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 construction. 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 unit (split or self-contained) to meet your requirements and space constraints, then sizing the unit 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.
The following guidelines are based on TMC recommendations under “average” conditions. If you are operating in very hot regions, you may need to consider extra capacity. Given current insulation conditions, these guidelines 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, in a battery-powered system more BTUs mean more batteries.
|
Day Cab |
42-48" Low Roof |
60" Flat Roof |
30-72" 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.
Batteries
To power your Dometic auxiliary air conditioner, 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.
For a day cab with limited engine-off times (less than 2 hours), we recommend replacing the existing bank of batteries 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 additional batteries. See the chart below for guidelines on the number of batteries required.
Hours Running on House Batteries
(AGM Group 31 Batteries) 7,000 BTU
|
Number of
Batteries |
100% Run |
60% Run* |
|
2 |
1.9 |
3.8 |
|
3 |
3.4 |
6.1 |
|
4 |
4.9 |
8.5 |
|
5 |
6.1 |
10.6 |
|
6 |
7.3 |
12.8 |
|
7 |
8.9 |
14.9 |
|
8 |
10.2 |
17.1 |
*Air conditioner runs 65% of the time, cycling off and on as needed to maintain desired temperature.
Hours Running on House Batteries
(AGM Group 31 Batteries) 10,000 BTU
|
Number of
Batteries |
100% Run |
60% Run* |
|
2 |
1.2 |
2.2 |
|
3 |
2.3 |
3.9 |
|
4 |
3.1 |
5.6 |
|
5 |
3.9 |
7.0 |
|
6 |
5.0 |
8.7 |
|
7 |
5.8 |
10.2 |
|
8 |
6.7 |
11.6 |
*Air conditioner runs 65% of the time, cycling off and on as needed to maintain desired temperature.
We suggest adding the extra batteries and tying them to the existing bank – essentially creating a single bank of 6 to 8 batteries. There are protective devices on the inverter and alternator/regulator that 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.
It is important to understand that the effect of adding more batteries 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.
Alternator/Regulator
Most trucks have a standard 135-amp alternator with internal regulator, which is adequate for keeping the batteries charged under normal conditions. This is not sufficient, however, for the heavier charging requirements for a battery-powered air conditioning system. Dometic specifies a high-output alternator with an external regulator. The regulator acts as a three-stage battery charger that charges and protects the batteries through numerous discharge cycles. These guidelines apply when sizing the alternator:
| Number of Batteries |
Alternator Size |
| 6 or less |
185 amps |
| 7 to 9 |
270 amps |
| 10 or more |
320 amps |
Inverter
The inverter takes the 12 Volt DC output from the batteries and changes it to 115 Volt AC current, which is fed to the air conditioner and other AC-powered devices. We have tested and evaluated dozens of different inverters and selected a 2,000 Watt unit with a modified sine-wave (MSW) output. This unit includes a 115 Volt pass-through circuit and built-in 50 amp charger to permit you to run the air conditioner and recharge the batteries from shorepower when available. It is also offered as an inverter-only model without the charger. We developed this inverter in conjunction with a company with wide experience in this technology, and we have confidence in its ability to start and run our air conditioning systems. Remember, whenever the air conditioning system cycles on there is a momentary heavy load that the inverter must accept and hold until the compressor gets past its starting requirement. It’s only a few milliseconds, but it’s very important.
We do not recommend running our air conditioning systems with any inverter not previously tested and
Shorepower
Although there are relatively few shorepower hookups available at truck stops, rest stops, and terminals, these will become more readily available in the future.
TMC recommends a minimum 20-amp circuit for shorepower connections. You should use any heavy-duty power cord with a standard three-prong plug. Remember that there will be a voltage drop across the cord, so you should use the minimum length necessary to reach your outside power source.
| Cord Gauge |
Distance |
| 12-gauge cord |
Up to 25 feet |
| 10-gauge cord |
Up to 50 feet |