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Old 02-21-2022, 02:42 PM   #1
Bourbon County
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Racetrack duct

Does anyone know what year Keystone started putting the racetrack duct system in their RVs? I have a 2017 model 3661RL Montana.

My unit has 2 ACs and apparently blow their supply side air into 2 common ducts, one on either side of the coach. I tried looking with my bore scope and really can't really see anything but duct insulation. I also turned both ACs on and tried using my thermal camera to find the ducts, with no success there either. Fortunately this does mean there are no duct leaks of any consequence.

I understand the idea behind the duct configuration; since the rear unit is a heat pump, it can blow warm air throughout the coach and on milder summer days possibly 1 unit could maintain temperature or if one unit happened to be out you still have some cooling.

The best I can tell, the racetrack design attempts to keep the air blowing in the same direction regardless if one or both units are on; sounds good in theory. I know Grand design has 1 or sometimes 2 crossovers in their racetrack ducting. My question is if you have 2 units separated by 30 feet or so and an AC on either end blowing in different directions towards each other, wouldn't that just create a lot of turbulence in the duct system between the 2 units?

I did a high tech test on airflow direction using a strip of paper. I removed a supply diffuser in the (front) bedroom and turned the fan on for that unit, the rear unit was off. The air was blowing towards the rear of trailer as I expected. Repeated the test with front unit off and rear unit fan on high, the air in same spot was definitely towards the front of the coach as I expected. I then turned the fans on both units on HI and the airflow was still flowing towards the front of the coach.

I'm not from HVAC background but have had training on residential duct systems and air balancing. One of the enemies to airflow and killer to AC systems is static pressure. Is my thinking wrong that this configuration would just add a lot of back pressure when both systems are running?
 
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Old 02-21-2022, 04:09 PM   #2
Carl n Susan
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Keystone started putting the racetrack duct system in their RVs in 2012 AFAIK.
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Old 02-21-2022, 04:15 PM   #3
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I looked thru some archived brochures. I am not sure when they started referring to the ductwork as “racetrack ducts”. The first screen shot below is from the 2018 brochure. The second screen shot is from the 2013 brochure and illustrates the same concept. I think they called it “whisper quiet”. The ductwork illustration did not appear in any of the older brochures going back to 2001 in the archives I looked through. I have no idea when the ducted ACs first appeared however.
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Old 02-22-2022, 09:52 AM   #4
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Old 02-22-2022, 09:52 AM   #5
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My 2016 has it.
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Old 02-22-2022, 10:12 AM   #6
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F.Y.I. my early 2012 (Aug 2011) doesn't have the Whisper Quiet system. It has the old style dump screen. But there is duct work that disperses A/C when the dump valve is closed.

Later 2012 builds came with the Whisper Air which only uses the duct work. But as I recall, even my 2005 Montana had duct work for when you closed the dump valve.
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Old 02-22-2022, 12:55 PM   #7
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My 2009 had the ducts. Close the dump system and the ducts blew cold air.
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Old 02-22-2022, 04:04 PM   #8
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My 2006 has the duct work and the dump louver.
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Old 02-22-2022, 07:08 PM   #9
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Bourbon County - now that we have explored the history of racetrack ductwork - I am not sure we answered your airflow or AC issue. I know that during hot weather lots of folks are surprised that the ACs can’t cool the unit down enough to satisfy the occupants. We address the issue with auxiliary fans pointed at DW and I in the easy chairs and dear dogs on the rear couch…and we run both units on high day and night ( you know - Texas heat!). Maybe someone else can address the technical air flow issue.
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Old 02-23-2022, 04:26 PM   #10
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Thanks to all that replied, very helpful.

I'm planning on doing some mods and upgrades when it warms up a bit and quits raining. I had stumbled across info on the Keystone "Blade" system that they seem to be very proud of. On the surface, it looks like a great idea since it diffuses the air in a unidirectional fashion and is much less restrictive than the directional diffusers that were standard on my model.

