All school bus manufacturers offer air conditioning options. Some install systems on the assembly line. Others offer systems that are installed by A/C manufacturer-qualified installers or the bus sales dealer, after the bus leaves the assembly line.
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OEM and after-market systems vary, from system installations that tie into existing OEM dashboard HVAC systems, to stand-alone complete air conditioning systems. The components that make up an A/C system are the compressor (mounted on the engine or a separate power source), dash evaporator (driver area), controls (switches, thermostat, electronics), hanging or flush evaporators, and rooftop or skirt mounted condensers.
There are so many different kinds of bus air conditioning systems to choose from, that it can be hard to figure out which one is just right for you.
Manufacturers are continually updating or adding new system configurator links through their dealer networks, so be sure that you pick the most economical and effective cooling system. Always work with your dealer to find the cooling system that is specific to your vehicle’s cooling needs.
Don’t compare apples to oranges, that is, “In-dash, tie-in systems to stand-alone propriety systems.” Get the right system for your bus, and be sure to consider the power draw that the A/C system will require, so that it does not impact your vehicle’s alternator capacity to maintain its battery charge.
Level the Playing Field
1. Have your system provider reference a performance pull-down test. The best and most important evaluation of a system is to measure the ability of the installed system to cool a bus interior.
2. Require BTU/hour system capacities be recorded in an industry standard format, such as the Society of Automotive Engineers (SAE).
The National Congress of School Transportation amended school bus A/C specification recommendations in 2015. The resulting National School Bus Specifications & Procedures document, starting on page 49, paragraph B, recommend that two performance standards be used when specifying school bus air conditioning systems:
• Standard Performance — The installed air conditioning system should cool the interior of the bus from 100 degrees to 80 degrees Fahrenheit in 30 minutes.
• High Performance — The installed air conditioning system should cool the interior of the bus from 100 degrees to 70 degrees in 30 minutes.
The American Public Transit Association has specified its requirements:
• Capacity and Performance Requirements — The air conditioning portion of the HVAC system shall be capable of reducing the passenger compartment temperature from 115 degrees to 95 degrees in less than 20 minutes after engine startup.
• Hotter Ambient Conditions — The air conditioning portion of the HVAC system shall be capable of reducing the passenger compartment temperature from 110 degrees to 70 degrees, plus or minus 3 degrees, in less than 30 minutes after system engagement for 30-, 35- and 40-foot buses.
BTU Capacity Considerations
• Gross capacity (most typically used) uses the weakest link (lowest capacity) among the evaporator, condenser and compressor to determine overall system capacity.
• Net capacity (rarely used and difficult to measure) uses multiple simultaneous equations until the system is balanced.
There is neither a universal BTU (British Thermal Unit) capacity rating method currently being used in the bus air conditioning industry, nor any governing association to police the industry!
BTU is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. Aside from the technical definition of BTU, bus air conditioning system BTU capacity is dependent on many variables. It requires that specific rating conditions be defined for each system component, since each has its own capacity.
Capacity ratings can change, higher or lower, depending on the rating conditions that are being used. So be aware that system BTU ranking is the lowest of the three component values: Compressor, condenser and the evaporator rating.
Theoretical BTU/hour system ratings, when using realistic rating conditions at the various vehicle engine/compressor speeds, is a valuable piece of information to assess the application of the system to the vehicle type.
However, the best way to measure A/C system performance is to install an air conditioning system in the vehicle, operate the vehicle into the above mentioned environment, and perform a standard or high-performance, pull-down test. This will then tell you that the air conditioning system you ordered to do the job is the one you got.
Editor’s Note: Reprinted from the March 2019 Issue of School Transportation News
Bob Pudlewski is the technical editor for School Transportation News magazine and has over 40 years of experience in the school bus industry. He retired as vice president of fleet operations, procurement and maintenance from Laidlaw and is a member of the NTSA Hall of Fame.