Tips for diagnosing performance issues found in RV air conditioners.
By Bill Hendrix, F761S
August 2010
At certain times of the year, many of us travel in climates where we depend on our RV air conditioners to keep us cool and comfortable. A simple temperature test can help determine whether an RV air conditioner is delivering the appropriate amount of cold air. However, in so doing, you also must make a few assumptions: that the air conditioner has been installed per the installation instructions; that the distribution ductwork is properly sized with a static pressure within the design specs; and that the return air path has the appropriate opening.
You may think that these are big assumptions, but all responsible motorhome manufacturers collaborate with the air conditioner supplier to ensure proper installations. Shortcuts in any installation requirements may result in poor airflow and reduced cooling, and could promote evaporator freeze-ups. It is in the best interest of the RV manufacturer to conform to the installation specifications, as trying to retrofit wiring or make ductwork changes would be extremely difficult, if not impossible, and very expensive.
There are only a few things that should be done to maintain the efficiency of today’s RV air conditioner, but those few things are very important. However, before delving into testing and maintenance, it might help to understand the mechanics of the air conditioner’s operation.
In order to produce a dramatic temperature change, the air conditioner takes advantage of a principle of physics: when a liquid evaporates, a significant temperature drop occurs. You can experience this simply by waving a damp cloth in warm summer air and pressing it against your face. This delightful cooling effect is created by evaporation of the water.
The air conditioner compresses the refrigerant; the refrigerant becomes hot and is then circulated through a high-pressure condenser coil. A fan blows away the heat, and the now-liquid refrigerant is circulated through a restrictor to the evaporator coil. When the liquid refrigerant passes through the restrictor (either an expansion valve or capillary tubes), it is allowed to evaporate into the larger chamber of the evaporator, which lowers its pressure and gives a drop in temperature. Different liquids have varying rates of evaporation. Virtually all sealed-system air conditioners used in the RV industry prior to January 2010 use refrigerant R-22, because its high rate of evaporation gives an excellent temperature drop. (See sidebar for refrigerant changes that began in 2010.)
To understand refrigeration and air-conditioning systems, we must recognize that there is no such thing as “cold.” Cold is an adjective describing a noun “” a cold drink, a cold breeze, etc. Heat is a noun, and it is heat we must deal with by absorbing it into a medium and moving it to another location. Heat is absorbed from the air by the evaporator, moved to the condenser via the refrigerant, and then discharged by exchange into the outside air.
Prior to conducting the temperature test, go through the basics. For a proper evaluation, you will need a relatively warm day “” not less than 70 degrees Fahrenheit “” with the unit running at maximum cooling with the fan speed on high and all the registers fully open for at least 15 or 20 minutes to allow the ductwork temperature and the refrigerant operating pressures to stabilize. Be sure the air filter is clean and is made of the correct filter material. If not, remove the filter for the test.
Operation basics. One of the more important preparation issues is to ensure that all of the cold air is being discharged into the living compartment. To do so, inspect the connection between the transition duct and the distribution duct (or ceiling assembly). The transition duct is the short, square vertical duct between the roof unit and the distribution duct (or ceiling assembly). With the air conditioner running, inspect for cold air escaping and “recirculating” to the evaporator. On most installations, this can be done by feeling around in the return air cavity. There will be air movement, but you should not detect any cold air escaping in this area.
Apply heat-resistant silver metallic duct tape (not cloth tape) to any area where cold air leakage is present. Some installations may be very difficult to access, but if an air leak is discovered, it must be corrected. It may be necessary to cut an access to the transition duct through the bottom of the distribution duct and use the silver tape to stop the leakage from the inside. Then repair the access and seal the hole with the silver tape. However, I would leave that job for a service center. Using silver tape to eliminate the air leak is most effective when done to the inside of the discharge air duct and to the outside of a return air duct. Never use tape on the inside of a return duct on a basement air conditioner if it is between the filter and the blower. If the tape were to come loose, it would be drawn into the blower.
A/C maintenance
Now that we have covered the basics of operation, let’s get to maintenance. Many years ago, a very wise air conditioner technician told me that there are three important points to remember about air conditioners: airflow, airflow, and airflow. The blowers, the coils, and the air ducts are all designed for a specific amount of air volume. When restrictions are present, dramatic changes in efficiency may occur.
First and foremost, the filter in the return air register should be clean. In our motorhome, we have developed a not-so-scientific way of determining when to clean the filter: we look at it. If the filter has dirt on it, it should be cleaned. It doesn’t matter whether we cleaned it just last week. If it is dirty, we clean it. True, this may be a bit simplistic, but especially in hot weather, a clean filter is very important.
Frost and ice forming on the evaporator often are caused by a combination of recirculation, low fan speed, a dirty filter, high humidity, and an unrealistically low thermostat setting. Some air conditioners are equipped with a “freeze control” that senses the evaporator temperature and cycles the compressor. In those cases, be sure the sensor is properly attached to the evaporator. This is not a 100 percent foolproof system, as the freeze pattern may sometimes avoid the sensor or the sensor may be outside the tolerance specs.
Every year or two, the condenser should be inspected. If airborne particles such as pollen or cottonwood seeds accumulate on the condenser, it won’t be able to discharge the appropriate amount of heat due to the reduced airflow. If the condenser can’t get rid of the heat, it will affect the amount of cooling available. Remove the air conditioner cover (shroud) and make a visual inspection. Any debris usually can be removed with a soft brush or blown away with low-pressure compressed air. While the cover is off, get rid of any little critter nests present. Also, inspect the condenser fins for damage. Fins crushed by a tree limb or by hail can be straightened with a fin comb.
