WD Catalog 2018

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Chapter 2 – System Operation
2-10 Go to o - Index
• On a cold day the temperature can drop below zero. The
engine may run cooler so the engine coolant is cooler when it
circulates through the heater. The heat in the cab moves out
of the cab faster (remember heat always moves to a cooler
area until both are the same temperature—nature’s law). To
maintain cab comfort you have to increase the flow of coolant
through the heater, increase coolant temperature, and/or
move more air through the heater core.
• On a hot day, an off-road vehicle experiences cooling at a
slower rate than an on-road vehicle. This is a result of high
sun-load, large wind
It is important for you to ke s in mind when you are
servicing or diagnosing heater or air conditioner systems. If you work in
Denver the altitude will affect system function and pressure. In Houston the
heat and humidity may lower heat transfer to the air at the condenser and
increase system operating pressures.
Chapter Review
• HVAC systems range from simple cab heaters to multi-function
combination systems. The multi-function system can heat and
cool the cab and sleeper unit, and have separate auxiliary
components and controls for driver and passenger comfort.
• Both heater (engine) coolant and air conditioner refrigerant
circulate inside sealed, pressurized systems. The normal air
conditioner operating pressure ranges from 150 to 250 pounds
per square inch, sometimes higher with a different refrigerant.
• Air conditioners have a high and a low pressure side within the
system. The compressor is the starting point of the high side.
Pressure drops at the expansion valve opening to the evaporator.
• The basic components of an AC system are the compressor-
clutch assembly, high pressure lines, condenser, receiver-drier,
expansion valve, evaporator, thermostat, blower assembly, and
suction lines. There may be other controls installed for more
complex systems.
• The basic components of a heater system are the inlet and outlet
hoses, a water valve and valve control, heater core and fan or blower
assembly. There may be other controls for more complex systems.
• An air conditioner system uses the “change of state” of refriger-
ant inside the system to move heat from the cab air to the
outside air. Refrigerant changes from a gas to a liquid in the
condenser, and back to a gas in the evaporator.
• A heater system uses the heat from the engine, carried to the
heater core by the action of the water pump, to warm the air in
the cab. There is no change of state within the heater system.
• Environmental conditions affect how both heaters and air condi-
tioners work. Weather, driving conditions, color of the vehicle are
factors. All contribute to heat gain or loss inside a cab and how
much heat energy must be moved to maintain occupant comfort.