Efficient evaporation is the key to a cool coach.
By Bill Hendrix, F761S
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 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 Freon refrigerant; the Freon becomes hot, and it is then circulated through a high-pressure condenser coil (see illustration). A fan blows away the heat, and the now liquid refrigerant is circulated through a restrictor to the evaporator coil. When the liquid Freon 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.
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 Freon, and then discharged by exchange into the outside air.
Different liquids have varying rates of evaporation. Most sealed-system air conditioners today use R-22 Freon, because its high rate of evaporation gives an excellent temperature drop. When the Freon returns to a gas, the evaporator gets colder, and a fan circulates that colder air throughout the living compartment. The spent gas is then routed back through the compressor to repeat the process.
The sizes of the compressor, the condenser, and the evaporator are engineered to produce a specific effect with the designed airflow, which is to give the circulating air a 20-degree-Fahrenheit temperature drop, at 50 percent humidity, as it flows through the evaporator coil. (Some marine systems are designed for a 16-degree-Fahrenheit drop, as they normally are used in higher humidity.) This means that if your coach is 100 degrees on the inside, you can initially expect only 80-degree air out of the registers until the temperature is gradually lowered to the thermostat setting.
The 20-degree-Fahrenheit temperature drop for roof-mounted air conditioners is a design criterion; however, this number may vary slightly with identical units, simply because of the manufacturing tolerances of components. The higher the humidity, the lower this number will be, since that cold water condensation from the evaporator coil is stealing some cooling effect that would otherwise be available to the living compartment.
A heat pump is an air conditioner with a reversing valve to make the Freon 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 furnace.
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 coach, 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 is a bit oversimplistic, but especially in hot weather, a clean filter is very important.
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, because of 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 and make a visual inspection. Any debris can usually be blown away with low-pressure compressed air. While the cover is off, get rid of any little critter nests present. Inspect the condenser fins for damage. Fins crushed by a tree limb or hail may be straightened with a fin comb.
The fan motor has sealed bearings and no lubrication is necessary, but even a small mud dauber nest on the condenser fan blade or blower wheel can cause a noticeable vibration.
If you are not comfortable being on the roof, or if you aren’t mechanically inclined, you should have this maintenance done at a service center. A fall from the top of an RV can be disastrous.
If you are primarily operating your RV air conditioner in climates with higher humidity levels, be aware of the danger of mold infestation. However, in my 20 years of performing service at various rallies, I have seen only two cases where mold was present in the air distribution ductwork. The owners became suspicious when little dark-colored particles occasionally were seen coming from the discharge registers. If you suspect mold contamination, remove a few ceiling registers and wipe the inside of the duct with a clean, white cloth. The difference between dust and mold is pretty obvious. If you suspect a problem, consult with people who specialize in mold inspection, duct cleaning, and sterilizing. Although the probability of occurrence is extremely low, the health risk can be serious.
Keep the system clean and the airflow moving, and you should enjoy years of cooling from your air-conditioning system.
RV Air Conditioner Maintenance
Efficient evaporation is the key to a cool coach.