SELECT THE RIGHT SYSTEM

An auxiliary air system can keep you cool in summer and warm in winter without idling your truck’s engine. Modern systems are designed for easy installation and low maintenance.

Before you can decide what sort of system to buy for your truck, it will be helpful to have a basic understanding of how it works.

HOW IT WORKS

The basic principle of an air conditioner is the transfer of heat from one place to another – in this case from the inside of your vehicle to the outside. This is accomplished by absorbing heat from the inside air into a refrigerant gas flowing through the unit’s evaporator coil. The refrigerant is then pumped to a condenser coil, where it releases the heat to the outside air.

As a part of the cooling process, the air conditioner also removes moisture from the inside air, which makes the area feel more comfortable and keeps the compartment dry.

HOW ABOUT HEAT?

The same system can provide heating as well as cooling, by adding electric heat strips, which blow hot air into the compartment through the same set of ducts and grills. Both heating and cooling are controlled by a single thermostat switch.

POWER REQUIREMENTS

Truck auxiliary air conditioners run on 115 Volt AC power. This can be supplied from an onboard generator (often called an auxiliary power unit, or APU), from a shore power hookup or from an inverter, a device that converts 12 or 24 Volt DC power from batteries into 115 Volt AC power to run the air conditioner and other electrical equipment in the sleeper. While a DC inverter solution is cleaner and quieter and uses less fuel than a diesel generator, it requires a substantial bank of deep-cycle batteries to keep the air conditioner running for your required hours.

SYSTEM COMPONENTS

There are two basic types of auxiliary air systems – self-contained and split.

The self-contained unit is designed to be wholly mounted inside the sleeper compartment – usually under a berth or seat or in a side storage locker. All of the system components, including the compressor, condenser, evaporator, heating strip, blower and electrical box, are mounted on a single low-profile chassis with an aluminum sound shield. The system’s refrigerant loop is pre charged and sealed at the factory, and no charging is necessary at installation. Inside air is pulled across the evaporator coil, and cooled air is blown through ducts to one or more grills for discharge into the living area. Hot air is discharged from the condenser outside the truck.

In a split system, the components are separated into two units. The outside unit contains the condenser and cooling fan. It is bolted to the underside of the sleeper or other convenient area. The inside unit contains the compressor, evaporator, blower and heating strip and is mounted under a berth or seat or in a side storage locker. The two units are connected by pre charged refrigerant line sets.

Each approach has advantages and disadvantages. A self-contained unit is simpler to install and maintain, since it does not have external refrigerant lines. It is also less expensive than a split system. On the other hand, it requires cutting two holes in the floor for air to and from the condenser. A split system has more installation flexibility, since the inside unit is smaller. It also is quieter, since the condenser fan is located outside the truck. Dometic’s refrigerant line sets are pre charged at the factory with the correct charge, and do not require any extra charging at installation.

OTHER COMPONENTS

In addition to the auxiliary air units themselves, the system includes ducts, air grills, condensate drains, thermostat controls and electrical hookups.

Also, you will need electrical hookups to your preferred source of AC power, including a shore power plug and connections to the onboard APU or inverter.

	Table of Contents	download and print (pdf)
	Air Conditioning Basics Battery Capacity Calculation How to Specify What You Need Available Battery System Components Examples Glossary of Terms Battery Terminology Battery Types and Capabilities Battery Configuration Battery Capacities Duty Cycle Battery Isolator/Battery Separator Power "Signatures" Alternators Inverter Types

How to Specify the System

Step 1: Determine HVAC capacity needed, based on sleeper size Step 2: Determine how many hours of operation are required Step 3: Use information from Steps 1 & 2 and Figure 4 to determine required Amp Hour capacity required Step 4: Using Amp Hour number from Step 3, divide by chosen battery Ah listed in Figure 3 Step 5: Choose a proper alternator Step 6: Decide on accessories: shore power capability, battery boxes, battery charger Step 7: Discuss all requirements from Steps 1-5, including system type (self-contained vs. remote), ducting with a Dometic Application Engineer

	Available Battery System Components
	Circuit breaker for mainline to inverter Battery separator for charging house bank from the alternator Battery boxes Batteries Inverter Cable pass-thru connections Charger Cab Power Wiring System Alternator

