A look at how motorhome air-conditioning systems operate.
By Lazelle Jones
With the emergence of production-built motorhomes (circa 1962), the need for refrigerant-based cooling systems for these new vehicles became apparent. Roger Aldrich, engineering manager at Dometic Corporation, explained that RV air-conditioner technology originally was borrowed from the residential and commercial industry. He went on to say that it was folks such as those at Duo-Therm (which later became one of the Dometic brand names) who initially rose to the occasion, designing and developing RV-specific systems.
The first motorhome roof-mounted air conditioners measured approximately 26 inches wide by 31 inches long by 15 inches high and weighed about 200 pounds. Today, low-profile air-conditioning units measure a stingy 7.5 inches high and weigh as little as 70 pounds. However, tailoring this technology to the RV was no simple task. It involved taking large fans, electric motors, a condenser, an evaporator, and a compressor and redesigning them so they could be contained in a small package and be installed on the roof of a motorhome.
RV air-conditioner technology has grown by leaps and bounds, with advancements taking place in the design of compressors (piston to rotary), air circulation devices (fans to blowers), and condensers (wet to dry). RV air-conditioner systems and components can now be located in the basements of motorhomes as well as on the roofs. However, not only have components evolved, but technologies such as heat pumps and electronic temperature controls have been added to the mix that’s available to the coach enthusiast.
What follows is an explanation of how a generic refrigerant-type RV air-conditioning system works, a buyer’s directory, and other related information. Even if you’re not in the market for a new air conditioner or you don’t need service on your existing unit, learning how an air conditioner works is interesting, plus being informed also means being empowered.
What Goes On Inside The Box
A refrigerant-based cooling device is a closed-loop system. This means the refrigerant is sealed in an environment (inside tubing, hoses, etc.) and continuously recirculates through this loop. A generic air-conditioning system includes a compressor, an evaporator, a condenser, fans, and electric motors.
The compressor, as the name implies, takes the gas refrigerant (normally R-22 in RV air conditioners) and compresses it. The refrigerant then flows to the condenser. A fan blows outside ambient air across the surface of the condenser tubing, thereby removing heat from the R-22. This heat is then discharged into the outside atmosphere. With the loss of heat, the R-22 gas condenses into a liquid. The flow of the compressed R-22 liquid is restricted by a small orifice (opening) in the flow path. As the R-22 passes through the orifice, a pressure drop occurs that reduces the temperature of the R-22 to approximately 40 degrees Fahrenheit.
The R-22 next flows through the evaporator coil. With the help of a blower or fan, air from inside the motorhome is passed over the surface of the evaporator tubing. This cools the air, which is then recirculated into the interior of the coach. As this air from the inside of the motorhome passes over the evaporator, heat from the air is transferred to the R-22, and the liquid changes back to a gas state. From here the R-22 flows back to the compressor where it is compressed and the process is repeated.
So what is the difference between an air conditioner and a heat pump? Not much. A heat pump, through solenoid-operated valves, turns the evaporator into a condenser and the condenser into an evaporator, depending upon the need for warm or cool air. Heat pumps do have a limitation. They work best when the outside ambient temperature is above 40 degrees Fahrenheit. If it’s colder outside, they are not as efficient. The nice thing about a heat pump is that when the coach is connected to shore power, the furnace doesn’t need to run. The heat pump uses shore power to warm the interior of the coach. This is also true of air-conditioning units with heat strips.
HVAC (Heating, Ventilation, And Air Conditioning)
Today’s cooling systems often are integrated with the coach’s HVAC system, with some (as noted with heat pumps) using the same components to generate warmth or cooling and the same plenum and register distribution system. The same temperature monitoring and control system also can be used for both the heating and the cooling system. These controls come in various levels of sophistication. The most basic type of control is a manual control that turns the air conditioner on or off and a switch to adjust the fan. Next are electronic systems (without microchips), followed by an electronic temperature monitoring and control system with microchips (built-in intelligence).
Other Important Information
Both motorhome and air-conditioning system manufacturers address the issue of having an adequate power supply (120 volts alternating current, 20 to 50 amps) to power the compressor and the fans/blowers. Remember, more amps are required during the startup of the air conditioner than are required after the unit has reached normal operating conditions. The power source can be shore power, a generator, or both. On occasion, luxury coach builders have designed air-conditioning units that operate off a house battery pack via an inverter.
Typical generator requirements look like this. A 2.5-kw generator can power a single air-conditioning unit that is rated at 13,500 Btus or smaller. A 3.5-kw gen set is needed to power a single 15,000-Btu unit. A 4-kw gen set can power two 11,000-Btu units, and a 5-kw gen set is needed to power two 15,000-Btu units.
Cooling/heating output is measured in Btus, and motorhome air conditioners can have ratings ranging from 2,500 Btus up to 26,000 Btus. The number and size of the air-conditioning unit(s) is determined by the interior size of the motorhome (cubic feet of living space). The number and size of the air-conditioning unit(s) dictate whether the motorhome needs 15-, 30-, or 50-amp electrical service.
Guidelines for the number and size of the air-conditioning unit(s) required to cool a coach are based on the number of slideouts, the amount of window area, and the insulation used. As a rule of thumb, some in the industry say that for every square foot of living space, 100 Btus of cooling is required.
Other considerations include cargo carrying capacity. The larger the air-conditioning unit, the more it will weigh, thus diminishing the capacity available for occupants and their gear. Roof air conditioners can range from 70 pounds up to 110 pounds; large basement units can weigh up to 200 pounds. Another consideration is the minimum and the maximum amount of cooled/heated air that can be delivered, measured in standard cubic feet per minute (SCFM). For example, the minimum/maximum SCFM on roof air-conditioning units may be rated at 250/335; 250/380; or 300/450 SCFM. The number of blower (fan) speeds also can vary; some feature two speeds, while others have three speeds.
