1. Air Distribution Control
Topics covered in this presentation:
Single and dual distribution assemblies
Blend-air and stacked core re-heat systems
Ducts and louvers
Single & dual systems

2. Introduction
The air distribution system is used to direct filtered air flow into the passenger compartment of a vehicle. This flow of air is controlled by the use of flap doors.
The temperature of the air flowing into the passenger compartment is controlled by a combination of reheated air through a heater core and outside air. Airflow through the heater core is controlled by the temperature selector.
A multi-speed fan is used to change the rate of airflow into the passenger compartment.
The driver can adjust the settings of the air distribution system by a control panel (head) mounted in the instrument cluster.

3. Air Distribution Section
Typical Heater & A/C Assembly Layout - 1
The heater and air conditioning distribution assembly contains an evaporator, a heater core, a fan and control doors.
Air Inlet Section
Plenum Section
Air Distribution Section
Filtered fresh air in
Air to defrost panels
Evaporator
Heater core
Fan
Floor air out
Intake door
Air mix
(blend) door
Recirculated air in
Air to panel outlets
The evaporator cools all air flowing through it and removes any moisture in the air. The heater core warms the air to a temperature determined by the driver. The fan is used to move air into, through and out of the assembly.
The position of the control doors are set by the driver via the use of control levers or knobs, so air flows into the vehicle as and where required.

4. Air Distribution Section
Typical Heater & A/C Assembly Layout - 2
The heater and air conditioning distribution assembly is split into three sections: Air inlet, plenum and air distribution.
Air Inlet Section
Plenum Section
Air Distribution Section
The air inlet section contains the fresh air and recirculated air inlets. Note that the fresh air inlet normally includes a filter to ensure that debris does not enter the system.
The plenum section contains the evaporator, heater core and air mix (blend) door, and the air distribution section contains the panel, defroster and floor control doors.

5. Air Distribution Assembly Location
The heating and air conditioning distribution assembly is typically situated at the rear of the engine compartment and fixed to the bulkhead.
Evaporator housing
Heater and A/C controls
Fan housing
The air distribution assembly is typically made from reinforced plastic or pressed steel sections.
System hoses
Air distribution
assembly
with heater core
Air conditioning system hoses are connected directly to the evaporator in the assembly.
Driver operated heater and air conditioning controls are connected to the assembly from the vehicle’s interior via cables and/or electrical connectors.
Compressor
Engine
Condenser

6. Air Flow Through Heater & A/C Assembly
In the air distribution assembly shown, only recirculated air from the passenger compartment passes through the evaporator.
Defrost air out
Filtered fresh air in
Air mix (blend) door
Intake door
The amount of evaporator and/or outside air that passes to the heater core is determined by the position of the intake door.
Fan
The air mix (blend) door combines the outside and heater core air in order to adjust the temperature to that determined by the driver’s controls.
Floor air out
Heater core
Recirculated air in
Air then passes, again via control doors, to the defrost/panel and floor outlets.

7. Blend-Air Reheat System - 1
Lines to A/C system
Filtered fresh or recirculated air in
In the blend-air reheat system shown, recirculated and/or outside air is firstly passed through the evaporator to cool it and remove any moisture.
Cold air from evaporator
Air blend door
This cold air is then passed through or around the heater core, or both.
Evaporator
Heater core
Hot water lines from engine cooling system to heater core
The amount of air that passes to the heater core is determined by the position of the air blend door, which is adjusted by driver controls in the passenger compartment.
Hot air from heater core
Blended air to vehicle interior

8. Blend-Air Reheat System - 2
Lines to A/C system
Filtered fresh or recirculated air in
When the driver’s temperature control is set to its hottest position, all of the air from the evaporator passes through the heater core.
Cold air from evaporator
Air blend door
When the driver’s temperature control is set to its coldest position, all of the air from the evaporator passes around the outside of the heater core.
Evaporator
Heater core
When the driver’s temperature control is set between its hottest and coldest positions, some air passes through the heater core, while the rest goes around it. The air then combines (blends) to produce the required temperature in the passenger compartment.
Hot water lines
Hot air from heater core
Blended air to vehicle interior

9. Stacked Core Reheat System
This reheat system has the evaporator and the heater core next to one another, so that all air passes through both the evaporator and the heater core.
Filtered fresh or recirculated air in
Lines to A/C system
Evaporator
The driver’s temperature control directly adjusts the heater core temperature using a heater valve. The valve then controls the amount by which the heater core heats the air from the evaporator.
Cold air from evaporator
Heater core
Hot water lines
Heater valve
The stacked core reheat system is not normally used in modern vehicles.
Reheated air to vehicle interior

10. Typical Control Head - 1
The climate control head is found in the passenger compartment and is used to control the flaps/doors and operation of the air distribution and air conditioning system.
Temperature control lever
Air conditioning control lever
The control head shown uses mechanical levers for the temperature, air conditioning and air distribution controls.
The air conditioning system is switched on and off by the use of an electrical switch.
Air distribution control lever
Air conditioning on/off switch
Fan control
The multi-position fan control directs electrical current through a number of resistors, dependent upon its setting. At its ‘low’ setting, all resistors are used. At its ‘hi’ setting, no resistors are used and 12V is applied to the fan.