Examples

1. Sleeper use You need to cool the sleeper of a 48" flat roof truck to meet TMC RP432, which mandates 10 hours of run time. A total of 8 hours of drive time are required for battery recharge.From Figure 1, you decide that the 7,000 BTU unit will meet the demand. You prefer to use a MSW inverter. Looking at Figure 2, you find that you will need 600 amp-hours of battery to be able to run for 10 hours. Using Figure 3, you can determine that you can use either three 8D batteries, four Northstar 8D alternatives, or eight Group 31 batteries. The weight of the three 8D batteries will be 474 lbs. The four Northstar batteries will weigspan24 lbs. The eight Group 31 batteries will weigh 552 lbs. The best choice for the weight would be the 8D batteries for this example. The alternator will need to replace 660 amp-hours within 8 hours. This equates to 82.5 amps per hour required. For this example, the alternator will need to be rated at least 165 amps. Now you can let Dometic help you specify the correct components for your system, including the HVAC unit, ductwork (if required), and battery system components. In contrast, if you had chosen the PSW inverter, listed in Figure 4, you could have reduced power consumption by 17%, and potentially reduced the number of batteries. This would bring the total amp-hour requirement down to 498 Ah. You would still need three 8D batteries (474 lbs.), but could get by on three Northstar (393 lbs.) batteries as well. 2. Day cab operation You choose the 7,000 BTU model. You determine that maximum down time is 1 hour, average down time while waiting to drop or pickup a load is ½ hour, and that your average time running between drops/pickups is ½ hour. Maximum amp hours is 75 required Average amp hour requirements will be 37.5 Choose two Gp 31 - 75 amp hour batteries Choose the 185 amp alternator with external regulator. The alternator will provide 88 amps per ½ hour. This will replace the 37.5 amps used in a ½ hour wait, or the 75 amps used in a one hour wait. Remember, there are inefficiencies everywhere (battery cables, inverter, alternator) and you do not get 100%. We have assumed inefficiencies in our calculations. Choose the 1800 watt inverter. You're ready to go.

