During the summer, temperatures start to rise and bake our boxes on wheels and most RVers are not satisfied with the efficiency of their roof air conditioners.  Just the other day I received a call from the owner of the RV we have been working on in videos for the past few years and they stayed in their rig for a week while their new apartment was being disinfected.  The outside temperature was up into the 90’s at times and it was brutal.  He stated; “I don’t understand it, I have cleaned the filter and all the vents and it’s still not cooling.  I probably just need to buy a newer, more efficient roof air?”  If the rig was 25+ years old I might agree with him as the newer models have more efficient compressors, however, his should be doing better?

What I did find was he had never gotten up on the roof and inspected the evaporator coil or condenser coil!  Just cleaning the cold air return filter is not enough.  A better understanding of how the system works is important to make the unit run at maximum efficiency.

This diagram shows the typical setup for most roof air conditioners.  The motor spins the fan, or “squirrel cage” which draws air from the interior of the coach up through the air return vent.  The air is pulled through the evaporator coil while the compressor pumps a refrigerant through the lines zig-zagging through the evaporator.  A chemical reaction occurs which flashes and draws heat from the air as well as moisture.  The heat is absorbed by the coolant which flows back to the condenser which draws air from the outside to dissipate the heat.  The moisture pools in the evaporator pan shown below and run out the two weep holes on either side.

When our owner told me it was not cooling, I asked him what was the ambient temperature inside the rig, and what was the temperature blowing out of the AC vents?  His reply; “ I don’t know, it was warm inside the rig because the AC wasn’t working, just blowing warm air?”   One thing we always stress at the RV Repair Club, when trying to troubleshoot always take specific measurements and not rely on generalities.  Temperatures, voltage, airflow, all that information helps identify issues and helps technicians understand what is happening without being there.

So the first thing I recommended was safely going up on the roof to inspect and clean the coils. Or, if you’re not comfortable getting on the roof, ask someone who is. This requires taking off the plastic covering and typically removing the metal cover over the evaporator coil as well.  As you can see in the previous photo, the coil is exposed and the hole is the return air going into your rig.  Use a shop vac to clean the coils but be careful you do not dent the fine coils.  If they are bent, you can purchase a coil comb from an RV dealer to straighten them which will allow proper airflow.  If the coils are extremely caked with dust or other material, you will need to thoroughly wash the coils with a low-pressure water source such as a garden hose and a light brush.  Cover the return air vent as water will enter the inside of the rig.  Check the water vents in the drip pan to make sure they are not clogged as well.  Then clean the condenser coil on the backside of the unit.

The next comment was; “Maybe I need to get it recharged?”  This is common in home and residential air conditioners, however the RV AC is a closed unit and cannot be recharged.

After verifying all components are clean and working properly, here are some tips to help keep your rig cooler.

Location of your rig.

If the outside temperature was over 90 degrees, the inside temperature could be even higher if the unit is sitting in the sun baking.  And sure enough, his rig was at the State Park Campground sitting out in full sun for most of the day because he needed a line of sight for his satellite receiver!  Ouch.  Most roof air conditioners can only “condition” the air down 16 degrees during a full cycle which could take over an hour.  So if you start with an ambient temperature inside of over 100 degrees it will take most of the day to get down to a comfortable interior temperature if everything is working properly!  Find a shaded area, this can reduce outside temperatures by over 20 degrees!  If you need to get a satellite signal, consider a portable dish that you can put anywhere.

Insulate and plug gaps

Another thing to consider is most RVs have very poor insulation, maybe a 4” thick roof that has an R12 rating and sidewall that are 2” thick with an R6 but have windows that reduce that even further.  Plus slide rooms have gaps in the seals which allows more moist warm air to enter the rig making it more difficult to remove moisture and cool the rig down.  Most residential homes have R19 walls and R24 or more attic insulation and are a completely closed system which means the interior air comes to the air conditioner and gets conditioned or “flashed” in the same manner, gets distributed to the rooms, and returns through the air return vents slightly warmer and can cool down and maintain a better temperature.  In an RV, warm moist exterior air is continually introduced and the conditioned air cannot be maintain as efficiently.  Therefore you need to inspect the entire coach and try to seal any gaps or areas that would allow the warm exterior air to enter.

Pull down the shades, add insulation to the windows such as the Reflectix aluminum wrapped foam and if you have a motorhome, pull the windshield curtain or get an exterior cover to reduce heat.  Use window foam strips around the inside of the slide room edges.  If you do not have dual pane windows, you can add a layer of insulation with the thin plastic home kits that are sealed with a hairdryer.

Reduce Humidity

You’ve heard the saying; “It’s not the heat, it’s the humidity!”   Warm air will hold much more moisture and that will affect our perceived temperature.  Here is how it works, as the temperature rises, your body sweats trying to cool by evaporating the sweat.  However, if the relative humidity is high, the air is saturated and your sweat will not evaporate.  As stated before, the evaporator coil not only flashes to pull heat out but also pulls out moisture.  If additional moisture or humidity is being added to the inside of your rig, the roof AC will not be able to keep up.

Limiting moist exterior air as described before is the first step.

• Reduce the amount of cooking with boiling water such as instapots, boiling water, coffee, and such.  This all adds steam/water.
• Additional moisture can be added by showers, washing machines, and hanging up wet swimming suits!

Proper Air Management

Because RVs do not have HVAC systems with several air return vents, we get pockets of cold and warm air, especially in rooms with heavy furniture.  Some RV manufacturers have been offering ceiling fans in the bedrooms, especially in 5^th Wheel units.  Use fans to get better circulation so all the air gets conditioned.

Some RV owners that have ducted roof vents like to close off some of the vents in the bedroom during the day to direct cool air to the living room.  This can be effective, however, make sure you do not close off too many vents as this could limit airflow and cause freezing, condensation, or other issues with the AC unit.

Open windows and use roof vents in cooler temps such as early morning and late evening to exhaust warm air and bring in cooler outside air.  This can also be done during the day if one side of the rig is in the shade.

Check Your Refrigerator

What does the refrigerator have to do with the air conditioning?  The more your refrigerator runs, the more heat it generates in the cabinet and inside the rig.  Check and clean the roof vent so hot air is able to rise and vent rather than sit and swelter inside the back cabinet.  Make sure the refrigerator is cooling at the highest efficiency so it’s not running all the time.

Reduce Heat From Interior Components

LED lights not only use 10 times less energy, but also produce 10 times less heat.  Consider changing your old halogens or incandescent bulbs or keep the lights off.  Limit the amount of cooking inside as the stove/oven produces tremendous heat and the microwave oven vents heat and moisture to the inside of your rig.  If you do need to use appliances such as the washer/dryer, water heater, and other items try to schedule their usage during early morning or late evening times when the temperature is cooler.

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 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.

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