WD Catalog 2018

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Chapter 3 - Heater Components and Controls
3-6 - -
AIR PRESSURE
FLOW
AIR OPERATED WATER VALVE
FLOW
CABLE CONTROL
CABLE OPERATED WATER VALVE
When a separate control panel is installed in the bunk area of a cab, there
may be one type of control for the cab and another type for the sleeper. The
bunk control is often a thermostat, cycling the fan on and off to maintain
a constant temperature. Motor switches usually have one to three speed
positions plus off.
In a typical heater system, other controls you may encounter position
the doors inside the ducts to direct air flow. The defroster control is the
main one you will find in basic heater systems. This is usually a cable
controlled door that diverts warm air to the windshield or floor.
In HVAC systems the number of doors in the system and their function
will vary depending on system design. Doors operate electrically, manu-
ally, or automatically, by cable, vacuum, compressed air or by some
combination of these controls. In certain HVAC systems, the defrost mode
activates the air conditioning system. The air conditioner will dry the
heated air flowing to clear the windshield. This is commonly referred to as
a “defrost interlock.”
Chapter Review
• In review, the heat energy for heating cab air comes from the
hot engine coolant. The coolant is a combination of water and
anti-freeze (ethylene glycol). The engine water pump circu-
lates coolant. The thermostat directs the flow of coolant in
the engine and to the radiator. The radiator pressure cap
allows the cooling system to operate under a preset amount of
pressure. Each pound of pressure increases the boiling point
of the coolant by three degrees Fahrenheit. The boiling point
drops by two degrees Fahrenheit for every 1000 feet of truck
operating elevation.
• Heater systems are designed to transfer heat energy from the
engine coolant to the air in the cab. A water valve controls
coolant flow to the heater core. A fan or blower forces cab air
through the fins of the heater core and heat from the coolant
is transferred to the air. The heater duct system and air
vents direct air movement in the cab.
Figure 3-8
This illustration shows air
and cable controlled water
valves.