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. In simple terms, here is what happens.
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 designed 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. The heat from the coach interior is now transferred to the Freon line, which sends it up to the condenser coil on the roof, where another fan blows across that coil, over the evaporator, cooling it down transferring the heat from the Freon into the outside air. It then continues the cycle and loops through the system. By expanding the gas as it enters the evaporator that evaporator will get very cold.
The compressor compresses the refrigerant into a hot, high-pressure gas. It passes through the condenser which cools it and it turns into a high-pressure liquid. From there it is squirted through a restriction device into the evaporator, which because of the restriction, is at low pressure. The pressure inside the evaporator is low enough to boil or “evaporate”, the liquid refrigerant. This evaporation of the liquid refrigerant is what cools the evaporator. This low-pressure gas is then sucked back into the compressor and the cycle repeats itself.
Think of it as boiling 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 the heat 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 condenser. 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 incapacity, 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.
- Did both AC shut off at exactly the same time?
- Did just the compressors shut down or did the AC fans stop also?
- Did other 120VAC items in the RV quit also?
The control problem that you have may not really be a problem at all. A good quality AC thermostat will have a delay timer built into it that will prevent the restart of the compressor within an allotted time, somewhere between 3 to 5 minutes. This feature will prevent the compressor from trying to start with the high side still under pressure, which would stall the motor. 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.
If you want a mini AC 101 course, read the rest of this post, otherwise, skip it.
Due to motor size, electrical loads, and cost, the electric motor inside the compressor is not large enough to start the compressor with pressure on the high side. The motor is just large enough to start it with both sides of the compressor at (or near) equal pressures. When the compressor is stopped, the refrigerant that is on the high-pressure side must still pass through the restriction device until both sides of the compressor are equal. If the compressor is powered up with high pressure, it will stall the motor. By waiting 3 to 5 minutes, this will allow the high pressure to bleed off and then the compressor will restart.
If you’ve ever had a momentary power loss in your home while the refrigerator is running, you may have heard a click a few seconds after the power came back on. That click is from a built-in overload (circuit breaker) that is mounted to the compressor. It monitors the current draw and also the temperature of the motor housing. If either, or a combination of the two, are too high, it will interrupt the power to the motor and let it cool down for a while. The overload is of the self-resetting type so after a few minutes, it will try a restart. The time to reset will depend if it was tripped from current overload, like a stall (a short reset time) or from overheating of the motor housing (a longer reset time). When a compressor stall at startup, the motor windings will heat up very quickly. Even with the overload, the life of the windings is shortened.
Try to avoid this at all times.
Your AC units may have shut down from a combination of low voltage (on an induction device like a motor, the low voltage will cause the current to increase) and the high ambient temperature. High ambient temps make the compressor work harder by higher high-side pressures (the condenser is not as effective) and higher low side pressures (refrigerant evaporates faster). This higher current draw and high motor housing temp will cause the overload to trip. But that should only shut down the compressor, the fan should have kept running.