WD Catalog 2018

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Environmental Effects
1. Heater Core
Heater cores are like small radiators. The fin and tube construction is
designed to route coolant flow for the best possible heat energy transfer
from coolant to cab air. Hoses from and to the engine are connected to the
core with clamps. The core outlet may be larger or the same size as the
inlet.
2. Water Valves
Water valves may be cable, vacuum or air controlled. The valve can be
either open, closed or set part way open. Some valves have a bypass
design to return coolant to the engine. Most are manually controlled
although electronic systems are now being installed.
3. Defrosters and Ducts
Defrosting is accomplished by directing heated dry air through ducts to
the windshield. The heater system serves the dual purpose of defrosting
and heating. Controls are used to route the air flow to the windshield and
occupant areas by opening and closing duct doors. Controls may be
manual, air or vacuum.
Many vehicles use a “defrost interlock” system which utilizes the air
conditioner to dry the defrost air and clear the cab windows more quickly.
4. Blowers and Fans
Blowers or fans are used in the system to move cab air through the heater
core and evaporator. Air can be pushed or pulled through the core depend-
ing on system design. Blower or fan speed is usually selected by the
operator.
Environmental Effects on System Operation
The environment outside the cab involves more than the weather. It may be hot
and humid or cold and dry. That is only part of the condition the HVAC system
must handle to maintain an ideal comfort range. A truck can be at idle, in
traffic or moving along for hours on the Interstate at 65 M.P.H. The load
condition on a trip can include going out full, coming home empty, or driving
across the Rockies or Kansas plains during the day or night. The truck color
and shape, the windows and angle of the sun are all variables that can increase
or decrease the “load” on the system. The following are a few examples of
environmental effect:
• A black cab-over (COE) with a dark color interior will be
more difficult to cool than the same vehicle with white paint
and a light colored interior. The black cab picks up and holds
the radiant heat from the sun more easily than the white one.
• In Florida or Houston the humidity in mid summer can be
very high with the temperature in the high 90’s or low 100’s.
The AC unit must remove a lot of moisture from the air in the
cab as the air moves through the evaporator fins. The more
moisture on the fins, the less effective the transfer of heat is
to the refrigerant inside the evaporator coil.