Temperature test
The temperature test is very simple and requires only a single probe-type thermometer. However, make sure there is no cold air recirculation as described above, or you will not get a valid temperature test. Probe-type thermometers are inexpensive and are available at most hardware and auto supply stores. Use the same thermometer for all your readings. The reason for doing this is that it eliminates the chance that individual thermometers may take slightly different readings. With only one thermometer, we look just for the temperature difference, so accuracy isn’t a major issue.
With the air conditioner running on maximum cooling and high fan speed and all the distribution registers fully open, place the probe of the thermometer in one of the discharge vents that has good air volume. Hold the thermometer in place for a few minutes until the reading stabilizes and note that temperature. Use the same thermometer and place it at the return air inlet (where the filter is). Allow it to stabilize and then note that temperature. Subtract the discharge temperature from the return temperature; this is the Delta T (temperature differential or temperature split).
The air conditioner is designed to reduce the temperature of the air 20 degrees Fahrenheit as it flows through the evaporator coil at 50 percent relative humidity. The actual temperature coming out of the registers is not the criteria, but the amount of temperature drop is the factor. If the return air is 100 degrees Fahrenheit, we would expect the discharge air to be around 80 degrees Fahrenheit, but the room temperature would be gradually lowered to the thermostat setting. If the humidity is very low, the temperature drop may be 23 degrees or 24 degrees. If the humidity is very high, the number may be 16 degrees or 17 degrees. When the humidity is high, the evaporator collects more moisture from the air and discharges it to the outside. This cold condensation steals some of the cooling effect from the air that could be going into the living quarters.
If the air conditioner passes this test, you are through. If the Delta T is significantly low, the air conditioner is not cooling properly and you must look for the problem. Be aware that the humidity level inside the coach may be significantly different from the outside humidity level. We cook, bathe, and breathe, all of which contribute to a higher indoor humidity, so take this into account when evaluating the Delta T.
The next step would be to make sure there are no restrictions in the transition duct or the distribution ductwork. Seldom will all the discharge registers have the same volume of airflow, but if there is virtually no air volume at any of the outlets, further investigation is necessary. Remove a near and far outlet register, and use a flashlight at one end and an inspection mirror at the other end to ensure clear passage. If you discover an obstruction or a collapsed duct and an easy remedy is not apparent, it may be necessary to install another vent at the problem area in order to gain access and effect a repair. Again, this type of repair should be done by a service center.
If the Delta T is low, and the air delivery is good, we would suspect a low refrigerant charge, a contaminated condenser, or a failing compressor. If the unit has lost refrigerant, an oily spot usually will appear somewhere along the tubing or at one of the coils. A visual inspection of the tubing and condenser can be done by removing the cover from a roof unit. As mentioned above, debris normally can be removed with a soft brush or low-pressure compressed air. While the cover is off, inspect the blower wheel for damage or for insect nests that may be causing vibration. There is no maintenance to be performed on the fan motor, as it includes permanently lubricated bearings. If the fan motor does not run, turn off the system, move the fan blower slightly, and try again. If the fan motor now runs, there is a “dead” spot on the motor and eventually it will stop on the dead spot again. Replacing the fan motor is the only option. Leaking refrigerant or problems within the sealed system require specialized equipment and a trained technician.
Sometimes the cost of repair will approach or exceed the cost of replacement, so get an estimate and compare that with the cost of a replacement air conditioner. I personally don’t like to spend major dollars on appliances that are more than 5 years old.
A heat pump is an air conditioner with a reversing valve to make the refrigerant circulate in the opposite direction. The evaporator now becomes the condenser (it gets hot), and the condenser now becomes the evaporator (it gets cold). However, effective operation for the heat pump mode is limited. Since the heat pump actually is extracting heat from the outside air and moving that heat to the inside compartment, as the outside temperature goes down to freezing, very little heat is left in the outside air. When outside temperatures are above 45 or 50 degrees Fahrenheit, the heat pump is very efficient. As those numbers go toward freezing, the amount of heat produced declines sharply. When outside temperatures drop below 40 degrees Fahrenheit, it is more prudent to use the motorhome’s furnace for interior heating.
We sometimes get the impression that the bedroom air conditioner works better than the one in the living area. Please realize that there is much more heat gain in the living area, especially if it incorporates slideouts, big windows, and a large windshield. Compare that to the small bedroom, with a much more limited heat gain. In hot weather, consider pulling in the slideout rooms, parking with the windows away from the sun, and taking advantage of any shade opportunity.
If you are not comfortable doing any of the above procedures, take the motorhome to a service center for professional help. Performing these checks should help to keep you and your air conditioners cool.
Refrigerant Changes
R-22 refrigerant is being phased out in the United States and Canada, incrementally, between January 1, 2010, and January 1, 2020, as it is suspected of making greenhouse gases, damaging the hole in the ozone layer, and contributing to global warming. (These are controversial issues not totally endorsed by all of the scientific community.) The listed replacement is R-410A in the United States and R-407C for Europe. For more information about this change, visit the Web sites of the U.S. Environmental Protection Agency (www.epa.gov) and Environment Canada (www.ec.gc.ca).