Conduction is the transfer of heat directly from molecule to molecule. An example of conduction is how the fasteners on the exhaust manifold get hot because they are in direct contact with the exhaust. Another example is a person accidently touching the hot exhaust manifold while the engine is running. The extreme discomfort felt by touching the hot exhaust manifold would be a result of conduction.
Convection is the transfer of heat by using a substance, such as air, water, coolant, or refrigerant. The engine cooling system is a prime example of convection because the coolant absorbs large quantities of heat from the hot engine block and cylinder heads and is then pumped to the radiator where the heat is released into the surrounding air. The AC system is another example of convection because the refrigerant absorbs large quantities of heat from the warm air in the passenger compartment and is then pumped out to the condenser, where the heat is released into the surrounding air.
Radiation is the transfer of heat through rays without heating the air as it passes through. Sunlight is the best example of transferring heat through radiation. This concept can be understood due to the fact that it is possible to have a sunny day during the winter while the outside temperature can still be very cold. Radiant heat also greatly changes the temperature inside of dark-colored vehicles due to how the sunlight is absorbed by the dark color. Light-colored vehicles reflect the sunlight instead of absorbing the radiant heat. This concept can be seen by measuring the cabin temperature of a dark-colored vehicle on a hot day and comparing the findings with that from a light-colored vehicle.
Air Handling System Operation
The air handling system controls the speed, the temperature, and the distribution location of the air being discharged into the cabin of the vehicle. The types of systems include manual control and automatic temperature control (ATC).
Manual Control Systems
In a manual control system, the driver or the front-seat passenger has the option to pick the blower speed, the air temperature, and the distribution location by moving the selectors on the HVAC control head to the desired position. A large portion of the vehicles on the road today, as well as older vehicles, use this type of HVAC control system.
This diagram shows examples of heat transfer. Conduction is the transfer of heat directly through a material. Convection is the transfer of heat through air, fluid, or gas. Radiation is the transfer of heat through heat rays.
Manual climate control heads are found on many vehicles of every era. This type of control head has a knob for blower speed, air temperature, air distribution location, air-conditioning, and recirculated air. This control head will also have a rear defogger switch if the vehicle is equipped with that option.
Manual climate control heads have been a reliable method to control the blower speed, the air temperature, the mode of air distribution, the air source, and the operation of the AC system for many years.
The duct box is where the action happens when the levers and knobs are moved on the control head. The duct box is typically located under the dash panel and is fairly simple in operation. Items that are located in the duct box include the blower motor, the evaporator core, the heater core, the fresh/recirculated air door, the blend door, and the mode doors. Here is an explanation of how each of the main items inside the duct box functions.
Blower Motor: This device is a multispeed electric motor that has a round impeller that forces the air to flow through the duct box and be directed to the desired location by the mode doors.
Evaporator Core: This device is the heat exchanger for the AC system that the air in the duct box always passes through. When the AC system is turned on, the evaporator core surface is cold and causes the heat and moisture to be removed from the air that passes through it, resulting in cool and dry air.
An overhead view is shown of the duct box that was removed from a late-model vehicle. This duct box must be removed and taken apart in order to replace the heater core or evaporator core. The blower motor, door actuators, and blower resistor are typically serviceable without having to remove the duct box.
Heater Core: This device is the heat exchanger for the heating system. Once the engine is warmed up, the heater core has hot engine coolant flowing through its passages. Heated engine coolant is routed to the heater core through hoses or pipes. The duct box air only passes through the heater core when the blend air door is set to a position that allows this to happen. As the air passes through the heater core, heat is released from the hot coolant and absorbed by the air, which results in warm air.
Air is routed through the duct box with a blower motor and a series of doors and heat exchangers. The blower motor is the device that forces air to be moved within the duct box. The recirculation door causes air to be pulled into the cab from outside or to be recirculated from within the cab. The blend door (also known as the temperature control door) causes the air to be directed through the heater core or around the heater core, which controls the temperature of the air. The mode doors cause the air to be directed at either defrost, vent, or floor ducts.
Fresh/Recirculated Air Door: This door controls where the air is pulled from by the blower motor. The two sources for air in the duct box are fresh air from outside the cab and recirculated air from inside the cab.
Blend Door: This door is located between the evaporator core and the heater core. This door controls the amount of air directed to the heater core that controls the temperature of the air that will be distributed by the mode doors. This door is sometimes called the temperature control door. When the door is in full heat mode, all of the air is directed to the heater core, which results in maximum heating in the cab. When the door is in full cool mode, it blocks air from passing through the heater core, which results in maximum cooling in the cab. Many times, the blend door is positioned somewhere between the maximum points, which causes a blending action to occur and results in the desired air temperature being sent into the cab. This door is controlled by the position of the temperature lever on the control head.
Mode Doors: These doors control where the duct box air is distributed in the cab. The different available modes include defrost, vent, and heat modes that are all controlled by the mode doors. Many systems have bi-level options that allow air to be distributed at two locations simultaneously, such as defrost and heat or vent and heat.
The doors in the duct box can be moved using cables, vacuum actuators, or electric motors. Cables and vacuum were the methods used on all early AC-equipped vehicles, and electric actuators are always used on late-model vehicles. Vehicles with cable-operated mechanisms to move the duct box doors typically had a method to adjust the cable length in case the door needed an adjustment. Vacuum-operated systems used either engine vacuum or some type of vacuum pump to create the negative pressure needed to move the doors. The electric actuators used on all late-model and current vehicles receive a command from the control head or logic device when movement is needed. The electric actuators seem to fail more often than the other devices used to move the doors.
A cable is used to move this blend door as the temperature lever is moved. Cables were used on many older-model vehicles and provided reliable operation of these systems.
Vacuum actuators are widely used on