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Old 09-14-2016, 10:19 AM   #1
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Roof ACs Not Cooling While Driving - Work GREAT Parked

This topic came up in a thread a year back, yet that thread is too old and will not accept replies. Searching for opinions here.

PROBLEM: My new (to me) Travel Supreme 42ME coach came with dual 15kBTU Penguin Duo-Therm heat pumps. Sitting parked, on generator or shore power, they will cool the coach down to <68 in 95-F ambient full sunshine. Driving down the road, though, they simply do not cool. The air coming out of the vents is >75-F and that allows the interior WHILE DRIVING to eventually become unbearable in 90+ ambient conditions.

From reading multiple other posts, there seems to be a disparity in performance between Class A roof-top AC systems while in motion: some work great moving or sitting still (like my Damon Outlaw's did); others only seem to work sitting still (like these Penguin Duo-Therms).

QUESTION: Why? What is causing the difference in performance and better...what can be done about it (if anything).

As one might guess by my moniker, I'm a pilot. In fact, a retired USAF Test Pilot and now work in flight test for the USAF. And worse for my DW, I'm an engineer. So...evaluating airflow and possible cause/results is in my DNA. That leads me to question a couple things:

1) Is the difference in performance perhaps related to some being 'low profile' units and some being 'standard height' (or non-low profile)?

2) Is the difference perhaps related to the design of the shroud?

3) Could the installation of other roof-top items, like a large satellite dome, disrupt the airflow and cause the problem?

Likely, as with most anything related to aero, it is a combination of things.

What would be helpful would be for those RV owners who run their ACs while driving to post a comment about performance (good or bad) along with the type of A/C unit and whether their is a satellite dome in front of it.

OBSERVATION: compare the shroud on the Carrier unit (discontinued) which is what I had on my Outlaw to the shroud on my 42ME Duo-Therms (in the roof pic and the separate stock Duo-Therm pic) to the 'New' design of the Duo-Therm shroud. Also note where the satellite dish sits in relation to my unit #1 and why that will affect airflow on my forward unit.

HYPOTHESIS: The design of the shrouds around the units is driving performance, as is the location of a dome directly in front of a unit.

Looking at the Carrier unit shroud, with the top fan exhaust and large side vents, evaporator performance would benefit from airflow over the coach roof while moving. Since more airflow = better cooling of the refrigerant, the ACs would work as well or better in motion. Conversely, the limited venting on the old Duo-Therm shroud, which has only a small vent area on the left (port) side near the base and none towards the top or back would create a high-pressure gradient along the base of the unit. This would trap air passing down the roof while in motion under the shroud thereby decreasing evaporator performance. Sitting still, the evaporator fan would not work against this gradient of airflow, so it is unaffected. Finally, a large disruption of the airflow in front of the unit would exacerbate smooth air flow away from the unit; hence, having a bulbous dome directly in front of the unit AC unit could actually decrease performance, as well. Perhaps this is why Dometic redesigned the Penguin AC shroud with more holes to allow better air flow out of the evaporator.

Look forward to other observations. Who knows...we might be able to come up with some workable solutions. I plan to remove the satellite dome and/or the A/C shrouds and do some test runs to see if there is a difference. Will post back results. In the mean time, perhaps others can provide some feedback on their personal setup and experiences?
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Old 09-14-2016, 11:51 AM   #2
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I have the same roof air units , with the shroud in the top right corner of the second photo; and a satellite dome.
When traveling , dash A/C is my first unit of use, because of poor air circulation from the roof A/C to the dash/drivers area, and I've only had to run the gen set and roof A/C's once in my travels ; driving west in the afternoon past Edwards AFB, at 95 degrees. They worked well in that situation. RH. less than 7%, sensor just said , low.
Is your problem is common on all trips ?
Or maybe only on days with high humidity , that may be freezing the roof air units up ; because the air flow disrupts the water draining out through the units drain pan openings .
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Old 09-14-2016, 12:19 PM   #3
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Quote:
Originally Posted by f15epilot View Post
This topic came up in a thread a year back, yet that thread is too old and will not accept replies. Searching for opinions here.

PROBLEM: My new (to me) Travel Supreme 42ME coach came with dual 15kBTU Penguin Duo-Therm heat pumps. Sitting parked, on generator or shore power, they will cool the coach down to <68 in 95-F ambient full sunshine. Driving down the road, though, they simply do not cool. The air coming out of the vents is >75-F and that allows the interior WHILE DRIVING to eventually become unbearable in 90+ ambient conditions.
f15epilot
Methinks you are expecting to much.

Air conditioners are only capable of put out air that's 15-20 degrees colder that the air they are drawing in.
See: For HVAC Systems, Efficiency Measurements Are The "Delta" Difference

And the cool air loss, (aka: heat gain), is far greater when driving than it is when the RV is sitting still/parked.
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Old 09-14-2016, 12:26 PM   #4
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Only have two data points drive from Little Rock, AR, to NW Florida after purchase; round trip to/from Orlando this past weekend. Dash A/C is another issue (works while sitting still at idle; does NOT work driving/running down the road; expansion valve replaced and system charge checked prior to purchase--which did not fix issue). Prior owner (Wurd on here) is yet to respond to PM; he had the coach in El Paso, which would be like Edwards (spent a year there).

Do not think it is freezing up as the issue as the air flow from the registers is fine.

