Finished the AC Mods - Here's the result

[5th_Time]

I spent the weekend completing the AC mods on my 36RL. I followed the excellent recommendations and pictorial by Robert (ROB_FLA) who really deserves the credit as the “pioneer” for these mods.

POOR COOLING – REMEDIES. I have always had decent velocity from the living room supply outlets (in the vicinity of the rear AC), but the bedroom supply outlets (in vicinity of the front AC) have always been deficient. Even though they are a few feet further from the unit, they only seem to flow about 1/3 velocity of the rear. Further, when I test, the airflow does not dramatically increase whether the front or rear AC is on. Ironically, the front AC is the unit I wish performed best – due to the much noted lack of insulation in the front cap heating up the bedroom.

First, I previously repaired the loose foam separating the supply and return chambers under both roof units by sealing with foil tape. This seems like your biggest gainer depending on the condition of the factory install. On mine, the rear was completely out and the front was just hanging. I followed the photos on the forum by Brad (Adam Ant) for this fix (took about 3 hours).

Next, I trimmed the excessive supply outlet collars in the ceiling. I removed all 9 ceiling supply outlets, sealed the complete hole with foil tape, trimmed the removable collars down to ¾”, and reinstalled. This is your most straight-forward modification after you review the photos by Robert (took about 5 hours).

Lastly, on the consensus that our AC units are starved for return air, I added additional AC return inlets. Robert’s (ROB_FLA) pictures of adding four additional return air inlets on the ceiling looks just like a factory install. I ordered the additional inlets from Redwood through Tiara in Indiana so they would be exact duplicates of my existing. I like Robert’s recommendation of cutting-in each additional duct 10” toward center at each existing location – effectively doubling the number of return air inlets and looking like a factory install. Another bonus is that each 4” x 10” hole you cut in the ceiling (and foil return air duct) brings you closer to the ducts of the rigid AC supply (discharge) ducts which bisect the foil return air ducts at 90 degree angles a few more inches further toward the centerline of the trailer. This helps you to make some corrections there.

CAUTION: HIGH RISK OPERATION. Cutting in the holes for the new 4”x10” return inlets turned out to be extremely high risk. At least on the RL/RE/FB model, you must use an oscillating saw to shallow-cut the 1/4" wood ceiling panel for the 4”x10” hole (spaced 10” from each existing inlet). If instead you use a saber saw, keyhole saw or any other deep cutting means, you will definitely cut through the 15-wire bundle running curb side just above the ceiling panel. In fact, as I pulled the freshly cut square out of the ceiling near the front AC, the entire wire bundle was centered directly above the piece I just removed. These are the ceiling wires for the 12v interior lights, outside lights, satellite prep, etc - any low voltage power for the rear half of the trailer.

On the street side near the front AC cut, if you do not use an oscillating (shallow depth) saw you will cut through (3) 120v ROMEX power lines just above the area of the cutout. These power the rear outlets and entertainment center. You can’t see these wire bundles ahead of time (by using your phone to take a picture inside the return ducts) because the wire bundles run between the underside of the foil duct and ceiling panel.

These same wire bundles continue toward the rear cap and present the same danger when cutting-in the additional return inlets at the rear ceiling location.
Once your holes are cut for the four additional return air inlets, you can stick down your wire bundles and seal the foil duct to the ceiling board with generous amounts of foil tape.

MY CONCLUSIONS:
Having completed all the modifications done above, my impression is that it’s only possible to improve the A/C cooling by a factor of about 10%. Each mod you do picks up a few percent here and there, but these mods did not result in increasing the bedroom outlet velocity in any dramatic way.
Like a racing engine, AC performance is mostly a function of air volume in/air volume out.

It’s apparent that the AC units are starved of return air to cool and send out the supply ducts. This seems due to two main reasons. First, the return A/C ducts are only about 14” wide and 4” high. Unfortunately, the 2½” discharge ducts cross through these ducts consuming the majority of space. Second, in order to seal the foil return duct where it is bisected by the discharge duct, the workers use gobs and gobs of expanding foam sealer which invades the return duct and then hardens. In my ducts, this huge amount of foam consumed about 20% of the remaining airflow space.

In the end, out of a 4+” return air duct, you have only ½” to ¾” above the supply duct and 0” to ½” under it for return air to flow to the roof unit. Given the above, installing additional return inlets is minimally effective (and risky!) OK- you’re basically reducing the backpressure entering the return duct by half, but because the supply duct constriction stands between the roof unit and the new inlet you just installed, the roof unit receives little to no additional air. Ideally, the additional inlet would be installed beyond the restriction caused by the bisecting supply duct – somewhere between the bisection and the return air plenum under the roof unit. Of course, the penalty would probably be increase fan or airflow noise.

