hot weather without the genny and AC. Our dog simply would not allow it.
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Old 08-08-2012, 08:38 PM   #3
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Bailey is pretty smart. We'll set the curtain up with enough room for her too if that is the setup we choose. She usually wants to be up with us anyway.
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Old 08-08-2012, 08:49 PM   #4
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I have to start by saying that I am an engineer that always has to know the most efficient way to get something done (even if I choose not to do it that way.)

I don't have the numbers handy, but it is more efficient to run just the dash AC. But at some point, it will not be strong enough, so efficiency takes a back seat to comfort and we run the roof AC. In Georgia in July, we run both.

And even though no one asked, it is less expensive to run the refrigerator on LP than use and inverter to power it on 110. Yes, I am a numbers geek.
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Old 08-08-2012, 10:19 PM   #5
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Jerry modern day compressors that are engine driven take about 6hp to turn when running. 6hp is going to take far less fuel than your genny.
You will have to make the decision if the dash air can make you comfortable or if you will need some occasional help from your roof AC unit(s). You can always shut off the roof AC(s) if need be.

tpm64, no disrespect meant with the question....Why is it that an engineer always has to let us know he/she is an engineer? Is that part of the schooling for your trade? By engineer...I am assuming you didn't drive a locomotive?
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Old 08-08-2012, 11:27 PM   #6
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I guess it would depend on the area you are trying to cool. My 29 footer is easily cooled by the vehicle A/C. But if I was traveling in 90+ weather I would give the engine a break and run the generator to power the roof A/C.
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Old 08-08-2012, 11:59 PM   #7
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If it is to hot for the dash AC to cool us down, I'll run the genny and the house AC for a bit. When I do this I make sure it is on MAX AC so the house air will recirculate through the dash AC. To just have the AC portion on will cause the inside air to be exhausted, which is not your cold air. (At least that is the way I understand it.) For me it is not about efficiency, it is about comfort.
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Old 08-09-2012, 07:38 AM   #8
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If it is to hot for the dash AC to cool us down, I'll run the genny and the house AC for a bit. When I do this I make sure it is on MAX AC so the house air will recirculate through the dash AC. To just have the AC portion on will cause the inside air to be exhausted, which is not your cold air. (At least that is the way I understand it.) For me it is not about efficiency, it is about comfort.
Running the cab A/C on max will not exhaust the inside air. You're recirculating it? Plus, your coach should be so air tight as to make this possible. That's just not going to happen. They all leak way too much, no matter how nice they are.

Here's something else to consider. My approach has been to run the cab AC and heater with the recirculating (max) door bypassed, locked, leaving the cab system with no choice but to recirculate inside air. I'm no longer trying to cool hot outside air in the summer, or cold outside air running the heater. Not my idea, but after testing it on a 6500 mile trip through the south and southwest, it's proven very effective!

When I'm driving into the sun or in very high temps, I can run the cab AC, and the roof AC. Doing this, because that door to the outside is always closed, the cab AC will pick the cool air from the roof AC up off the floor, cool it some more, then blow that in my face. That process is so effective you'll often run with the blower on medium or lower - even when temps are pushing 100.

With the outside air door closed there's also the potential to run the cab air on the heat side (so cab AC compressor is not running), temp control set to cold, roof AC running. Because the outside door is locked closed, the cab heater will be pulling air through the same channels it would if the AC were on cool , set to max. This allows the cab heater to pull that nice cool air up off the floor - though it's running on the "heat" side. Allows nice cool air blown on your face - without the engine compressor running.

The heater will work normally - though it will be pulling air from inside the coach, as described above.

Outside air door is usually under the hood on passenger side, operated by a vacuum diaphragm that's maybe 3" in diameter. I used a zip tie to hold the door closed. Can't get much simpler.
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Old 08-09-2012, 08:14 AM   #9
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What I generally do in the hottest part of summer when we are traveling is run gen set and put temp set point at 78-80 , set on auto, and run dash A/C on A/C and not set on max A/C just to help to maintain a reasonable temp throughout coach, it's too hard to cool down once temps get over 88' , and when you do that your gen set might not run over 50% loaded always crack at least one window for a little fresh air in A/C units.

