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Old 07-15-2018, 07:43 AM   #1
louielouieohoh
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AC Register Design

I have a 2017 Cougar 326RDS. We have never had an issue with heating the unit in the winter (low of 25°).

The summer time cooling is another story. We live and play in California. When the temps get up to 95-105° outside, the AC unit has never quite done its job. I recently went through the whole system (Dometic Brisk Air II, 15k BTU with 5 registers). I followed about 6 threads in this forum, fixed a couple small leaks and opened the slots between the AC unit and the ducts (not very thorough manufacturing practice) and had the block the end of the longest duct run. Tested after the fixes and still have low flow out of every register. When we open the vent at the AC unit ... there is ample flow! Loud and strong!

From that point, I looked for and found the installation instructions from Dometic. According to the instructions, the ducting is sized correctly and we have only 5 registers (instructions call for 4-8 registers). The unit requires that every register have a minimum of 14 sq. inches of free unblocked area to work properly. It appears to me (I still need to check) that the surface area of my registers are a little small like 8 sq. Inches. I am wondering if this is why some posters have "opened" their registers and had success with more cool air flow?
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Old 07-15-2018, 08:57 AM   #2
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Well ... I pulled a register off and did the math ... the register has 8.6 square inches of free air flow (versus required 14 sq. in.). My next test ... run the unit with all registers closed except the farthest one from the AC unit and check the air flow. Wish I had a slack tube to measure the flow.
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Old 07-15-2018, 02:16 PM   #3
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Ugh, hearing these ac stories on top mine is making me rethink my purchase. I too am getting very weak airflow through my 2019 cougar
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Old 07-15-2018, 03:49 PM   #4
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I truly believe they shouldn't sell them without 2 AC units.
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Old 07-15-2018, 06:49 PM   #5
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That will be my argument when i bring it back tuesday
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Old 07-15-2018, 08:26 PM   #6
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From a engineering perspective with the poor R factor in the average RV the one or two AC's are totally inadequate with full sun on a 80 deg day or hotter in May June July and August. The heating ability of the Sun has more to do with it than outside temperature. You can go through the Thermodynamic equations and you will come up short on AC a lot short. Heck, the average car has twice as much AC output than the two AC's in our RV's.
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Old 07-16-2018, 08:41 AM   #7
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Originally Posted by Frank G View Post
From a engineering perspective with the poor R factor in the average RV the one or two AC's are totally inadequate with full sun on a 80 deg day or hotter in May June July and August. The heating ability of the Sun has more to do with it than outside temperature. You can go through the Thermodynamic equations and you will come up short on AC a lot short. Heck, the average car has twice as much AC output than the two AC's in our RV's.
I was surprised to see how much insulation there really is ... I removed two vents and saw about 1-1/2" of insulation batting in the ceiling.

My issue with the one unit is air flow ... I see no blockages, bad taping, etc. There were a couple small duct taping issues that are "fixed". If I open the register vent directly on the AC unit ... air blasts out. So I have air flow at the unit.

I did have one end of the farthest duct extend into oblivion (roof curved at front cap so phone camera can't see the end of the duct). I blocked that run with some closed cell insulating foam.

The registers in my unit don't have enough open area as per the Dometic instalation instructions (8.6" free area versus 14.0 required). I found a copy here. Since my RV has only 5 registers, I thought it might be logical to assume that the registers could have an even large free area (instructions say 4 to 8 registers). Assumption on my part ... but logical.

Therefore ... my next test is to remove a vent completely (farthest one from the unit) then close off all the other registers. The open area of the register hole is about 18.66 square inches ... the flow of air hole from that hole should be pretty good unless something else in the ducting is restricting flow or allowing the flow to escape somewhere else.

I am hoping the big issue was the unblocked duct as others in this forum have found.

Hopefully I can get out to the RV this week (in storage).
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Old 07-16-2018, 09:17 AM   #8
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Originally Posted by louielouieohoh View Post
... my next test is to remove a vent completely (farthest one from the unit) then close off all the other registers. The open area of the register hole is about 18.66 square inches ... the flow of air hole from that hole should be pretty good unless something else in the ducting is restricting flow or allowing the flow to escape somewhere else.
You might want to do some research on "duct back pressure".... There are more factors that affect airflow in ductwork than just the "blower on one end and the size of the hole on the other".... That "end run ducting" beyond the final vent opening is there for a reason. Whether Keystone engineered it correctly or not is something I can't answer, but blocking it "right to the vent opening" with foam may inhibit airflow more than it helps. I'm far from an expert in duct design, in fact, I have no training and very little experience in the area, but have read a number of articles that point to the necessary "end run beyond the last vent" as being essential to quiet, efficient ducting..... YMMV
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Old 07-16-2018, 11:11 AM   #9
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You might want to do some research on "duct back pressure".... There are more factors that affect airflow in ductwork than just the "blower on one end and the size of the hole on the other".... That "end run ducting" beyond the final vent opening is there for a reason. Whether Keystone engineered it correctly or not is something I can't answer, but blocking it "right to the vent opening" with foam may inhibit airflow more than it helps. I'm far from an expert in duct design, in fact, I have no training and very little experience in the area, but have read a number of articles that point to the necessary "end run beyond the last vent" as being essential to quiet, efficient ducting..... YMMV
Excellent points! The manufacturer recommends 4 to 8 inches past the last register. The duct size is optimumized as per Dometic. My RV, while not the longest is tall along the complete length of the RV so the area needing to be cooled is quite big as compared to my Iold Itasca Spirit. It had a 13500btu and worked well in 100° weather.