Location, Location, Location
This is not about real estate. This is the decision about where to locate the air-conditioning equipment — on the roof or in the basement. Some say that because cool air falls and warm air rises, the air conditioner should be placed on the roof. Others say that cool air can be distributed efficiently from a floor register system and be equally comfortable.
Those who prefer basement air-conditioning systems note that today’s state-of-the-art insulation and ducting materials permit cooled air to be delivered from the basement upward, with only minimal losses. Proponents of roof air conditioners say this is not necessarily so. They also point out that with basement storage space at a premium in a coach, housing air conditioners there is not always the best use of space.
Boyd Vanover, vice president of engineering at Newell Coach, noted that with coaches now offering more and more slideouts (up to four), designing a delivery system that originates in the basement can present an engineering challenge. It must deliver sufficient quantities of cold air through a ducting system that winds up and around the slideouts. But some luxury coach enthusiasts simply don’t like the way a roofline looks when disturbed by air-conditioning units, so the discussions continue.
Service And Maintenance
Should an RV air conditioner be run periodically, just to keep it in shape? Roger Aldrich of Dometic said that in some areas of the country, snow may fill up the outside portion of the air conditioner cavity. This snow and ice may lock the fan blade, so it cannot turn when power is applied. While there are safety devices inside the motors that will prevent damage to the motor when the stator becomes locked, it is not recommended that this be relied upon as a normal course of action. Mr. Aldrich noted that in cold weather (temperatures below 65 degrees Fahrenheit), the compressor will not start on an air conditioner. Most heat pump systems have an ambient sensor that will not allow the compressor to start when temperatures are below 32 degrees Fahrenheit.
Mr. Aldrich indicated that Dometic’s state-of-the-art RV air-conditioning units have no seals that can dry out, become brittle, and leak refrigerant as might be the case in an automotive air-conditioning system. An RV air conditioner can sit for extended periods of time without damage to the system. However, Dometic does recommend the following preventive maintenance actions:
1. Keep the evaporator inlet air filter clean and dirt-free (inside filter).
2. Keep leaves and branches out of the outside portion of the unit.
3. Plastic parts may be washed down with soapy water.
Companies that design and build air conditioner components and systems for motorhomes include Carrier, Dometic, and RV Products. Some products and models are installed by the manufacturer as original equipment, but most also are available in the aftermarket.
50 Grumbacher Road
York, PA 17402
Carrier Corporation, 50 Grumbacher Road, York, PA 17402
(866) 464-2478; www.airv.com
Model Name Btus Air Flow (CFM) Watt Rating Weight (lbs.
AirV 13,500 325 90
AirV High Capacity 15,000 325 90
AirV Heat Pump 15,000 365 94
AirV Low Profile 15,000 300 1,582 watts, cooling 88
High Capacity 1,600 watts, heating
Dometic Corporation, P.O. Box 490, Elkhart, IN 46515
(574) 294-2511; www.dometicusa.com
Nominal capacity Nominal Unit Watts SCFM, high-speed Installed weight Btu/hour max/min (lbs.) 1 unit-2 units
Brisk Air Rooftop Air Conditioners
57908 7,100 900 325/250 75
57912 11,000 1,150 325/250 83-94
57915 13,500 1,475 325/250 84-94
59516 15,000 1,575 350/250 94-108
Penguin Rooftop Air Conditioners
600312 11,000 1,425 335/250 95
600315 13,500 1,625 335/250 95
620515 13,500 1,625 335/250 100
620526 15,000 1,750 380/250 104
Brisk Air High-Efficiency Air Conditioners
59530 High-efficiency 1,150 not available 108
Brisk Air Rooftop Heat Pumps
59136 15,000 1,600 350/250 100
59146 15,000 1,600 450/300 100-110
Penguin Rooftop Heat Pumps
630025 13,500 1,625 335/250 99
630516 15,000 1,750 380/250 104
630515 13,500 1,625 335/250 100
Basement Heat Pumps
41001 12,000 not available not available 70
39335 15,000 1,750 not available 102
39524 25,000 1575/2825 not available 205
Basement Air Conditioners
39045 15,000 1,400 not available 102
RV Products, P.O. Box 4020, Wichita, KS 67204
(316) 832-3400; www.rvcomfort.com
Capacity Btu/hour SCFM Watt Rating Weight (lbs.
Polar Mach 13,500 310 1,395 cooling 96
Polar Cub 9,200 200 1,010 cooling 88
Mach 3 P.S. 102% of Mach 3 Plus 310 1,147 92.5
Mach 3 Plus 13,500 320 1,595 84.5
Mach 15 14,800 325 1,748 90
Mach Roughneck 13,500 320 1,617 93
Heat Pump HP High Performance
Polar Mach 13,500 cool/11,600 heat 310 1,390 100
Polar Cub 9,200 cool/8050 heat 200 910 96
Packaged Air Conditioner
2-Ton Plus 10,000 stage1; 600 2,468 cooling 200 20,000 stages 1 & 2
Packaged Heat Pump
2-Ton Plus 13,500 stage 1; 600 1,500-2,550 cooling; 200 27,000 stages 1 & 2 1,521-2,563 heating
2-Ton 12,000 stage 1; 600 1,650-2,784 cooling; 200 High Efficiency 24,000 stages 1 & 2 1,521-2,563 heating
9000 Series 9,200 cooling; 210 1,050 cooling; 94.5 8,150 heating 1,100 heating