The HVAC system uses hot engine coolant to move heat into the passenger compartment when the heat mode is chosen on the control head. The HVAC system uses refrigerant to remove heat from the passenger compartment when the AC mode is chosen on the control head. To understand how this transfer of heat takes place, an explanation of the three main types of heat transfer is needed. The three types of heat transfer include conduction, convection, and radiation.
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.
