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Old 07-27-2018, 11:04 PM   #61
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To steer a bit away from the whole propane-while-towing issue, I have an observation and a question.

Dometic stated that 12VDC operation of the 3-way fridge in our Casita would produce a lower wattage than operation on 120VAC or LPG, resulting in less effective cooling; the fridge should be initially operated on 120VAC or propane to effectively cool it down before towing it and operating on 12VDC.

My question is what have folks observed when operating on 12VDC while towing in hot weather conditions? Was this satisfactory in terms of keeping things cool?
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Old 07-27-2018, 11:51 PM   #62
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I initially was going to order a 3-way fridge, but after learning how inefficient 12V was, I changed my order to a propane/120V fridge.
That was ten years ago, and I haven't regretted my decision.
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Old 07-30-2018, 07:41 PM   #63
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We run the fridge on 12v while towing with great results even in summer heat. The fridge maintains its temperature well.

The power line in our 7-way umbilical provides adequate power to heat the internal element.

I installed a relay off the fridge's 12v power cord which requires the trailer running lights to be on for 12v power to get to the fridge. This way I can control the fridge from the tow vehicle and the fridge doesn't pull power from the tow while parked, unless I leave the lights on.
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Old 07-30-2018, 08:12 PM   #64
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Quote:
Originally Posted by Markz View Post
We run the fridge on 12v while towing with great results even in summer heat. The fridge maintains its temperature well.

The power line in our 7-way umbilical provides adequate power to heat the internal element.

I installed a relay off the fridge's 12v power cord which requires the trailer running lights to be on for 12v power to get to the fridge. This way I can control the fridge from the tow vehicle and the fridge doesn't pull power from the tow while parked, unless I leave the lights on.
so the fridge only runs with the parking lights on the tow vehicle? huh.
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Old 10-12-2021, 12:24 PM   #65
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Compressor fridge

My strategy:

Replace refrigerator with similar sized 120 volt unit. These only cost $100 up to $600

Attach 400 w inverter to house battery. I use pure sine wave but I'm not positive that's required.

200 to 400 Watts of solar on the roof.

Attach solar charge controller to solar panels and the house battery.

Even if I only visit the RV once a month the beer is always cold.

Note that the 12 volt compressor refrigerator / freezers are definitely more efficient. But it's hard to find a good one that is not an ice chest style and they are more expensive than the high volume dorm fridges.

Just Google YouTube replace RV refrigerator with 120 volt compressor fridge.
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Old 10-12-2021, 12:42 PM   #66
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WAY higher power usage from a home fridge, than any of the 'danfoss compressor' based DC compressor fridges. also noisier.
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Old 10-12-2021, 02:10 PM   #67
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Interesting all the conversation in this thread, once it got back on the technical side of things.



I actually thought, initially, that 110v/12v on a 2 or 3 way fridge actually used a compressor, but to my surprise they don't. No wonder they're so inefficient.



I actually have one of those 12v (110v -> 12v for at home chilling) "cooler" style, and I'm contemplating trying to find a similar size, but as the "built-in" fridge for whatever trailer I end up getting (haven't purchased yet).


Do those danfross style need any kind of external venting? The 13' Burro I'm probably going to be looking at tonight/tomorrow currently just has a ice box with drip tube. Think it'd be possible to reasonably replace that with a danfross style fridge, or freezer/fridge? I do plan on LiFePO4 battery(ies), and a ground-mount solar setup for off-grid camping as needed.



Unfortunately that means waste heat from the compressor/heat-exchanger will have to be vented from the bottom/top of the fridge, and thus heating up the trailer. And that means that in a shut up trailer with hot weather & sun I worry that after a point it won't be able to keep up, and I'll end up with being >40F for too long.
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Old 10-12-2021, 03:28 PM   #68
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My new 10 Cu ft Furrion compressor refrigerator draws about 11 amps at 12 volts when running. But it only runs about 20% or so, of the time. It vents with a fan to the interior of the trailer. I'd say the loss/gain of an uninsulated trailer affects the interior temp much more than the waste heat from the fridge. Assuming a coefficient of performance of approximately 3-1, the heat emitted to the interior of the trailer would be 11 X 12 or 132 watts X 3 = 396 watts X 20% = 79.2 watts X 3.4 BTUs per watt = 270 BTUs per hour average. An electric heater on low (750 watts) emits about 2,550 BTUs into the room, or nearly 10 times as much.

Then factor in the loss/gain of a propane fridge with two large vents cut into the trailer wall.

Then consider that the compressor fridge runs off the sun with no propane required.

These fridges are much more efficient than a 120 volt dorm style fridge and quieter too. I have a dorm fridge in the house and it runs a lot, plus I can hear it in the room on the other side of an insulated wall. If someone were to run one from an inverter, there would be another 10% loss in energy and more heat added to the interior of the trailer from the inverter inefficiency.

At this point, I don't intend to ever plug the trailer in, and meanwhile the fridge is running and the freezer is at about 0 degrees F.

12 volt refrigerators also cool immediately, the same as a household fridge does. No waiting for 4 hours to begin to feel it cooling. No debating about the safety of a flame while driving either, although I never worried about that with my propane fridges.

These 12 volt compressor fridges are not the same thing as running an absorption fridge on 12 volts. The compressor fridge is running a small motor, the absorption fridge is running an electric resistance heater with a much higher electric load.
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Old 10-12-2021, 03:44 PM   #69
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I'm less worried about heat _loss_, more heat _buildup_. So for example, camping out in the desert during the summer. Ambient air temps >110F occasionally, LOTS of sun, even if I do get some shade/shade cloth over top to block most of the direct sun. So even if I have all the windows/doors/etc fully open, is ~110-120F ambient (assuming there will still be some heat buildup inside), that'll raise the duty cycle significantly, and so it'll be outputting a lot more heat, even with the insulation.


And if I don't want dust/dirt blowing in, everything will be closed up fairly tight. Even if we assume minimal direct sun warming, probably 120-130F easily, if not more, sometimes. Possibly even more with the fridge going at a high duty cycle.



Or without doing math, and I over-estimating the heat contribution from the fridge, even if it ended up going 100% (unlikely) for multiple hours?


BTW, what fridge/brand did you get that you're happy with? I'll have to see if they have one that'd be sized for whatever trailer I end up getting.
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Old 10-12-2021, 06:43 PM   #70
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A,

I named the brand in the first sentence, if you are referring to me.
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Old 10-12-2021, 06:50 PM   #71
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Oh there it is, sorry. Was reading the rest of your comment a few times to digest it all. Thanks!
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