Summer Solstice is behind us the temperatures have been climbing! At home, with your car or truck, it would be time to have a technician check your coolant and recharge it. However, the roof air conditioners in RVs are a completely sealed system with no ports to check or recharge anything. But there are some maintenance items you can do to make your roof air conditioner run more efficiently.
Clean The Filter, Evaporator, and Condenser Coils.
Whether you have a direct flow air conditioner or one that is ducted throughout the ceiling, they all have a filter that needs to be cleaned periodically. Typically it is in the cold air return and is a woven plastic that can be washed with soap and water. Make sure that it is thoroughly dry before putting it back as a wet filter can mold or restrict airflow.
When you pull the vent, from the return air, down to access the filter you should be able to see the evaporator coil from the inside. The interior air is pulled up through the return air vent by the fan motor and it passes through the evaporator coil where the coolant flashes and draws out heat and moisture. It can also get clogged with dust and other items that make it past the return air filter. The image below is a unit that quit working and we pulled off the top cover and the evaporator shield and found it caked with body powder!
You should be able to use a shop vac to clean the coil from the inside. Otherwise, you would need to get on top of the unit and take the plastic cover and evaporator metal cover off to access it. You must have clean airflow through the coils for the unit to cool.
On the back side of the air conditioner is the condenser coils which draw outside air through the fan motor and help cool the hot copper tubing of the coolant as it makes its way back to the compressor. This needs to be clean and the coils or fins straight. Some models are exposed to the elements and hail or branches can flatten them restricting airflow. Some models have redesigned the condenser coils at an angle to reduce exposure to hail and branches.
Check Airflow At The Unit And At The Vents
Most air conditioner manufacturers make a model that can either be used as a direct vent out of the unit like a window air conditioner in a house, or can have a baffle placed in the cavity to divert air to the ductwork placed in the ceiling. If you have a direct vent model coming down from the unit, check to make sure there are no obstructions such as insulation blocking the flow of air.
If you have a ducted model, it is important to check the diverter or baffle as the air conditioner manufacturers send a generic size and the RV manufacturers are required to cut it to length and fasten it. I have seen several units where the baffle was cut too short, a single piece of duct tape was fastened to the top, and the baffle was actually laying sideways, so very little air was getting back to the vents. If the baffle is not cut to length or the sides are also secure, air will circulate in the cavity and just return back through the return air and sometimes gets so cold it will shut down the system as there is a thermistor or temperature sensor at the evaporator coil. It is important to inspect the entire system as you might need to add HVAC duct tape to reinforce areas that have gaps and loose ends. This is a unit on a 2015 Thor Challenger and although the baffle looks good, there are gaps on the side and top that should be sealed reduce leakage.
I recommend getting an anemometer with an airflow and temperature gauge. Your roof air conditioner can only cool the incoming air at the return air vent 16 degrees so if you let the rig sit in sweltering heat in the summer when not in use and fire it up to cool things down, you will not be very satisfied. If the temperature inside the rig is 100 degrees, the air coming out of the vent will only be a balmy 84 degrees! So having the correct temperature data is important when it comes to troubleshooting. I also like the airflow tester as it tells me the highs and lows of certain vents. Naturally, the vent closest will have a higher CFM (cubic feet per minute) however you should be able to tell if there is a difference between the front and the back.
Some designs are just not very efficient due to the boxed design of the cavity, and instead of providing the maximum airflow to the vents, it creates a circulating effect that provides insufficient airflow and cooling which oftentimes causes condensation as the cool moist air hits the metal sides and ductwork. You can add additional insulation to the sides and create a “V” shaped design, however, a company called RV Airflow has designed a ready-made insert that increases airflow and efficiency. This product captures 100% of the airflow and directs it to the ductwork as well as reducing friction which reduces the stress on the unit which can prolong the life of the compressor and motor as well as reduce noise. You can find the design that fits your model here.
Reduce Initial Start Up Amps
When your roof air conditioner starts the cycle it can draw up to 40+ amps for each unit! This is a quick spike that typically doesn’t trip the 30 amp breaker but does weaken components over time. However, if you have two roof AC units and they start the cycle simultaneously on a 30 amp service, you will have a problem. AND…if you are getting your rig ready for a trip and have it parked at home, plugged into a 20 amp outlet in the garage… another problem.
The Softstartrv is a product that is installed on the air conditioner and will reduce the initial amp draw down to approximately 14-18 amps which help with energy management but also extend the life of the compressor and motor. You can find information on how to install and purchase 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 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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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.
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.
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.