Glossary of Terms

HVAC Unit Heating, ventilating, air conditioning unit - machine which contains components to cool, heat, and move air – the air is "conditioned." Battery Terminology Battery Types and Capabilities AGM - Absorbed glass mat - a type of lead-acid battery in which the electrolyte is absorbed in a type of fiberglass mat which is sandwiched between the metal plates of the battery. Flooded or lead-acid – the liquid is free to move around within a cell of the battery. Others – There are many other types of batteries, even more brands. For this particular application we only recommend deep-cycle absorbed glass mat (AGM) batteries. Standard "starting" or even standard "flooded lead acid deep-cycle batteries" are not recommended, as they will not have a satisfactory life. Battery manufacturers whose products we have tested and are using are: Deka, Northstar, Lifeline, and Optima. Certain hybrid deep cycle batteries have the ability to maintain a superior cranking ability. We encourage the use of this type. The Optima is one of these. Battery Configuration within the Truck Starting bank – A set of batteries used primarily for consistent starting of the truck’s primary engine. Generally, they are also used for historical other "typical" loads – lights on the trailer, 12V apparatus in the cab or sleeper. With the advent of heavier loads – for instance, air conditioning - an alternative bank should be considered. This would protect the starting bank from possible too great a discharge, resulting in the inability to re-start the truck. House Bank – A set of batteries which are used only for house or hotel loads, such as an HVAC unit or a microwave. Battery Capacities - Battery capacity is very important to get the desired run time. Capacity is often defined as a "Group," for instance: Group 31 batteries are the typical truck battery. A group 27 is a typical automobile battery. The battery sizes that we most frequently use are Group 31 and 8D. The Group 31 battery will fit into a standard truck battery box. The 8D is a larger battery, and needs a special box. However, for large capacity systems, the 8D makes more sense, as there are fewer batteries and fewer connections. Figure 3 Approximate capacity (in amp/hours) vs. approximate weight and size: Type Amp Hours Weight Height x Width x Length Group 31 75 Ah 69 lbs. 9.4" x 6.8" x 13" 8D 200Ah 158 lbs. 10" x 11" x 20.8" Northstar 8D Alternate 170Ah 131 lbs. 12.6" x 4.9" x 22" Duty Cycle - The percentage of run time of a device based on an average over the entire run time of the system. Battery Isolator/Separator – Any time one has two banks of batteries, two sets of batteries that have separate functions and a common charging system. One wishes to prevent back flow of current and one must use one of the following. We recommend the use of the separator because there will be no voltage drop across the separator. Battery Separator – This device prevents back flow of current. It is a device composed of a high ampacity solenoid and an electronic control package. The separator will allow amperage flow based on the presence of a charging voltage. The separator may be bi or unidirectional. Battery Isolator – A device which prevents back flow of current using diodes. This is strictly a one-way device. There is always a voltage drop across a diode (.7 – 1.0 volt), and, given this "penalty," it may not allow one bank to get charged to 100% capacity. Power "Signatures" – The "best" power is directly from land based (shore) power. Power from small generators or inverters is generally not "clean" therefore it is not as efficient as shore power. This is due to a departure from the normal sine wave of alternating current available on a land-based grid. We have often found that "air conditioning not working" means that the generator is not running at proper speed, or there is some other generator fault ("dirty" sine wave), or the inverter is not of good quality or the correct size. Alternators These replace energy into the batteries. Alternating current air conditioning systems use more power than is typical in current truck electrical systems and the alternator requirement for a truck with a battery-based auxiliary HVAC system is greater. Typical alternators have an internal regulator and are sized at about 135 amps. These are susceptible to failure in heavy use and high heat situations. We recommend an alternator with greater capacity and with an external regulator. Alternator size should be based upon amp-hours to be replaced divided by drive time. You must replace at least 110% of the amp-hours removed. The typical truck is said to use approximately 80 amps for operation. See example below. Alternator size – The alternator with the external regulator is rated at 185 amps. If the chosen alternator has an internal regulator, a minimum of a 200 amp alternator is suggested. Dometic applications engineering should be consulted. External regulator – The regulator we will recommend – manufactured by Leece-Neville – acts like a three-stage battery charger, and because of its construction has a superior life cycle. Inverters Inverters change DC current to AC current. So, an inverter takes DC current out of the battery, changes it to AC current, and it is then fed to the air conditioner (or microwave or TV, etc). There are many inverter manufacturers. We cannot test each one, so we will speak from experience on what we recommend. There are two main types of inverters that are available through Dometic for use: a) Modified Sine Wave inverter (MSW) - The MSW is less expensive, but also less efficient than the PSW. This means that the MSW inverter will require more DC amps for the same AC output. See the PSW inverter, below. The MSW inverter is more readily available. b) Pure Sine Wave inverter (PSW.) - PSW inverters will increase run time (all other things being equal) by approximately 17%. They are more expensive than the MSW type, but may mean fewer batteries, and less weight. Two inverters from different manufacturers, labeled "1800 watts" will not act the same. All inverters are not created equal, and are definitely not labeled using the same kinds of criteria. The only way one can make an informed decision on an inverter/HVAC combination is to first test the combination. A less efficient inverter means that more amps are required to "run" the inverter. This means – more battery capacity is required for the same end result as a PSW inverter. It is entirely possible to operate a good battery based air conditioning system on two batteries using a PSW inverter where in may take three batteries for MSW inverter. Also, the quality of the inverter will also affect its life. Again, we have recommendations. Dometic has already performed such testing on some units, and can provide approved units to the market. Here is a list of potential inverters, some available through Dometic: Figure 4 Inverter Selection 1,800 Watt MSW (7k BTU unit only) 1,800 Watt PSW (7k BTU unit only) 2,500 Watt MSW inverter/120 amp charger (7k or 10k BTU units) 2,000 Watt PSW (7k or 10k BTU units) Shore Power Power available from a typical 115V outlet in a terminal (rest area, truck plaza, garage, etc). A shore power connection (in your truck) is a transfer mechanism that is wired to move alternating current electricity from a shore-based source to the air conditioner or to an AC load distribution panel. Shore power can be as simple as a properly-sized cord that is plugged from a 20 amp wall circuit and directly to an air conditioner (a ground fault circuit interrupter is required).