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Old 07-26-2013, 03:55 AM   #21
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Is it possible to run the roof air off the engine power only? Maybe by upgrading to a larger inverter??
No. The thermostat is 12vDC, but the Airstream-installed air conditioner is 120vAC. The engine alternator puts out 12vDC. That means the air conditioner would have to run off the inverter, EXCEPT the inverter doesn't put out enough power to run it, and even if it did, the alternator wouldn't put out enough power to keep up with the battery drain.

The Mercedes factory rooftop air conditioner on the other hand is a 12vDC model, and will run off the alternator.

I really don't know why Airstream doesn't just use the Mercedes a/c instead of replacing it.
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Old 07-26-2013, 04:24 AM   #22
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Or why don't they just use a larger compressor and a second evaporator in the rear like my TahoeTahoe.
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Old 07-26-2013, 04:51 AM   #23
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Or why don't they just use a larger compressor and a second evaporator in the rear like my TahoeTahoe.
Can you run the second evaporator while parked? If not, that's why no larger compressor and second evaporator. Need to be able to run it while camping as well, not just while driving. Not a criticism of your comment… thinking outside the box is never a bad idea. And even in my case, probably 70% of the ideas I ever have go nowhere, but the other 30%…

Anyway, the Mercedes rooftop a/c should run while parked with the engine off, especially if one installs a relay (or even just a manual switch next to the thermostat, for that matter) to switch it to operating from the house batteries when parked, and from the main/auxiliary chassis battery while driving.
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Old 07-26-2013, 07:11 AM   #24
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I agree re: running off batteries but wouldn't it take quite a bit of capacity to power 2.5 KW plus a much larger inverter?

The chassis version I was proposing would work for me since most of my use is traveling.
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Old 07-26-2013, 07:46 AM   #25
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I agree re: running off batteries but wouldn't it take quite a bit of capacity to power 2.5 KW plus a much larger inverter?
Forget I said anything. After giving the matter more thought, the 12vDC Mercedes rooftop a/c in place of the 120vAC Airstream-installed a/c brings with it more problems than it solves.

Either unit would require hooking up to shore power or firing up the generator to run the a/c. But with a 120vAC a/c unit, when you're on shore power the inverter/charger is in charge mode, not invert mode. The a/c doesn't drain the batteries at all. When the generator is running to power the a/c, the inverter is in invert mode, but it's inverting the generator output, not battery output.

With a 12vDC a/c unit, it's different. When on generator power, the a/c would be running off direct generator output without going through the inverter. So far, so good. But running on shore power, the a/c unit would be drawing off the batteries, and you would need a larger inverter/charger to keep up with the demand without depleting your house batteries, and the amount of current flowing through the batteries would be problematic.

Running on battery power alone, the 12vDC a/c unit would kill the batteries in no time, kind of like the tank heaters that can kill them stony dead in four hours or less.

Guess it really is better to use the Mercedes 12vDC rooftop a/c as a supplement to the 120vAC a/c unit, not as a replacement.
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Old 07-26-2013, 08:53 AM   #26
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I believe the generator input is treated like shore power. Inverting from the house batteries is only done when no shore or generator 120vAC.
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Old 07-26-2013, 08:56 AM   #27
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I believe the generator input is treated like shore power. Inverting from the house batteries is only done when no shore or generator 120vAC.
My mistake. Forget I said that, too.
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Old 07-26-2013, 01:10 PM   #28
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I agree with the idea of the Mercedes rooftop A/C as a supplement. I think the rest of our discussion was a misunderstanding all around.
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