And is your satellite directly in front of the unit? If so, how close. Being only 14in and directly centered, mine is making an impact. Plan to temporarily remove the dome and see if that makes a difference.
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Old 09-14-2016, 03:18 PM   #5
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We have had several different MH over the years and all of them have cooled great whether parked or going 60 mph. I sometimes shut the dash unit off, as the roof units keep us comfortable. Wish I could help you, but have never had your problem. Are you certain that the compressors are coming on when in motion?
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Old 09-14-2016, 03:34 PM   #6
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I have 3 of the 10 year old DuoTherm 15K units on our Essex. I routinely use the front 2 on genset while driving in 95 degree weather. They will bring the front area down to around 75. However, the Essex (Newmar) has ductwork that is NOT covered by the slides. Is your ductwork covered or uncovered when the slides are in? Do you drive with the window shades down to add more insulation?
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Old 09-14-2016, 04:09 PM   #7
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Pilot, Interesting problem. Aerodynamic effects are interesting.....

First. I could not find a good photo of your AC with the bonnet removed, so a couple SWAGS.

It appears to have a plenum surrounding the evaporator section. It appears to use a squirrel cage fan to draw air through the pan vents and through the coil, exhausting the now hot air out the port(?) side pan vents. The plenum is "sealed" to the bonnet using foam stripping.

RV A/C mfgrs are not known for for using high quality, durable materials (especially on the unseen insides!). Exposed to the heat & sun, it is likely the foam gaskets have gone south and the the fan is not drawing air through the coil as intended. Airflow on the roof while under way maybe be causing stagnation, or reverse flow in the plenum.

If the foam gasket is gone....debris have FODded the squirrel cage or the exit ducting.

https://eldonrv.wordpress.com/2012/0...s-3308046-006/


Putting "holes" in the rear sides and end of the bonnet might improve airflow across the condenser. Note that newer versions have venting on the sidest. A simpler solution might be to see if the newer bonnets with the side holes can be reto-ed to your unit.

Or maybe just some holes on the exhaust side?
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Old 09-14-2016, 05:37 PM   #8
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Definitely plan some basic tests of the aero effects to include sampling temps and maybe a go-pro with some tufts to capture wind flow at the surface.

What would also help would be some first-hand reports from other owners. Rather than trying to diagnose my AC issues, I would hope to gather basic data from the multitudes of other owners who (have) run their roof units while driving:
1) If you have done this, did they cool the RV?
2) What type of unit(s) do you have?
3) Is there an obstruction (e.g. sat dome) in front of any AC and if so, how close and how big?

The data will help...
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Old 09-14-2016, 06:22 PM   #9
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My dome is approx 24" ahead of the first roof A/C , many people with dash A/C units have had to install a manual water flow shut off valve , in the heater hoses to completely stop hot coolant from the engine getting into the dash while A/C is in use.
Don't have a roof top picture , side on is the best I can do.
No A/C problems that day.
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Old 09-14-2016, 06:32 PM   #10
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As posted they cool 15 degrees or so.

That is air flowing through the unit.

One ma| assume driving would be better as plenty of fresh air passing by the condenser.

It may be possible the compressor is not running maybe due to bad connections.

One could get a clamp on amp meter and have it placed where a helper can watch it while driving down the road to see if the current drops off to indicate compressor stopping.
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Old 09-14-2016, 06:43 PM   #11
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Hi All!

Our 40' coach in 90* plus heat requires two 15k roof units and the dash air to keep us comfy.

And in the cold weather,,,, it take more heat than we can find to keep us warm. I think is is due to the lack of an air tight area??? Too many vents, cracks around slides, etc..

At lower speeds,,,, both situations are better.

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Old 09-15-2016, 06:32 AM   #12
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Interesting problem. I might choose to do some other things first.

1) determine whether the temperature delta is adequate (cold side air is 20 degrees colder than warm side air).

2) assuming the delta is correct, but the warm side doesn't continue to get colder, look for sources of outside air entry or inside air loss.
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Old 09-15-2016, 10:30 AM   #13
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My Dometic Penguin roof AC units cool fine while driving the rig, if I have my generator running.......

One is 21 years old. One is 12 months old. Both are ducted Heat Pump units.

Both are behind my Sat Dome.

I doubt the configuration of your AC covers or their location affect their cooling capability.

However, I cannot run both roof AC units at the same time using my generator if I have some other high draw appliance operating.

My generator only provides 30A of current.
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Old 09-15-2016, 10:55 AM   #14
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OK--first thing to understand is that the 20-25 degree A/C differential is a measure of ambient INSIDE air going into the air box vs cooled air leaving the dash vent--eg, if your inside ambient air is at 90, you should expect cooled vent air to be at 65-70. As your cab cools to 80, then vent air should drop to 55-60. Accordingly, if you draw in outside air rather than recirculating inside air [eg, set controls on "Max"] you are always trying to cool 90 outside air. Good points were made about air leaks in the ceiling duct work, and under the roof shrouds that allows warm and cool side air to remix. On really hot days, we added a shower curtain [designer if you like] hung behind the driver/pass seats--yes, rest of coach gets/stays warm but cab area does fine with just dash A/C. Ironically, some of this also applies to winter dash heating. Use max cool setting but set heat knob to hot--this will recirc and heat only inside air. Also many coaches are designed with front caps and aerodynamic dams that create a partial vacuum in the area where the dash heater is attempting to draw air--in this case you will notice that the heater works great while stopped or driving thru towns, but degrades at hi-way speeds.
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