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Family RVing Magazine

Coach Cooling

June 1, 2005

A look at how motorhome air-conditioning systems operate.
By Lazelle Jones
May 2005

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.

Heat Pumps
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.

Manufacturers
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.

Carrier Corporation
50 Grumbacher Road
York, PA 17402
(866) 464-2478
www.airv.com
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
High-efficiency               

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.

Low Profile                 
Polar Mach               13,500                               310                1,395 cooling           96
Polar Cub                 9,200                                200                 1,010 cooling           88

Standard Height                 
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

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