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Old 08-03-2012, 05:00 PM   #1
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Cab AC vs Roof AC economics

My cab AC quit working (low freon??) and I ran my generator and two roof ACs to be comfortable. It made me curious about the AC costs?

I have had a problem with my side radiator fan running at top speed at all times. Eventually the problem was diagnosed to a wiring connector to the pressure sensor of the cab AC. It turns out that my fan is designed to operate at top speed if it senses that the AC is running and the wiring problem caused the ECM to believe that it was.

I keep complete records of my mileage so I was able to calculate the mileage difference with the fan operating correctly.

Before fix: 6.23 MPG for 24,933 miles. Yields 9.63 GPH @ 60 MPH
After fix: 7.25s MPG for 8,216 miles. Yields 8.27 GPH @ 60 MPH

Therefore it will cost me 1.36 GPH to run the cab AC.
Onan generators normally use about 0.75 GPH to run two Roof ACs.

So I save over 1/2 gallons of diesel per hour by using the roof ACs and the generator needs to be excercised anyway.

Just thought it was interesting - Darrel
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Old 08-03-2012, 05:17 PM   #2
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Good Post.
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Old 08-03-2012, 07:11 PM   #3
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Great details...but you forgot one more measure: The Dash A/C can only be a fraction of the BTU's of the roof units.

I don't know how you could figure the BTU's of the dash unit, but I bet that makes your figures even stronger.
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Old 08-03-2012, 07:25 PM   #4
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You left something out of the equation.

How much fuel does the generator burn every hour when it's running the air conditioners?

I can tell you I see a great difference when that big fan kicks in too (Gasser)

But how much does the Generator burn?
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Old 08-03-2012, 07:26 PM   #5
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Figure the BTU rating of a dash unit to be 20 - 25,000 BTU/hr. They are bigger than you think, and can draw several kW of power from the belt drive. Remember that the typical dash AC has to cool the car/truck/whatever from "stupid hot" to comfortable in much less time than the one in your house!
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Old 08-03-2012, 09:13 PM   #6
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Darrel, if the dash a/c compressor was not running when you gathered your data then before the fix your engine was probably running cooler than it should much of the time and operating less efficiently. So if the compressor was running, a warmer engine would be more fuel efficient. But, if the dash a/c compressor was not running then you don't have the added loss in mpg due to the compressor running either before or after the fix. There are lots of variables to look at.

It is probably hard to coax out all the data you need to get to an accurate fuel savings but this is the first data I've seen and it appears to support the general feeling that running the generator and house air is more fuel efficient than running the dash air.

Thank you for sharing this information with us.
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Old 08-03-2012, 11:09 PM   #7
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modern day round style compressors will use about 6hp to turn them. That is the compressor only...no draw from the fan. 6hp is nil
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Old 08-04-2012, 08:44 AM   #8
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modern day round style compressors will use about 6hp to turn them. That is the compressor only...no draw from the fan. 6hp is nil
A 6 HP gasoline engine is enough to run a 3KW constant power (not peak)generator (about 500 watts/HP). That's enough to run one roof mounted AC.

Fran
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Old 08-04-2012, 08:56 AM   #9
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Quote:
Originally Posted by wa8yxm View Post
You left something out of the equation.

How much fuel does the generator burn every hour when it's running the air conditioners?

I can tell you I see a great difference when that big fan kicks in too (Gasser)

But how much does the Generator burn?
He said Onan generators normally use about 0.75 GPH to run two Roof ACs.
My 5.5 onan uses about the same with both on high,and refridgerator also on ac
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Old 08-04-2012, 11:12 AM   #10
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Interesting! I am in the process of repairing my dash AC. My freightliner has a separate condenser and fan under the motorhome. So only added load would be the compressor and the alternator to add current for the condenser fan. Thinking that would not affect fuel mileage as much as running genset. Currently getting around 8 and if I subtract out the 1/2 gallon per hour for the genset at half load I get about 10.5 which is what I get without gen running.
I guess I will continue to repair dash AC and let you know what I find.
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Old 08-04-2012, 04:29 PM   #11
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The Onan manual for my 8k generator says that at half load the generator will use about .5 gal/hr. According to my a/c manual, each of my 13,500 btu/hr ac uses 15.5 amps which includes the fan to distribute the air. That means that two a/c's will draw 31 amps or about 3500 watts or just under 1/2 the generator capacity.

As for me, I'm going to continue to run the generator and house a/c
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Old 08-04-2012, 04:52 PM   #12
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Is your side fan Hydraulic? Your telling us that your dash AC compressor and fan puts that kind of load on a diesel? Side fan has to run constantly when the AC is on.
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Old 08-04-2012, 05:02 PM   #13
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Perry, I have a hydraulic fan on my side radiator. Should I run my dash air, the fan controller will open the solenoid valve all the way and the fan will run at high speed. One of the three sensors attached to the fan controller is a pressure switch in the dash a/c compressor. When the compressor head pressure builds up, the switch closes and the fan goes to high speed. This is no secret. If you notice a side radiator coach come into a campground blowing up a dust storm it is because the dash a/c is on and the cooling fan is running at high speed.

As an aside, if you are overheating because the fan controller isn't doing its job, you can remove the power from the fan controller and the fan will run at high speed. Useful to get you home should you suffer overheating because of faulty fan controller behavior.
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Old 08-04-2012, 05:07 PM   #14
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Great. Mine is belt driven with an electric clutch. Can't see much difference with it on or off. It's power off clutch on.
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