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Old 06-01-2014, 09:04 AM   #1
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Making a 2-way Fridge into a 3-way Fridge?

In theory, one could add a 12v, 275 Watt heating element to a 2-way fridge to make it a 3-way fridge. Any thoughts? I haven't looked at the "boiler" where the element goes as mine was recently replaced by the previous owner. Could an element be added in there somewhere and be effective? Clearly, only one heat source would be used at a time.

The idea being that, while moving down the road, 12V from the TV would supply the energy for the fridge thus eliminating the concerns about keeping propane on and burning safely. 12V 275 Watt may not be as effective as propane, but in the right conditions, it should work to at least keep the boiler somewhat hot and the fridge somewhat cold.
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Old 06-01-2014, 09:30 AM   #2
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In theory, one could add a 12v, 275 Watt heating element to a 2-way fridge to make it a 3-way fridge. Any thoughts? I haven't looked at the "boiler" where the element goes as mine was recently replaced by the previous owner. Could an element be added in there somewhere and be effective? Clearly, only one heat source would be used at a time.

The idea being that, while moving down the road, 12V from the TV would supply the energy for the fridge thus eliminating the concerns about keeping propane on and burning safely. 12V 275 Watt may not be as effective as propane, but in the right conditions, it should work to at least keep the boiler somewhat hot and the fridge somewhat cold.
You should be aware of the 12 volt current drain.

W = E x I

I = W over E

That spells out you current drain, which will be 23 amps.

It's not worth it to make that conversion.

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Old 06-01-2014, 10:34 AM   #3
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Andy's right - 23 amps is a heck of a lot of power, especially considering that you're trying to get that current from the tow vehicle alternator, though the vehicle wiring harness, through the tow connector, then through the trailer fuse box and wiring and over to the fridge. In my ever-so-humble opinion, you'd want to use 4 AWG wire, or take a chance on something getting way too hot.
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Old 06-01-2014, 12:37 PM   #4
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12VDC conversions are possible as most cooling units already have a second steel sleeve to hold the DC heating element. You would have to install larger DC wiring for this element and a new control board. Still, the 12VDC option is only for maintaining the fridge temp and will not cool down a warm fridge.

A better option might be a marine fridge that uses a Danfoss compressor. AC or DC use and very efficient with Los amp draw.
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Old 06-01-2014, 12:51 PM   #5
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The idea being that, while moving down the road, 12V from the TV would supply the energy for the fridge thus eliminating the concerns about keeping propane on and burning safely.
Just run the propane. Propane fires/explosions are vanishingly rare in RVs. Most fires are electrical in origin. By the numbers you'd be better off for safety by turning off the 12v at the battery disconnect and leaving the propane on.
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Old 06-01-2014, 01:01 PM   #6
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Eight or ten gauge would suffice for 25 amps @ 25 feet.

A simple switch rated at 25 amps would do in a pinch. What the computer don't know.....

http://www.offroaders.com/tech/12-vo...gauge-amps.htm
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Old 06-01-2014, 01:29 PM   #7
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The real watts from the Dometic web site is 150 on the 12VDC and 12.5 amps. That's on the RM2353 and the 120VAC watts is 175 for 1.5amps.
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Old 06-01-2014, 01:38 PM   #8
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Fwiw, I agree with using the propane.
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Old 06-01-2014, 04:04 PM   #9
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There is a reason for 3-way fridges otherwise they wouldn't make or sell them. That being stated, I was hoping to get to the practicality of adding 12V for whatever reason one had to do so. The risk of running propane was just one example. Perhaps the 12V element could be smaller in wattage? I believe there are also combination elements out there.
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Old 06-01-2014, 04:07 PM   #10
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I think the idea will work so long as the fridge has a spot to install the element.
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Old 06-01-2014, 05:44 PM   #11
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There is a reason for 3-way fridges otherwise they wouldn't make or sell them. That being stated, I was hoping to get to the practicality of adding 12V for whatever reason one had to do so. The risk of running propane was just one example. Perhaps the 12V element could be smaller in wattage? I believe there are also combination elements out there.
The 3 way is much more practical in a motorhome as you are much closer to the batteries and alternator.
Even at that it is a big load and only maintains the temp.

Just run with the propane on
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Old 06-03-2014, 09:01 AM   #12
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There is a reason for 3-way fridges otherwise they wouldn't make or sell them.
For a time from the 1970s to the 1990s, small 3-way fridges were popular in motorhomes and, to a lesser extent, truck campers and pop-ups. That is because, during that era, RV fridges did not have electric ignition. It was common for the propane flame to blow out while running down the highway, so 12v operation was more reliable.

A secondary reason they were used was that small motorhomes tended to have limited propane capacity, and their owners found it more difficult logistically to fill propane than do trailer owners since it motorhome tanks are typically not removable (there are exceptions).

3-way fridges have never been widely used in travel trailers (except pop-ups) because it is rare for the charge line from the vehicle to provide sufficient power for them to work well.

