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Originally Posted by cghsatx
Really appreciate this info. I have the exact same issue (among others) on my 2019 3731. One question is what does "collapsed" mean on the duct-work. I took off a vent cover and there was about 1.5 inches of space, but I could push up the top part of the duct another inch or so. I am in Texas and have never been able to get the inside temp below about 78 in the hottest part of the day in the summer. "Auto" wont cut it and have run on High to have any change of cooling. Already have new thermostats and dealer gave me the working as designed speech, and wanting to sell me a 3rd AC unit, which might be a good idea.
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The issue is the returns are collapsed. I don't think my supply duct is. If you pull those covers and the foam filter and stick your hand inside, mine I could barely get my fingers through. Most folks have used 1.5" PVC pipe sections. I used 1.25 and they were touch to work in without damaging the duct. 2' on either side of the rooftop unit is all you need, a coupler, and install them in 12" sections at a time. I went from the roof and had to pull my units (easy to do). There was another post in this forum where the guy found that Keystone didn't align the units over the supply and return box properly. Mine was the same so I ended up opening the return side as 1/2 of it was blocked. It is all within the seal on the bottom of the Dometic, so no leak problems and I didn't have to make new mounting holes. It's easy to install the PVC from the roof on the 3820FK as there's plenty of room. The other thing that people have found is the supply air diffusers are restrictive. That is true. I've pulled all mine off and just have open holes waiting for the new ones to arrive.