MIL-B-45367AA
interior temperature of the bus from 0°F to 50°F in 20 minutes (with the bus soaked in an
ambient air temperature of 0°F for 15 hours and with no more than a 10 minute engine warm up
time). Temperature rise shall be measured 36 inches inward from the side walls, windshield and
rear walls. Heat shall be evenly distributed in the aisle area. The system shall also be capable of
providing 20 percent fresh air and 80 percent recirculated air or 100 percent recirculated air with
controls for proportioning the amount of fresh air from 0 to 20 percent. Blower capacities shall
provide a ventilating airflow of at least 1,500 cubic feet per minute (cfm). Heaters shall be
located to assure flow of heated air to the driver's area, to all parts of the passenger compartment,
and so that no objectionable flow is directed toward any passenger. Heater hoses shall be
shielded from the passenger compartment and shall be located in protected locations. The
heating system shall include not less than three heater units: one as specified in 3.5.10.2.1, one
near the midpoint, and one near the rear of the bus. The system shall be provided with
thermostatic controls. Cut-off valves shall be provided near the engine to control water inlet and
outlet to heaters. A booster pump shall be provided for proper water circulation.
3.5.10.1.1 Auxiliary heater. When specified (see 6.2), and in addition to requirements of
3.5.10.1, an auxiliary heater conforming to Federal Motor Carrier Safety Regulation 393.77 shall
be furnished. The heater shall operate on the same fuel as the vehicle engine and shall have a
rated capacity of not less than 60,000 British thermal units per hour (Btu/hr.) The heater shall be
mounted in the engine hot water system with a circulating pump of not less than 6 gallons per
minute capacity. Controls for operation of the heater shall be located in the driver's area.
3.5.10.2 Defroster. Defroster outlets with suitable connections, a blower for the driver's
side and a fan or blower for the right side of the windshield shall be furnished and shall be
installed to provide a flow of heated air to each section or side of the windshield. The
windshield defrosting systems shall conform to SAE J381 and SAE J382.
3.5.10.2.1 Windshield defroster heater. A separate heater shall be furnished to provide
heated air to the driver and to the windshield defrosters. The heater shall have a capacity of not
less than 30,000 Btu/hr. A manual control shall be provided to control the amount of fresh air
passing through the heater. A water shut-off valve is required to permit use of the blower for
power ventilation. Manual control shall be provided to adjust the flow of air to the windshield
defrosters as desired.
3.5.10.3 Ventilator. Type III buses shall be provided with four power ventilators when
air conditioning is not furnished (see 3.5.11). Type IV buses shall be provided with four static
ventilators when air conditioning is not furnished. The ventilators shall be located in the roof,
one over each wheelhousing. Outside openings for heaters and ventilators shall be so located as
to minimize entry of fumes, rain, road dust, and road wash. Ventilator openings shall be
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