I’d like to improve the efficiency of our roof air conditioner. The factory shrouds are black. I’d like to paint them white. Second, I’d like to insulate the pipe from the compressor to the evaporator. Thanks. —Joe, 2019 Coachmen Pursuit
There are several things you can do to increase the efficiency of your roof air conditioners; however, I do not believe painting them white will add much to that. If there was a significant difference in temperature between the white and black, I doubt any of the major roof air conditioner manufacturers such as Airxcel, Dometic, or Lippert would offer anything in black. There are so many vents in the side and back of these units that they don’t build up much heat. Plus, I would also doubt the paint would stick very well to a plastic shroud that is subjected to the weather elements and the flexing of running down the road!
You are better off looking at the operation of the components and making sure everything is clean and working as designed and ways to improve the airflow.
How the air conditioning system works
The warm, moist interior air is drawn in by the fan through the air return in the ceiling of your rig and passes through the evaporator, which is a series of fins with the evaporator coils zig-zagging through it. As the air passes through, the coolant solution in the coils “flashes” and heat and moisture are removed. The air is then directed back through the chamber to either the direct air vent on the unit or ducted roof air throughout the coach.
The return air vent typically has a filter that needs to be inspected and cleaned periodically or it will restrict airflow. This is typically a plastic cell-type filter that can be cleaned with a vacuum or soap and water.
Once the return air passes through the filter, it is drawn through the evaporator coils, which can also get clogged with dust, pet hair, and even body powder, which the filter does not catch, such as this unit that was completely covered.
Periodically clean and inspect the evaporator coils and make sure the fins are straight and not smashed, which would also restrict airflow. You can typically access these through the return air and use a portable vacuum to clean the fins. Just be careful not to damage the coils.
Air conditioner efficiency
One of the most important issues to help with air conditioner efficiency is plugging any gaps in the air chamber, diverter, and ductwork. Most air conditioner manufacturers build their units with a standard air chamber that can be used for direct airflow coming straight out of the unit, or ducted vents. They supply a generic diverter, which is simply a flat panel made of rigid foam with HVAC foil on each side. The manufacturer cuts the panel to fit their ductwork and inserts it into the air chamber. Most of the time there are gaps around this diverter that allow the cold air to “escape” away from the desired path.
Remove the air return shroud and inspect the diverter and all vents going through the roof. Use HVAC “foil tape” to secure the diverter and plug any gaps and cover any exposed metal to reduce condensation. I have found several of the diverters that were installed with a single piece of duct tape that are either flapping around in the chamber or have come off completely. That allows the air to go back to the intake and just create a whirlwind inside the roof air and not much cool air going to the vents.
There are a few aftermarket products that can be installed such as the RV Airflow system, which is designed specifically for your unit, that create a sweeping path and also provide more efficient airflow. You can find them here.
You might also enjoy this from Dave
RV ‘Gremlins’, Part 4: RV roof air conditioners: Sometimes they work, sometimes they don’t
We have had quite a number of questions come in concerning the efficiency and overall operation of RV roof air conditioners. Some don’t cool, and some just don’t work at all. Once again, it’s important to understand how the roof air conditioner works to help find the “gremlin.”
Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”
Read more from Dave here.
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I’m a retired mechanical engineer (MIT), former Safari Motor Coach plant engineer (3.5 yrs), and director of operations for a custom air conditioner manufacturer (2.5yrs) so I have intimate familiarity with both industries. I have studied RV A/C for some time and can state with a high degree of certainty that RV A/C will never be anywhere as efficient as residential. There is no incentive to do so. Residential A/C is far more efficient because of government mandates and because owners are sensitive to electrical bills. Neither is the case for RV A/C.
I have a number of YouTube videos (YouTube.com/WorkingOnExploring) analyzing and comparing the efficiencies of various types of RV and residential A/C units as well as a comparative spreadsheet on my blog, (WorkingOnExploring.com/TechDocs). As Tom E. says, mini-splits are ‘where it’s at’ if you want efficient A/C, (which is also what I do). I run my 120V, 9kBTU mini-split A/C 8 hours per day from solar in my custom adventure truck.
You nailed the comment I was going to make, plus your professional experience. Having installed a minisplit in my house this past year, I remain SHOCKED at the BTUs/watt — a piddling 500W on my ammeter produces arctic or inferno. There is huge angst and effort getting RV AC units to run on a 2KW genny, when you could run FOUR of my whole-house (inverter) unit on that power WITHOUT a soft start? You could get FIVE TIMES the heat the electric fireplace puts out for the same power? There’s voodoo in this tech, I’m telling you…
I have Dometic Penguin IIs. When the top cover broke I replaced it with same which has no vent holes in the side. Later I noticed some shrouds of the same shape have those holes. Should I have gotten the ones with the holes? Should I drill holes in my current shrouds?
There is a very significant difference between the surface temperature of black and white in direct sunlight. With a heat gun I have measured black as high as 193 degrees in 95 degree ambient temps. That kind of baking heat on the surface of an AC has to transfer to the inside and affect efficiency. I have measured cream color within inches of black and found it to be 60 to 70 degrees cooler.
Net, no black AC’s or smoke colored vent lids or covers on my RV.
The time to think about this is when you are looking at that dark colored RV paint job vs a lighter one. Color does make a difference on hot days when it comes to keeping it cool inside.
And you failed to mention that right off the manufacturing floor, an RV AC unit has 1/2 the SEER rating and is more than 2X as loud compared to a mini split. I got tired of the noise and low efficiency of our 7 year old RV roof AC, installing a 22 SEER mini split at less than the cost replacing our roof unit with an RV heat pump/AC. WOW! What a difference. No more 2800 watt start up. No more 70+ decibel roar. AND it heats down to sub-zero temperatures outside, so no more inefficient furnace ROAR waking us up in the middle of the night. So, the real question here is: Why won’t RV manufacturers insist on higher efficiency, roof mounted AC’s? Well, for the same reason, we’ve been stuck with NH3 absorption fridges, jet roaring inefficient furnaces, single pane windows….as they rake in the profits and deliver the poorest quality products we consumers purchase today. They all may have buried their futures, selling sub-quality RV’s to the next two generations of potential buyers.
Absolutely right. RV manufacturers are doing what auto manufacturers do. Selling the vehicle at near cost, with a minimal set of low-performing equipment and making a profit on repairs, options, and upgrades. The real money in The RV industry is service centers and ‘factory’ options. Don’t kid yourself by what they claim, educate yourself and see what they DO!
I’m envious of your rig! I have an almost new rig now, so won’t replace AC preemptively, but if ANYTHING goes wrong with the RV AC I’m replacing it with a minisplit… In my home, my 23 SEER, 24000btu unit claims it COULD draw 10A/240V or 2400W but I’ve never seen the meter much over 500W at any setting I can tolerate. No startup surge, almost dead-silent (I’m VERY irritable to noises), scalable output when I don’t need FULL power… I’m impressed as punch!