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About Insulation.
Before we go any further, let’s talk briefly about insulation. Most truck sleepers today are insulated with a value of
approximately R 1 to R 1.25. Compare this to the insulation of R 30 that is required in most new housing con-
struction. Even the “cold-weather packages” offered with some new trucks only have insulation values of R 2. TMC
recommended practices call for R 4.2 as a standard insulation package and greater than R 4.6 in a premium package.
This standard, once adopted by the industry, will go a long way toward improving the performance of auxiliary air
conditioning and heating systems.
If you are buying a new truck, you should order it with additional insulation. There are also a number of measures
you can take to reduce the heat load in your truck. If you have skylights, cover them. Insulate the floors with foam
under the carpet. Add insulation to the doors. Get heavy-duty curtains to separate the sleeper from the cab.
Cover the windshield and windows with reflective shades when parked. Park in the shade whenever possible.
Switch off heat-producing appliances in the sleeper.
Air Conditioning Units
Your first step will be determining the type of air conditioning units (split or self-contained) to meet your require-
ments and space constraints, then sizing the units to meet the expected heat load.
Sizing the system can be a complicated affair, since it depends on a number of factors, such as the size and shape of
the sleeper, the amount of insulation (see above), the size and location of windows, the color of the truck (darker
colors absorb more heat) and the ambient temperature outside. It also depends on how long you will need to run
the system without starting your truck’s engine. The following guidelines are based on TMC recommendations for
“average” conditions. If you are operating in very hot regions, you may need to consider extra capacity.
Given current insulation conditions, this will usually keep the occupant cool from head to feet when standing, but
will not cool a high-rise sleeper all the way to the ceiling.
Remember that in a battery-powered sys-
tem more BTUs means more batteries.
Batteries
To power your Dometic auxiliary air con-
ditioner, you will replace your truck’s
existing batteries with Group 31 AGM
batteries, which are designed to support
the heavier loads and frequent
discharge/charge cycles.
SPECIFYING THE SYSTEM
For a day cab with limited engine-off times (less than 2 hours), we recommend replacing the existing bank of batter-
ies with an equal number of AGM 31 batteries.
For a typical sleeper, you will replace the existing bank of four batteries with AGM 31 batteries, and also install addi-
tional batteries. See the chart below for guidelines on the number of batteries required.
We suggest adding the extra batteries and tying them to the existing bank – creating in effect a single bank of 6 to 8
batteries. There are protective low voltage devices on the inverter will shut down electrical loads whenever the
batteries reach a pre-set voltage, thereby ensuring that sufficient power is retained for cranking the engine.
In some cases, it may be desirable to install a separator to isolate two of the batteries from the others. This creates
two separate banks – one for “house loads” and the other exclusively for cranking the engine.
One must understand that the effect of adding more batteries on the system. The relationship is not linear. For
instance, if two batteries give you 1.5 hours, it does not follow that three batteries would give you an additional .75
hour. This is why – given battery voltage safeguards – we recommend a single bank of batteries without separators.
Engine Off
Day Cab 42-48" low roof 60" flat roof 60-72" high-rise
7,000 BTU 7,000 BTU 7,000 BTU 10,000 BTU
Note:14,000 BTU may be used in certain situations where engine-off time is minimal.
Engine Never Off (No Engine-Driven Air Conditioner)
Day Cab Vocational
10,000 BTU 10,000 BTU
Hours Running on House Batteries
(AGM Group 31 Batteries)
# Batteries 7,000 BTU 10,000 BTU
100% Run 60% Run* 100% Run 60%* Run
2 1.9 3.8 1.2 2.2
3 3.4 6.1 2.3 3.9
4 4.9 8.5 3.1 5.6
5 6.1 10.6 3.9 7.0
6 7.3 12.8 5.0 8.7
7 8.9 14.9 5.8 10.2
8 10.2 17.1 6.7 11.6
*Air conditioner runs 60% of the time, cycling off and on as needed to maintain desired temperature.