Dometic TJ22 12V Compressor Fridge (long)

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Dometic TJ22 12V Compressor Fridge (long)

Postby bdosborn » Tue Sep 20, 2011 12:09 pm

The 3 way absorption fridge has been replaced with a 12V compressor fridge in the Boxcar. I've been having trouble with the 3 way in hot weather lately. Interior temps would go over 40F whenever the outside temperature went over 90F or higher. I've tried every fan/ventilation trick I could find on the internet and finally threw up my hands in defeat. I found a great deal (they were discontinuing the fridge) on a Dometic Tundra TJ22.

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This is a 2.2 CF marine fridge formerly built by Tundra before they were bought by Dometic. We used it during our trip to South Dakota and pulled the fridge through 101F heat in Wyoming. The fridge maintained 36F during the entire trip. We were plugged in the whole time camping so I didn't watch the amp-hr usage. To get an idea of the draw I ran the same tests I ran on the Waeco cf-40. The Tundra uses the same Danfoss BD-35 compressor as the Waeco but it's twice as big on the inside. I sampled amps, amp-hrs and exterior temp and logged them every minute for 24 hours without ever opening the fridge door. I didn't log the interior temp but I did measure it using a wireless thermometer. The Tundra is remarkably consistent and the interior temp doesn't vary more than +/- a degree over 24 hours.

The first test I ran was with the interior temp set at 36F and the outside temp at 78F. I wasn't happy with the amp-hr usage as it was higher than I expected compared to the Waeco. 31.9 amp-hrs is higher than I'd like.

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An interior fan helped a lot with the 3 way fridge so I tried that. The interior temp went up to 38 but I imagine its more uniform inside. It didn't help at all, in fact the energy usage went up a little. Its not worth the energy penalty of running the fan (I only measured the fridge usage, the fan is not included in the measurements). I don't know why it went up, maybe it increased the heat transfer to the walls?

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Then I remember that I had set the Waeco at an interior temp of 40F when I tested it so I tried that. That made a huge difference in the energy usage. Now we're down to a number that I can live with at 24.1 amp-hrs total or 1 amp-hr average.

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Then I tried it with a 12 pack of beer inside to see if that helped by adding some thermal mass. The amp-hrs is down a little but I think that's because the exterior temperature was down at 75F. The beer was already cold, the fridge was off and warm at the beginning of the test. I wanted to see the effect of adding already chilled items to the fridge. All other tests were with the fridge already cold. Adding the beer and starting up the fridge didn't affect the usage much. It took 2 hours for the temperature to get to 40F.

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For comparison, here's the Waeco test results. The Waeco runs less often than the Tundra but runs longer when it does. It also has electronic temperature control while the Tundra runs off an mechanical thermostat.

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The Tundra is almost exactly twice as big as the Weaco and the energy usage reflects that. I've added some extra insulation to the Tundra but using twice as much energy for twice as much interior room seems about right. I don't care much what the usage will be during the day as I have solar panels that can easily run the fridge. The night time usage is manageable for me at around 1amp-hr (assuming I keep it at 40F overnight). I've added 10-15 amp-hrs to my night time usage, which is around 25 amp-hrs now. These aren't real world numbers since I didn't open the door but it should give a better idea of usage over guessing based on the fridge specs.

The good news is that the fridge just works. I'm not having to watch the fridge temp to make sure our food doesn't spoil. I don't think I'd buy a compressor fridge if we lived where it didn't cool off at night but it does where we camp. It's not for everyone but it seems like it will work for us. I'll post an update after we boondock with it.

Bruce
Last edited by bdosborn on Wed Jun 10, 2020 9:31 am, edited 1 time in total.
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Re: Dometic TJ22 12V Compressor Fridge (long)

Postby RJ Howell » Wed Jun 10, 2020 6:25 am

Awesome job! A great baseline to work from!
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Re: Dometic TJ22 12V Compressor Fridge (long)

Postby Cosmo » Wed Jun 10, 2020 6:31 pm

Great research Bruce! Love the data.

It is assumed (by me) you are working with the food already chilled to the desired temp which saves lots of energy.

I tested using my fridge with square water containers. While testing the Dometic-35 without a fan I found the water containers on top were several degrees warmer than the containers at the bottom. Adding a fan made all the containers close to the same temp . Fan or no fan may depend on the desired result and available space. I usually bring a fan and when I eat enough to create room I deploy the fan.

Searching the web opinions seemed to show it wont matter much if the fridge is full or not. More air space and less food chill and warm faster faster, causing more duty cycles with compressor running. More mass less duty cycles but compressor runs longer with each cycle. I did not find a clear answer.

Thanks so much for sharing the useful data!

=Cosmo



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Re: Dometic TJ22 12V Compressor Fridge (long)

Postby bdosborn » Thu Jun 11, 2020 4:52 pm

Boy, its hard to believe I posted this almost 9 years ago. I'm glad you found it interesting, I sure did when I ran the tests.

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Re: Dometic TJ22 12V Compressor Fridge (long)

Postby Shadow Catcher » Sat Jun 13, 2020 6:02 pm

I wrapped our Waeco in Reflectix as much to reflect sun. And yes I remember the study.
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