Air Conditioning

Overview:

Air conditioners are standard equipment on today's motorhomes. As a matter of fact, most have two and some have three. Air conditioners utilize mechanical cooling. An electric motor driven compressor pumps compressed Freon through a closed loop recirculating system. A radiator like device, called an evaporator is located inside the RV while another similar device, called a condenser, is located outside the RV. The system is design to compress and expand the Freon gas at key points in the system. When the gas is compressed it gets hot and when it's expanded it gets cold. By expanding the gas as it enters the evaporator that evaporator will get very cold. A blower fan is then used to blow the warm interior air over the evaporator, cooling it down. The heat from the coach interior is now transferred to the Freon line, which sends it up to the condensor coil on the roof, where another fan blows across that coil, transferring the heat from the Freon into the outside air. It then continues the cycle and loops through the system. Think of it as bailing water in a rowboat with a bucket. Except, instead of removing water from the boat, we are removing heat from the interior of the RV. Air conditioners do not "make" cool air, they only remove it from one area and send it someplace else, namely outside.

Air conditioners have their limits. They operate on a temperature differential. In other words they can only cool the evaporator to a point that is "x" number of degrees below that of the condensor. A cheap air conditioner may only have a temperature differential of 20 degrees while a high quality unit may have a temperature differential of 40 degrees. If it's 100 degrees outside and you want to cool it to 70 degrees on the inside you will need a temperature differential of at least 30 degrees. Naturally, the better units will cost more and they will require more amps to run. You can help the situation out in extremely hot weather by cooling your rooftop air conditioner. Parking in the shade is a huge help over sitting in the hot sun and some RVers have even taken to running a cool water hose over the unit. These tricks all help to keep your unit within the needed temperature differential. Unfortunately, units are not published with temperature differential specs in their literature.

A/C units are rated in capacity however. They are rated in BTUs (British Thermal Units), which are a measurement of the quantity of heat that they can transfer. Most rooftop units are either 13,500 BTUs or 15,000 BTUs. The higher the rating, the faster you will cool down your RV and keep it cool. Larger units will draw more amps and starting surge can be quite large on an air conditioner, especially if the compressor is trying to start loaded, against a high Freon pressure. For this reason most units will have delayed start features to eliminate trying to restart the compressor after an immediate stop. So, if you are fiddling with the thermostat and wondering why the air conditioner isn't kicking in, have patience. It'll start up soon enough.

Heat Pumps:

Heat pumps are an option on many motorhomes. All they really are is extra mods made to the existing air conditioners. In effect, the heat transfer is backwards and the heat from the "outside" is being pumped, or transferred, to the "inside". Heat pumps are not a very efficient method of heating. But, when it isn't all that cold out and you have shore power available, it can be a source of free heat. Heat pumps also work on a temperature differential and their basic inefficiency makes it even tighter than when cooling.

Heat pumps do not work in cold temperatures. Once it gets down to 40 degrees you'll be hard pressed to get the coach warmed up to much more than 60 degrees inside. As the temperature outside drops even further, so does your ability to heat the interior. Furthermore you can damage the compressor when running at low temps. The oil in the compressor is designed to lubricate it at high ambient temperatures, when it's hot outside. Accordingly, it must be a pretty thick lubricant to keep it from breaking down in the heat. That same heavy viscosity works against it when the temps go down and the oil gets very thick. Running in the heat pump mode at very cold temperatures is not only inefficient, but damaging to the compressor. For this reason heat pump manufacturers put low temperature cutoff switches on their systems. Typically these will prevent operation below 40 degrees.

Submitted by Mark Quasius and Mike Sundberg - 3/15/06
 

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