OK to run fridge short-term on AC & DC combined?

[JArry]

I have the small cube three-way "mechanical" fridge (no control board, automatic mode changeover, etc.) - I know many in this A-Frame forum are familiar.

When I am running on AC at home or in a campground and getting ready to leave, I would like to change over the fridge to 12V DC, since that's what it is designed for. However, I believe there are separate AC and DC heating coils (correct me if I'm wrong), and when ambient temp is high, there seems to be quite a rise in fridge temp around the point of changeover. I am assuming this is because there is a period during which neither coil is heated sufficiently to cool the fridge.

(I use a wireless digital thermometer to monitor the temp.)

I recall that the fridge manual has a grave warning about running the fridge on two modes at the same time. But I am wondering whether it would really be all that bad to run AC and DC at the same time for an hour or two to smooth the transition. I suppose there is also the option of cooling the fridge on DC to begin with, before leaving home, while the AC is still plugged in, but I gather that might put unnecessary stress on the converter/charger (?).

I know there is also the propane option, but I would prefer not to run on propane while towing since the pilot light can extinguish and since it may be illegal in some places.

Also, maybe I am wrong about the reason for, or even the existence of, a rise in temp when changing over, in which case I would welcome any other thoughts.

[Flybob]

In most 3way fridges, the 12V side is not as efficient as the 120VAC or propane side as the 12V heater has a lower heat capacity than the 120V. The 12V side is primarily to be used when towing when you do not want to run propane. If you have shore power( 120VAC) why would you want to run on DC. A much better solution to your issue when the ambient temp is high is to ass a small ( quiet) 12VDC fan on the exhaust side of the fridge vent to remove the hot air more efficiently.

[IsleDog]

The 3 way we had in our pop up specifically said not to run both at the same time or damage would result. What model do you have?

[Bama Rambler]

The issue with running two sources at the same time is that you're running two heat sources into the same chamber, so the chamber is going to get a lot hotter and possibly cause a cooling unit failure or even a fire. I wouldn't recommend it.

It's not like a water heater where the heating systems heat separate locations.

[JArry]

Originally Posted by IsleDog

The 3 way we had in our pop up specifically said not to run both at the same time or damage would result. What model do you have?

I don't know the model offhand, but mine says the same thing. Just wasn't sure if it's legalese or if both modes could be run awhile using common sense. The building consensus seems to be I should listen to the instructions.

It is possible I am not seeing a rise in temps as a result of an AC-DC "transition period," but rather it is just the lower effectiveness of the DC coil as mentioned by Flybob. I'm curious to know whether anyone else has experienced the same thing, though.

[pgandw]

If you dig past the front page you'll find a thread I started called I'm liking my A-frame fridge. In it, I detailed the steps I took to make the fridge more efficient and more consistent at cooling.

Very quickly, in stock form, the fridge cools inconsistently for a variety of reasons: heat gets trapped in the top of the compartment, the vents don't naturally vent very well, there isn't enough height for a true chimney effect, the wind is from the wrong direction and blocks airflow, etc. Mostly, there isn't enough air flow across the cooling fins under adverse conditions.

I found that once I started driving at 40MPH+, the airflow made the fridge work really well on DC. When stopped, the fridge would heat back up (measured with a wireless thermometer).

The best answer is a computer case fan attached to the exhaust grill (I used tie wraps) to blow the hot air out when there is not enough wind, although the other measures certainly help.

I'm now looking at repeating the mods for my new A-frame.

Fred W
2019 Flagstaff T21TBHW
2014 Rockwood A122
camping Colorado and adjacent states one weekend at a time

[JArry]

Originally Posted by pgandw

I found that once I started driving at 40MPH+, the airflow made the fridge work really well on DC. When stopped, the fridge would heat back up (measured with a wireless thermometer).

And that's the only time I use mine on DC. Typically it's not hot enough in my area for the ventilation to be a problem, but I have been thinking about the fan mod.