I'm still kicking myself for not adding the second AC ... I added every other option from the dealer at that time.
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Old 07-17-2018, 11:04 AM   #10
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Well fellow RVers ... I had the time so went out to RV and re-inspected the system. This time I was very meticulous about taping every little crack and crevice in both the intake and outflow of the AC unit. It was taped pretty good but found several spots with openings to the attic including the holes where the wiring passes.

Additionally, inspected every register opening (they were all taped very well). I made sure that the "end of plenum" locations were blocked off about 6" past the last register.

I removed all the register openings (louvered covers) and started the AC up. Wow! What a huge difference in air flow! I would say the farthest vent away from the AC unit was 5 times stronger than before. Can't wait to test a 105° day!
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Old 07-17-2018, 11:10 AM   #11
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Additionally... the campground host was driving by and we were chatting. He said most every RVer he's known has had to do the same thing. He also said his "AC service guy" said to take the cover off the unit (rooftop) and add more insulation around "the box". He said it might have some already but to add more helps. Has anyone ever heard or tried this?
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Old 07-17-2018, 03:40 PM   #12
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also own a 326 RDS with poor flow

I have a 2016/2017 326 RDS. We got it last fall and I just got it back out of storage. I haven't had good camping test, but I have not been impressed with the air flow either. I recently had an electrical problem that resulted in me opening up vents and what not, and I also noticed that for the most part the taping of duct work was ok. I did some optimizing of the taping and fixed some areas where I thought leaks might be happening - mostly ont he main unit itself where air first gets directed down the ducts.

I found that the air flow, especially in the bedroom where it's the weakest improved (couldn't really say by what factor). I"m hoping, especially based off of your results that I can improve it even further.

But I have been thinking about installing the bedroom A/C unit none the less.
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Old 07-17-2018, 04:15 PM   #13
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Originally Posted by datasponge View Post
I have a 2016/2017 326 RDS. We got it last fall and I just got it back out of storage. I haven't had good camping test, but I have not been impressed with the air flow either. I recently had an electrical problem that resulted in me opening up vents and what not, and I also noticed that for the most part the taping of duct work was ok. I did some optimizing of the taping and fixed some areas where I thought leaks might be happening - mostly ont he main unit itself where air first gets directed down the ducts.

I found that the air flow, especially in the bedroom where it's the weakest improved (couldn't really say by what factor). I"m hoping, especially based off of your results that I can improve it even further.

But I have been thinking about installing the bedroom A/C unit none the less.
Wondering ... do you have only 5 registers as well? Two in LR, one each in the kitchen, bath and bedroom?
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Old 07-17-2018, 05:10 PM   #14
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Yep. Same layout, sounds like.
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Old 08-19-2018, 01:48 PM   #15
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Id like to thank all of you for the help and support!

Finally got the time to test my unit. Here are the facts and results.

Outside temperature during test = 97° (and rising)
RV in full direct sunlight.
Started testing at 12 noon.
Inside temperature of RV = 91°

Started AC unit, set to 68°
After 15 minutes, return air duct at 84 and AC vent in bedroom (farthest from AC unit) 66°.
After 30 minutes, return air at 78, same vent at 57°.
After 45 minutes ... temp inside stayed around 78°.

Captain Kirk ... I dont think she's got any more!

I do see now that the skylight in the shower is a giant heater! I like the idea of "spraying" the outside with sealant to reflect some of the heat and/or the velcro pad thing too. I pointed my temperature probe at the "tinted plastic" during the test and it was 105°!

As always ... comments welcome!
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Old 08-19-2018, 03:14 PM   #16
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Additionally... the campground host was driving by and we were chatting. He said most every RVer he's known has had to do the same thing. He also said his "AC service guy" said to take the cover off the unit (rooftop) and add more insulation around "the box". He said it might have some already but to add more helps. Has anyone ever heard or tried this?
Have added Reflectix insulation an all my a/c covers underneath and always made a difference
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Old 08-19-2018, 03:48 PM   #17
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I have an older Cougar 27' 5ver. It was not wired for a 2nd A/C so I first replaced the fixed A/C vents and reinstalled new foil HVAC tape to close the void between the vent and duct so cold air wasn't escaping into the void. I sold my old 13.5K btu Dometic and bought a 15K Brisk II. I also closed off the open ended ducts at the end (have only four ducts) but didn't put the pieces of foam material back a bit as suggested by this thread (didn't see this). I actually have great flow through the vents. When installing the A/C, I found that the ducts are on the sides of the outflow part of the A/C and the foil keeping cold air out of the void under the top unit was not in place nor had it ever been in place in all likelihood. I fixed that with more foil tape.

At this point, the system was working about as well as it could but it wouldn't drop cabin temps below 80F when the outside temps are in triple digits when the camper is in direct sun. I also tried a portable 8K btu unit which exhausts out the window and that helped keep the cabin where you could live in it but not real comfortable. I added two 16" fans in the cabin and now it was OK but not great. Thought my summer camping wasn't going to happen.

Then I bought a roll of adhesive 20 percent window film and put it on all the windows. It is now darker inside and the windows are barely warm compared with before. Cabin temps are also in the mid-70s even in direct sunlight on triple digit days; it is HOT today along the Nueces River but we are very comfortable. I never really understood why the bedroom area was cooler than the cabin on hot days but it makes sense as there are only two fairly small windows to heat up. We have single pane windows.Just had to fetch a blanket for my wife who is napping in a Lazyboy and it is HOT outside. Maybe something I droned on about will help someone.
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