As it turns out, the "Blade" is really not that unique at all. There are a few plastic round adjustable diffusers that operate the same way. The "Blade" has a lower profile sleeve that extends through the ceiling into the duct and does have an adjustable scoop extending into the duct that can be adjusted to maximize airflow through that diffuser. I went to a local Keystone dealer to see if I could buy some as repair parts; I got the clear impression that I was trying to buy a state secret. The parts guy called his boss in and after they looked at the monitor for a few minutes proceeded to whisper back and forth to each other. They asked why I needed them and what model Keystone product I owned. They finally said they couldn't get them. I was fully expecting to find numerous black Chevy Suburbans and guys wearing suits and Ray Bans waiting on me in the parking lot. If they were called, I've evaded them so far.

My ACs actually work quite well. Mine are totally ducted, there is no return or dump dampers on the ceiling directly under the units. Everyone wants their RV to cool down quickly on a very hot day, but they are actually better at maintaining temperature once the coach does cool down, that's just the nature of the beasts. We camped in FL a few years ago and it was in the mid 90s every day, worked very well after initial cool down.

We had a day in the high 60s here last week and I checked the temperature across the evaporator coil, measured return and supply temps. I know it wasn't a good test with outdoor temps that mild, but I was getting a 22 degree drop across the coil which is outstanding.

I have an anemometer and will be doing some actual airflow measurements in the coming weeks and experimenting with different diffusers. I'll try to post my findings when I'm done.
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Old 02-27-2022, 06:21 PM   #11
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Duct improvement

My 2013 RL had collapsing ducts, especially in the intake. Improvement found here https://www.montanaowners.com/forums...ad.php?t=83870

I didn’t realize the racetrack blew air in a circle…I thought it was just a 2 spoked system that both ac units fed.
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Old 02-27-2022, 06:37 PM   #12
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This system is similar to what the Redwoods and Rushmore lines from Crossroads have and they are not without their issues. I also experienced trouble keeping the camper cool on a 90-degree day and started to do some investigating.
A couple of notes on what I found and corrections I have made, hopefully that can help some of you out.


1. I have two 15K- AC's ducted into one racetrack similar to what is above. The supplies are ran above the returns and are very problematic to crushing the returns, not allowing for proper air flow.



2. The original design only allowed for 2 return ducts per 15K AC, which did not allow enough return air flow to the units, causing freeze up issues.


3. The round supply diffusers, included a round "baffle" that went up into the supply duct to capture the air. The supply diffuser also by design, allowed for about a 1/2" gap between the duct and inside roof panel.


Steps to Correct...I snapped some photos of inside the return ducts and noted the smashed ducting, doing the old fashion tissue test, when running it would barely move the tissue.


A. I first cut 2-additional 6x9 return vents on EACH AC return duct and also foil taped the area where I cut. From there, I used rigid styrofoam to push the crushed section where the return goes under the supply back into shape....this must be done from each side of the return. A test of the additional return ducts and re-supported ducting with the tissue test, the return now almost took the tissue out of my hand.


B. I remove each of the supply diffusers, removed the plastic baffle and proceeded to foil tape every opening around the opening ring of the supply.


I also did some temperature checks and I improved my delta T temps by several degrees. I am seeing with one AC running about a 20-degree difference between return and supply. When two AC's are running, the number jumps to 25-30 degrees. I have seen supply air temperatures in the mid to upper 40-degrees when things really get rocking.



It should also be noted that prior to this modification, I cleaned both AC coils at the beginning and half-way through the camping season.


Again, hope this helps...
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Old 02-28-2022, 01:35 PM   #13
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Thanks gossamar for the detailed feedback.

Normally in a residential AC system you can accurately assume that the majority of systems are lacking in return. In the limited airflow measurements I've taken so far show that I have way more return than supply which makes me question my measuring process. I've made up some "capture boxes" out of rigid foam insulation and will be doing some more measuring later this week.

I think Keystone has a different definition of "racetrack" than does Grand Design. Grand Design builds theirs like an oval racetrack to keep air movement in the duct system going in the same direction with 2 or even 3 AC units running at the same time. Keystone's design is more like a drag race strip (which I suppose qualifies as a racetrack) only starting the race cars on both ends of the track destined to collide somewhere near the center point. Grand Design claims a 23% increase in airflow with their design which is quite substantial.

https://www.google.com/url?sa=i&url=...AAAAAdAAAAABAJ
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