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Old 06-06-2019, 05:02 PM   #1
mwemaxxowner
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Dometic AC Return VS Exhaust and Ducts

My camper has a Dometic Duo Therm AC unit, with ducts in the ceiling. The unit itself works great, it blows very cold air.

However, there are a few things I've found that seem peculiar to me. It has a grate with a diverter that I can open for the "quick cool" option, and directly beside that is a larger grate where the filter goes.

None of that is a big deal, but this next part confuses the heck out of me, when compared to the conventional AC knowledge that I THINK I have.

The unit blows air OUT of the filtered side, and draws air IN through the side with the diverter. If I'm not mistaken, that "quick chill" and diverter slide mean that air should come OUT right there. When the diverter is open air should come out there all at once to quickly cool down the living area. Close the diverter and air is then forced through the ducts. As it is, when I close the diverter all that does is choke air flow INTO the AC unit.

It also perplexes me that the air coming OUT of the unit is what's being filtered. I will attach a few photos and videos showing what's going on.

I discovered all of this because I've never had a unit with ducts in the ceiling before, and was investigating why little to no air comes out of the ducts. Then, I was trying to figure out where the filter should go (it is sans filter right now). The more I looked, and the more I googled about this particular unit and where the filter goes and what the "quick chill" feature is, the less and less sense things made to me.

In one photo you can see the paper being sucked into the right side of the larger grate. It seems air is being inhaled on that right side and exhaled on the left side. It almost seems like air is rushing in from the bottom right there because it's heading into the ducting, not necessarily returned into the AC. Not really sure. Click image for larger version

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Old 06-06-2019, 05:38 PM   #2
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https://youtu.be/y4ajkKiGEWg

https://youtu.be/-n3rrbBgx94

https://youtu.be/fB5XYJmeL3o
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Old 06-06-2019, 05:55 PM   #3
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I think I see the problem. That metal mount appears to be mounted backwards! Remove the four long bolts that bolt that thing to the air conditioner on the roof. Rotate that metal diverter thing around 180 degrees. Bolt it back down. The ducts should be on the opposite side of that relay control box. Bizarre. Did you buy this trailer used? Install the AC yourself?
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Old 06-06-2019, 05:57 PM   #4
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I did buy it used, and no I definitely did not install that myself. Lol.

I actually at one point wondered if that plastic cover was mounted backwards, then pulled the cover and ruled that out. What you said makes perfect sense!

I was just happy it's cooling EXTREMELY well, but would like the vents to work. Otherwise the bedroom isn't going to get any cool air with the doors closed.
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Old 06-06-2019, 06:06 PM   #5
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Yup. After watching your videos, that is most definitely your problem. That metal plate is rotated 180 degrees in the wrong direction. Look at the sides of your square hole. You'll see rectangular holes on each side. Those are the ducts leading to your roof vents! Since your metal plate is turned around the wrong way, it's essentially cavitating when you close that plastic diverter. The larger hole in the AC itself is the intake and the smaller one is the exhaust. The diverter should be on the same side as the small hole. When you close that, you're actually sucking air in through the vents for a bit but it has nowhere to go. Anyways, it's a super simple fix. Unbolt those four bolts on the metal plate, rotate 180 degrees being careful not to tear up your wiring, and bolt it back down. Problem solved!
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Old 06-06-2019, 06:12 PM   #6
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Here's mine. Some photos I took when I was making a write up on how to install a second AC. This is looking forward. Big hole is forward. Note the orientation of the metal plate and where my ducts are for the AC.


Looking forward.


This is the intake side. On the forward part of the AC. The ceiling ducts are on the aft side of the hole, not visible in this photo.
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Old 06-06-2019, 06:17 PM   #7
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Whoever installed that unit had no idea what they were doing. Probably the previous owner. Those wires should not be pinched between the AC and the metal wall. When you rotate it around, that problem will be solved too. Rotating just the plastic piece won't work either. Let me know how it works when you turn that thing around!
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Old 06-06-2019, 06:31 PM   #8
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Awesome sauce! Many thanks! Maybe this means my AC isn't a 2005.
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Old 06-06-2019, 06:39 PM   #9
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I’ve had two units come from the factory “backwards”. Not bad considering the number of units I’ve seen.
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Old 06-09-2019, 11:50 AM   #10
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I have another question regarding the AC. After it runs a while, I get some condensation build up that drips inside the RV. This occurs right at the exit for the cold exhaust air, and may be par for the course with very cold air passing over metal in high SC humidity. If so that's fine, I can certainly live with it. I just want to be sure.

I get plenty of runoff from the AC on top of the camper, so I'm inclined to think that there is no drip pan clog, but I can't say for sure. If you guys think it looks in the video like that COULD come from a drip pan clog I'll climb up there and take a look. This camper doesn't have a ladder, my old one did, so I'm a little nervous about climbing on top.




https://youtu.be/fGHZ5qNpPwE
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Old 06-11-2019, 05:41 PM   #11
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It doesn't really have a dry pan like an AC in your house. Just a channel under the coil that drains off to the side. Wouldn't hurt to check it out though. From the roof, remove the plastic cover. Mine is held on with four screws. Then just pull off the styrofoam cover. You can see everything there.
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Old 06-13-2019, 11:25 AM   #12
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Good deal. I'll climb up there and pull the cover then to have a look. It's probably a good idea to inspect it regardless. It's the first thing I did to my old camper, but it had a ladder and a walkable roof.
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Old 06-24-2019, 11:58 AM   #13
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Since our RV doesn't have a ladder, you might want to lay a piece of plywood on the roof, to spread out your weight over a larger area. Roof thicknesses have decreased over the years, and some RV instruction manuals tell you not to climb on the roof.
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Old 06-25-2019, 11:14 AM   #14
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Quote:
Originally Posted by johnlewis View Post
Since our RV doesn't have a ladder, you might want to lay a piece of plywood on the roof, to spread out your weight over a larger area. Roof thicknesses have decreased over the years, and some RV instruction manuals tell you not to climb on the roof.
Yeah, that's why I keep mentioning that my roof has no ladder and is not deemed "walkable". Otherwise I would have already been up there and pulled the cover off. I probably would have done that regardless. I like the idea of using a piece of 2" thick pink foam board under me up there!
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Old 07-10-2019, 07:18 AM   #15
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Spinning that assembly around did the trick. I now have air coming out of all vents and the bedroom stays cooler. It's not a ton of volume, but once the quick chill gets it cool it is much better now on the ends.
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