Happy RVing James
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Old 08-09-2012, 08:14 AM   #10
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[QUOTE=ahicks;1271101]

Here's something else to consider. My approach has been to run the cab AC and heater with the recirculating (max) door bypassed, locked, leaving the cab system with no choice but to recirculate inside air. I'm no longer trying to cool hot outside air in the summer, or cold outside air running the heater. Not my idea, but after testing it on a 6500 mile trip through the south and southwest, it's proven very effective!

To aHicks
I had read your post on this from some time ago when I did a search on this topic. Good idea. This almost gives the benefit of running both dash and roof A/C but without robbing some power from the engine, especially when climbing grades.

As I see it, the only downside with the damper door for outside air locked in the closed position are those cooler days when you want cool outside air blowing through the vents without the A/C running.
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Old 08-09-2012, 09:41 AM   #11
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tpm64, no disrespect meant with the question....Why is it that an engineer always has to let us know he/she is an engineer? Is that part of the schooling for your trade? By engineer...I am assuming you didn't drive a locomotive?
No offense taken with the question.

Mainly because this was an engineering question. If it was about the best route through Atlanta, I might say that I am a native Georgian. If it was about treating a broken bone, I would not answer, but would put more weight in a doctor's answer than an accountants.
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Old 08-09-2012, 11:10 AM   #12
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ahicks...your making the most of your AC systems although 1 thing can occur with locking the recirculation door shut. Some fresh air is required to aid in keeping windshields from fogging during cold/cool/high humidity conditions. Some fresh air is needed when using the heat mode as well. When on recirculate mode window fogging may occur especially in tightly sealed vehicles. The fresh air coming into the HVAC system keeps the fogging to a minimum or keeps it eliminated. I too always run on the "max" setting for AC but let the HVAC system go to "fresh" or outside air mode for the cool,cold, high humidity conditions in late fall and early spring
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Old 08-09-2012, 11:32 AM   #13
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When traveling and if it is hot outside I run the generator and house air conditioning because I want the entire RV cool when I get where I'm going. If I don't do it that way the rear areas of the RV are hot and it takes quite a while to cool that area after stopping and hooking up to electricity. Beings as the generator is running I also use an electric fan to better circulate the air.
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Old 08-09-2012, 08:59 PM   #14
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ahicks...your making the most of your AC systems although 1 thing can occur with locking the recirculation door shut. Some fresh air is required to aid in keeping windshields from fogging during cold/cool/high humidity conditions. Some fresh air is needed when using the heat mode as well. When on recirculate mode window fogging may occur especially in tightly sealed vehicles. The fresh air coming into the HVAC system keeps the fogging to a minimum or keeps it eliminated. I too always run on the "max" setting for AC but let the HVAC system go to "fresh" or outside air mode for the cool,cold, high humidity conditions in late fall and early spring
With all due respect, not too concerned about operating in this fashion with a "tightly sealed vehicle"? My experience is these things leak like a sieve? I would totally agree if we were talking car or pickup.

Point made though. That point is unproven, though I have my doubts about whether or not that will actually happen to the point it's an issue? My thought is that there's a huge volume of air in these things (thinking DP here), along with the fact that air is changing constantly due to air leaks.

Historically, the only window in our coach that ever develops condensation is the windshield, and then only with the windshield curtain closed, while sitting still in cooler weather. Pretty sure it would do that with the door in either position? Once moving, running the defrosters (or the AC?) will have the AC evaporator acting as a dehumidifier, and we should be OK.

Absolute worst case? I find I have to go out and snip that tie wrap (allowing the door to open) for operation in cooler (but not cold!) weather. I'll suffer that indignity for proven comfort in 100 degree weather.

CaptainJerry - I get where you're coming from too. I just open my window? That and a breeze coming from the vents usually does it for me.

Bottom line, I've found something that's working pretty good for me (so far!), and wanted to share it. If you can use the idea, then great!

OP, the thought occurred to me what I'm doing has nothing to do with the cost of operation either way. I was thinking efficiency when I wrote the post.... My apologies for dragging this off on a tangent that may not interest you. -Al
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