, , , , ,

With record temperatures hitting almost every part of the nation, I’m getting quite a number of questions on how to make the old Coleman and Dometic roof air conditioners cool more efficiently. I know we have featured this in the past however, there are some new products on the market as well as a little trick I found when it comes to the standard air diverters in ducted roof air conditioners.

First, Regular Maintenance

Most RV owners don’t know there is some maintenance required to keep the roof AC running efficiently. After all, they never had to do anything to their unit at home why is it any differently? Let’s take a look at how the RV roof air conditioner works and what we can do periodically to help it work as designed.

When the thermostat calls for cooler air, the motor starts in the system and draws the warm, moist interior air up through the air return in the ceiling.


Whether your unit is a direct flow like this older unit or it is ducted throughout the ceiling, the air return will have a thin filter to catch dust, dander, or other objects. This filter must be checked and cleaned periodically or it will block airflow and not only make the unit run less efficiently but could ruin the compressor and motor.

The warm moist air comes up through the return and is pulled through the evaporator coil at which time the coolant has been sent from the compressor and “flashes” which pulls heat out and moisture. The evaporator coil needs to be checked and cleaned periodically as well.

Here is a unit from the local campground that stopped working. The evaporator coil was caked with body powder! This unit sat directly above the bedroom area where the owner created a cloud of powder every morning! The evaporator needs to be checked and cleaned at least once a year. You can access this from the inside by pulling off the return air shroud and looking up through the hole.

This is what a good evaporator coil should look like. You can use a small portable vacuum to clean the coils however if they are really dirty, you will need to go up on the roof, take off the shroud, and then take off the evaporator housing and clean it. If you use a garden hose, cover the return air vent or you will get water inside the rig. If you get water or moisture spraying out when the air conditioner is blowing, it could be the drain holes are plugged and moisture is trapped in the drip pan. Check these periodically as well.

Next, the coolant flows back to the condenser coil where outside air is drawn into it down so it can go through the cycle again. Even with 110-degree outside heat, the air drawn in is much less than the high temperatures the coolant can reach. Make sure the condenser fins are not blocking airflow and are straight. It’s not uncommon to see flattened fins from hail or tree branches especially the older units with flat open backs. Some manufacturers are putting them at an angle or in a circle with a covered vent.

Create Inside Airflow

The roof air conditioner of your RV can only cool or “condition” air down 16 degrees from what it is pulling in. As your unit sits in the sweltering sun, inside temperatures can get 20-30 degrees hotter than the outside temperature and sometimes even more. And since hot air rises, the temperature at the inside ceiling of your rig could be 120 degrees! That means your roof air conditioner will pull it in and blowback 104 degrees into your rig. Use your roof vents, windows, and fans to move air around and pull as much warm moist air out as possible if your rig is sweltering.

Create A More Efficient Ducted Airflow

If you have a direct flow unit in which the air comes directly out of vents on the unit, there is not much you can do to improve the airflow as it is basically dumped out like a residential window air conditioner. However, if you have vents in the ceiling and air is distributed from the unit through ductwork there are some modifications you can perform.

Both ducted and non-ducted roof airs are basically the same unit with a few additional components. For ducted models, the unit is shipped with a generic baffle or thin piece of material, typically foam that the manufacturer needs to cut to length and insert into the opening. This is what diverts or directs the air coming from the fan to the ductwork rather than vents on the bottom of the unit.

Here is the diverter on a 2015 Thor Challenger we worked on recently. Notice the back of the unit, behind the diverter, has a side duct going back to the vents. Also, notice all the gaps above and on the side of the diverter! This allows air to blow back into the air return rather than going to the ductwork. Also, when the air comes off the fan it goes straight down and hits the shroud or plastic cover it creates a whirlwind effect and again, limits the airflow. All these gaps and connecting points can be covered with duct sealing tape that you can get at any home improvement store. I would even suggest getting the insulated foam tape and covering all exposed metal as this will also create condensation if left exposed.

To help even further, RV Airflow Systems has developed an aftermarket kit that can be installed to take away the whirlwind effect and increase airflow by 40%. At least that is what they claim. Next month we will be installing one on this unit and will let you know how well it works but it does make sense! You can find more information on their website here: rvairflow.com/collections/all

Lower Your Initial Amp Draw And Use Two Roof AC Units

When the thermostat calls for cooler temperatures and your unit’s motor and compressor kick in, the initial amp draw can be as high as 50 amps or more. Normally that is more than a 30 amp circuit at a campground could handle however, it is just for a short amount of time and typically does not blow the breaker at the pedestal. But it does limit you from using two roof air units on a 30 amp service if they both kick in at the same time. Plus it is a constant issue when you bring the rig home to get it ready for the next camping trip and plug it into a 20 amp garage outlet. Also, if you have a trailer and are using a portable generator rather than shoreline power, you can’t use one of the smaller units just because of the high amp draw at start-up.

Last month we installed and tested a new product called Soft Start RV that reduces the initial amp draw by almost 70%. It also eliminated the annoying “thumping” at start-up and our initial tests showed a startup amp draw of only 10 amps! It is easy to install with just 6 wires and no cutting of any existing roof air conditioner wires. If you can crimp on a few spade connections and get on the roof, you can install this in less than 1 hour. And they have excellent USA-based technical support!

Just a note: SoftStartRV or any other amp reduction product on the market will not make your roof air conditioner run more efficiently during normal operation. It is just during the initial start-up which has a high amp draw.

Check out the video and get a $30 discount through the RV Repair Club here.

About the author: Dave Solberg: Managing Editor, RV Repair Club

For the last 25 years, Dave has conducted RV maintenance and safety seminars, developed dealer and owner training programs, written RV safety and handyman articles authored an RV handbook reference guide, and logged over 100,000 miles on the road in an RV.

RV Repair Club

RV Repair Club is your go-to online resource for enthusiasts who want quality RV maintenance, repair and upgrade information – a community where passionate RVers can come together to gather knowledge and share their experiences.

Learn more about RV Repair Club.

Adam H. ~“We had a tire blowout.  I want to thank the staff at Coach-Net for their quick response in finding an excellent tire shop with the correct size and brand very quickly. Communication with Coach-Net, the tire shop and us was excellent.  A bad day turned out pretty well as we were back on the road very quickly thanks to your help!”