[sundancer330]

I tried the 12v once after precooling overnight, our drive to the campground was just under 3 hours, when we arrived the frig was pretty warm. Now for the past few years we only use the propane when traveling to keep the frig nice and cold, never had the pilot go out and uses very little gas, I cant believe how small the flame is.

[aircommuter]

I worked on a 3 way refrigerator once because the 12v wasn’t cooling as well as the owner wanted. I found the voltage at the fridge was considerably lower than 12v, turned out the wire feeding it was too small. Increasing that made the difference. As far as the fan goes it even helps in cool climate because removing heat from the coil is the key.

[DouglasReid]

Originally Posted by JArry

I recall that the fridge manual has a grave warning about running the fridge on two modes at the same time. But I am wondering whether it would really be all that bad to run AC and DC at the same time for an hour .

It is in the manual for a reason.

DONT DO IT!

[JArry]

Originally Posted by aircommuter

I worked on a 3 way refrigerator once because the 12v wasn’t cooling as well as the owner wanted. I found the voltage at the fridge was considerably lower than 12v, turned out the wire feeding it was too small. Increasing that made the difference. As far as the fan goes it even helps in cool climate because removing heat from the coil is the key.

A poorly wired Forest River trailer?! Knock me over with a feather! Thanks.

[Alben]

I saw that warning in the manual and I've been careful to never have more than one heat source (12VDC, 120VAC, propane) on at the same time. What puzzles me is why Dometic didn't put some mechanical interlock in place between the 12VDC and 120VAC switches so that two can't be on at the same time. Interlocking with the propane knob would be considerably harder to do. One of the best mods I have done is install two computer case cooling fans. Here is my writeup: http://www.forestriverforums.com/for...ml#post1825956

[JArry]

Originally Posted by Alben

I saw that warning in the manual and I've been careful to never have more than one heat source (12VDC, 120VAC, propane) on at the same time. What puzzles me is why Dometic didn't put some mechanical interlock in place between the 12VDC and 120VAC switches so that two can't be on at the same time.

Interesting you should say this. I inadvertently tested my theory, or whatever, on my last trip. After setting up, I turned on 120V and somehow forgot to shut off 12V. Usually it's automatic for me. But I was in a hurry, and when I came back to the camper a few hours later, the fridge was running in the 50s. With BOTH power sources on. So this was either directly causing the fridge to overheat or preventing the cooling system from working properly. Granted, it could have been unrelated to either of these, since the ambient temp was high (in the 80s) and I had also not properly set the air conditioning. Still, needless to say, I will not be running 120 & 12 at the same time anymore, for ANY period of time. And I will be more thorough when setting up in the future!

Luckily, this episode doesn't seem to have damaged anything, as at least 120V and propane are working fine. I probably won't be using 12V anymore since it is so ineffective.

[DouglasReid]

Originally Posted by JArry

Interesting you should say this. I inadvertently tested my theory, or whatever, on my last trip. After setting up, I turned on 120V and somehow forgot to shut off 12V. Usually it's automatic for me. But I was in a hurry, and when I came back to the camper a few hours later, the fridge was running in the 50s. With BOTH power sources on. So this was either directly causing the fridge to overheat or preventing the cooling system from working properly. Granted, it could have been unrelated to either of these, since the ambient temp was high (in the 80s) and I had also not properly set the air conditioning. Still, needless to say, I will not be running 120 & 12 at the same time anymore, for ANY period of time. And I will be more thorough when setting up in the future!

Luckily, this episode doesn't seem to have damaged anything, as at least 120V and propane are working fine. I probably won't be using 12V anymore since it is so ineffective.

The purpose of the heat in the absorption type refrigeration is only to get the liquid in the sealed system to begin to flow upwards. Once it reaches the top it falls and cools in the cooling fins on the back, then runs through the portion where the cooling fins are located, cooling the interior of the refer.

With two heat sources you are probably heating that liquid in the system so hot that the cooling fins cannot remove all that heat and the refer does not cool.

Im no refrigeration engineer but I know that any cooling unit is only capable of removing X amount of heat. Exceed that capacity and it will not cool.