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That being stated, I was hoping to get to the practicality of adding 12V for whatever reason one had to do so. The risk of running propane was just one example. Perhaps the 12V element could be smaller in wattage? I believe there are also combination elements out there.
It's a hassle and I'll bet you'll never actually build it and use it. You have to add the element and then figure out a way to control it. If you have the 2-way model of a fridge that is also sold as a 3-way then maybe you can add the element and the relay and replace the control board. The larger fridges aren't made in 3-way models so you'll have to make up some of that from whole cloth.

A few months ago someone wanted to run the 120v side of their 2-way fridge off an inverter citing the same "I don't like gas, gas explodes" line of reasoning. They never built it either.
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Old 06-03-2014, 09:59 AM   #13
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Damn, must most every thread be a war of wills?

I would run propane going down the road, but that isn't what the op asked.

He asked whether it is possible. I say that if the fridge has provisions for a 12 volt element it should be possible.

And since it is pretty much consensus that a 12 volt element is marginally sized, meant for maintenance, I don't see why its "control " couldn't simply be a toggle switch.

The refrigerator control would never even need to know that the 12 volt element is even present.
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Old 06-03-2014, 10:03 AM   #14
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Making a 2-way Fridge into a 3-way Fridge?

By the way, there is a 7.1 cuft Norcold on eBay, so apparently three ways are not exclusive to tiny trailers with tiny refrigerators.

Most tow vehicles have alternators that produce well over 100 amps. On GM vehicles the trailer hot line is fused at 30 or 40 amps.

In my Sovereign I run a 10cu ft residential fridge on an inverter, my tow vehicle keeps up with and gains on the draw going down the road.

There is more than one way to approach an issue.
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Old 06-03-2014, 11:29 AM   #15
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Damn, must most every thread be a war of wills?
Maybe, but this thread appears to me to be a discussion of facts together with sharing opinions, at least some of which are based on personal experience or expertise.

Quote:

I would run propane going down the road, but that isn't what the op asked.

He asked whether it is possible.
He asked, "Thoughts?"

Quote:
And since it is pretty much consensus that a 12 volt element is marginally sized, meant for maintenance, I don't see why its "control " couldn't simply be a toggle switch.
The 12 volt element in 3-way fridges has a reputation for being suitable only for "maintenance" because most RV 12 volt systems will not deliver adequate power to the fridge. Since 3-way fridges do not regulate the DC voltage, they must be designed to withstand around 13.5 volts which is what they typically see in a well-executed motorhome installation while the engine is running.

Installations that use undersize wire or that have loose connections at any point will only deliver 11-12 volts to the fridge. This includes most trailer installations, in large measure because of the resistance in the charge line circuit to the tow vehicle alternator. Heat produced is proportional to the square of the voltage. At 12 volts you get 80% of the heat you get at 13.5 volts. At 11 volts you get 66%. 80% of the rated heat will give you little cooling. 66% will give you none at all.

This is why 3-way fridges have a reputation for failing to cool properly on 12 volts.

But if the fridge is properly wired, and the charge circuit works the way it's supposed to, and you start with fully charged batteries in the trailer, then yes, you could get 13.5 volts at the fridge. And it will cool just the way it would on gas or electric. With just a switch , on a cool, cloudy day, you're not only going to freeze the lettuce, you're going to freeze the beer.

There also has to be an interlock so that two power sources aren't on at once, or the boiler will overheat, and may crack and leak.

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By the way, there is a 7.1 cuft Norcold on eBay, so apparently three ways are not exclusive to tiny trailers with tiny refrigerators.
As I noted upthread, they were once common in motorhomes.

Quote:
Most tow vehicles have alternators that produce well over 100 amps. On GM vehicles the trailer hot line is fused at 30 or 40 amps.
Try drawing 30 or 40 amps from the connector and see what voltage you get.

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In my Sovereign I run a 10cu ft residential fridge on an inverter, my tow vehicle keeps up with and gains on the draw going down the road.

There is more than one way to approach an issue.
It is rare for charge line circuits to work that well, though not unheard of. Mine is not stock and will deliver 40-50 amps. Also, inverters typically have the ability to compensate for input voltage swings within a reasonable range.
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Old 06-03-2014, 11:42 AM   #16
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I guess one of my points was that if my tow vehicle will run my fridge that has an actual compressor, and recharge my batteries going down the road through the stock tv wiring, which it does, I think that probably the tow vehicle will run the heating element in the fridge.

Wiring with proper gauge wire was discussed, I even posted a chart, and yea, it would be important.

From what I have read about the three ways, I can't say for sure, but I don't think that there is a lot of risk in the low power element freezing things in the fridge.
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Old 06-03-2014, 12:50 PM   #17
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If anyone is interested I will add a comment about my method. I have the old fridge, manual start, in my moho. I run exclusively on propane. It's propane consumption is next to nothing and changing to electricity at a park and relighting the propane is just more hassle than it is worth. Unless I am staying for a week or more I just leave it on propane. It has never blown out on the highway.
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Old 06-03-2014, 01:29 PM   #18
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Another option might be to plug the fridge into a 12v/110v inverter when going down the road. With a strong alternator, a good 12v battery setup and the proper inverter, it should work just fine.
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Old 06-03-2014, 04:37 PM   #19
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Good point Jim. That would be easier.
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