After you have cut the additional return inlet holes, you might possibly pick up a few percent volume by studiously trimming the expanded foam back to the sides of the duct with a long razor knife. Since you can only reach one side of the foam where the supply duct crosses, this would be a lot of work for an assumed benefit (ie. you can’t see or feel the sealer applied to the far side (inside) of the ducts but it's a lot!.)

BTW, don’t bother to fastidiously clean the return inlet filters. After three years, mine were barely colored by dust and you can certainly see through them even dirty. Their filtering capabilities are minimally effective and even when dusty, they certainly don’t impede any airflow. If pets travel with you or you full-time, your experience may be different...

[Shane Wood]

Nice write up Mike thank you

[Steve & Linda]

Wow, a very in-depth report...thank you. Bottom line is very little return form a bunch of work.

[5th_Time]

If you're desperate, 10 or 15% gain is a necessary improvement. Also, from what I've read here, the FL owners seem to be more satisfied after the mods. A cfm measurement would tell the tale...

[johnboytoo]

And IMHO,
should not HAVE to be done with the proper design from the manufacturer.
I truly think they design the climate control for the moderate temps and not for the extremes.

[rob_fla]

Originally Posted by johnboytoo

And IMHO,
should not HAVE to be done with the proper design from the manufacturer.
I truly think they design the climate control for the moderate temps and not for the extremes.

I would agree

[rob_fla]

Originally Posted by 5th_Time

MY CONCLUSIONS:
Having completed all the modifications done above, my impression is that it’s only possible to improve the A/C cooling by a factor of about 10%.

BTW, don’t bother to fastidiously clean the return inlet filters. After three years, mine were barely colored by dust and you can certainly see through them even dirty. Their filtering capabilities are minimally effective and even when dusty, they certainly don’t impede any airflow. If pets travel with you or you full-time, your experience may be different...

I agree, the improvement's for all the effort is only 10% to 15%, seems like a lot of effort for such a small improvement but it does make a performance difference and every little bit helps

As far as the filters, yes I fulltime and even with the added filters, (now have 4 return filters on each unit) I still have to clean them about once a month here in the south during the summer since the A/C units run continuously during the hot part of the day

[mhs4771]

I don't know, we've cleaned our four filters a few times in the year and half that we've had this MB and everytime they were pretty full of dust. I guess we're some of the lucky ones, haven't done any of the AC mods yet and can still keep the inside at 72 with 90+ outside. On our RF we maintained 72 inside in Parumph, NV with outside temps over 100.

[Char & Steve]

I'm one of the FL owners and all the mods are completed on the AC by either myself or factory. So far so good but next month will be the test down South. I haven't added the additional returns since I want to see how it works before adding. Have any FL owners explored the front cap on how well it's insulated? I haven't yet but this winter will get into it.

Steve

[rob_fla]

Not sure how to get access to the front cap on the FL to explore it.

Nothing to remove except the whole wall panel on my FL, any ideas Steve ?

[Char & Steve]

That's exactly what I plan to do Rob. I have a few interesting ideas for the front wall and also tossing around a coffered ceiling in main areas. This will also allow me to fix all the screwed up duct work. As much as I need a garage and looked at the GD with that small one (not the big garage) we just have not found a wow?

So it's time this winter to change a few things and make it what we want. I got Char all worked up when I said it might be time for a roof top deck with spiral stairway were her W/D goes so she went out and bought the new set quick.

Steve

[rob_fla]

Originally Posted by Char & Steve

That's exactly what I plan to do Rob. I have a few interesting ideas for the front wall and also tossing around a coffered ceiling in main areas. This will also allow me to fix all the screwed up duct work. As much as I need a garage and looked at the GD with that small one (not the big garage) we just have not found a wow?

So it's time this winter to change a few things and make it what we want. I got Char all worked up when I said it might be time for a roof top deck with spiral stairway were her W/D goes so she went out and bought the new set quick.

Steve

I look forward to a review of your winter mods, sounds way beyond my capabilities

[travelin' texans]

20160807_115711.jpgMy foam filters crapped out & found these at Ace Hardware about $5 a dozen, just trim length to fit & toss them when dirty, it's amazing how much more dust they trap.