11. Typical Control Head - 2
When the air conditioning lever is set to NORM, outside air is drawn into the system and when in the MAX A/C position, recirculated air is drawn in. For both settings, air is output to the defrost, floor and panel outlets.
Temperature control lever
Air conditioning control lever
With the A/C system off, the air flow is controlled by the distribution control lever. The lever shown can be set to:
Panel - Most air goes to panel outlets, some to the defrost outlets.
Floor - Most air goes to floor outlets, some to the defrost outlets.
Air distribution control lever
Air conditioning on/off switch
Fan control
Mix - The air is divided between the panel and floor outlets.
Defrost (indicated by windscreen symbol) - Most air goes to defrost outlets, some to the floor outlets.

12. Control Head Cable Connections
The control head shown has cables connected to the air conditioning, air distribution and temperature control levers. Moving a lever from side-to-side moves a control arm, which in turn moves the cables backward and forward.
Air conditioning control lever
The control cables then operate cams connected to the air distribution control flaps, causing them to open and close.
Temperature control lever
Air conditioning control cable
The air conditioning on/off and fan switches have electrical connectors at the control head.
Fan control connector
Air distribution control cable
Many modern control heads operate the air distribution flaps by the use of vacuum valves or electrical solenoids.
Air conditioning on/off switch connector
Temperature control cable

13. Heating System Inlets and Outlets
The diagram below shows the inlets and outlets of a typical air distribution system.
The inlets typically consist of a filtered fresh air inlet found behind the engine compartment and a recirculated air inlet found in the foot well of the
passenger compartment.
Windscreen defroster nozzle
Filtered fresh air in
Side window outlet
Side window outlet
The most common outlets are the windscreen defroster nozzle, instrument panel outlets/louvers (that direct air to the passengers) and the floor outlets.
Instrument panel outlets
Floor outlets
Recirculated
air in
Additional outlets may also be found that are used to defrost the side windows.
Instrument panel outlets

14. Louver direction control
Louver Construction
The outlet louvers (registers) are typically made from a reinforced plastic.
The vent itself is a grid-type opening through which air flows into the passenger compartment. It has a direction control that allows the air flow to be controlled, either left or right.
Air door thumb wheel
A thumb wheel can be used to operate an air control door, which determines how much air flows through the outlet dependent on its position.
Air control door
A housing contains the louver, air control door and air door thumb wheel. This housing also allows the louvers to be moved in the vertical axis, so that air can flow upward or downward as required by the passenger.
Louver housing
Louver direction control
Outlet louver

15. Front & Rear Passenger Compartment Ducts
The passenger compartment ducts are typically made from reinforced plastic. However, some vehicles have sheet metal or wire-reinforced flexible ducts.
Side window outlet
Front compartment ducts
The diagram shows ducts that direct air
to both the front and rear passenger compartments. However, some vehicles only direct air to the front passenger compartment.
Defroster outlets
Instrument panel
outlets
The ducts must be
designed to be compact
to fit confined spaces,
quiet in operation and
leak free.
Side window outlet
Rear compartment outlets
Rear compartment ducts

16. Dual-Zone Distribution System - 1
The dual zone distribution system allows the driver and passenger to select different temperature settings to suit individual needs.
The system splits at the heater core so that the driver and passenger have individual blend doors for controlling temperature.
Driver defrost door
Driver air mix (blend) door
Filtered fresh air in
Driver mode door
Heater core
Evaporator
Fan
Intake door
Passenger defrost door
Passenger mode door
Passenger air
mix (blend) door
Recirculated air in

17. Dual-Zone Distribution System - 2
The air inlets for the dual-zone system are the same as those for a single-zone system, the air coming from outside or recirculated from the vehicle interior.
The air outlets are again the same as those for a single-zone system (floor, windscreen and instrument panel), however, the ducting is split so the required temperature is provided to the driver and passenger individually.
Driver panel outlets
Filtered fresh air in
Driver windscreen outlet
Driver floor outlets
Passenger windscreen
outlet
Recirculated air in
Passenger
panel outlets
Passenger floor outlets