HVAC: Heating & Air-conditioning

HVAC basic principles	HVAC legislation	In most countries, any person who services an air conditioning system must be separately accredited to comply with local environmental protection legislation, or under close supervision of an accredited person.
	Vehicle heating & ventilation systems	To improve passenger comfort and safety, fresh air is drawn from outside ducts and directed to the passenger compartment. Incoming air can be heated by passing over a small heating core connected to the engine’s cooling system.
	Basic air-conditioning principles	Heat transfer can take place by one of three methods, conduction, convection and radiation.
	Air-conditioning capacity	Latent heat is the amount of heat required to be added to a substance or removed from a substance to bring about a change of state.
	Air-conditioning refrigerant	The refrigerant removes heat from the vehicle interior and transfers it to the outside air in a continuous cycle of vaporization and condensation.
	Humidity	Reducing the temperature of the air also reduces its humidity. Cold air will not hold as much water vapor as warm air.

Fixed orifice tube air-conditioning system	Fixed orifice	In a fixed orifice system, the orifice tube controls the flow of refrigerant into the evaporator. An accumulator acts as a storage device and also contains filters and desiccants to remove moisture and impurities from the refrigerant.
	Control devices	Control devices are fitted into the system to ensure maximum efficiency of operation and to protect components from damage.

Thermal expansion valve air-conditioning system	Thermostatic expansion valve system	In a thermostatic expansion valve system, a receiver drier is located between the condenser and the evaporator. It acts as a storage volume for liquid refrigerant and also contains a filter and desiccant material.
	Thermal expansion valves	A thermostatic expansion valve controls the amount of refrigerant entering the evaporator coils. It also causes a drop in pressure to take place without causing a change of state.

Air-conditioning components	Air-conditioning compressors	The compressor circulates refrigerant through the air conditioning system. Axial piston compressors are one of the most common types used.
	Condensers & evaporators	The condenser transfers heat from the refrigerant to the outside air. The evaporator draws heat from air in the passenger compartment.
	Receiver drier	The receiver drier acts as a storage volume for liquid refrigerant. It also contains filters and desiccants.
	Lines & hoses	Hoses and pipes must be capable of withstanding the high pressures and temperatures that are found in the air-conditioning system.
	TX valve construction	The thermal expansion valve controls the amount of refrigerant entering the evaporator coils. It also causes a drop in pressure with out causing a change of state.
	Temperature monitoring thermostat	The thermostat monitors the temperature of the evaporator fins and controls system operation within set limits.
	Refrigerants	The refrigerant is used to absorb, transfer and release heat in the air-conditioning circuit.
	Pressure switches	Pressure switches protect the system from excessively high or low pressures.
	Heating elements	Most vehicles contain a heated rear window.
	Heater cores	The heater core uses heated coolant to blow heated hair into the passenger compartment of the vehicle.

Climate control	Air-conditioning ECU	The electronic control is connected to the sensors and actuators by a multi-plug, and through wiring to the main vehicle wiring harness.
	Ambient air temperature sensor	The cabin air temperature is continually measured by the cabin air temperature sensor. When the cabin air temperature has been changed to be within 2°C or 35°F of the desired selection, the fan speed is gradually reduced.
	Servo motors	Vacuum servos consist of a sealed housing divided by a flexible diaphragm. The diaphragm seals the housing into two separate chambers.
	Electric servo motors	Electrical servos incorporate limit switches to open circuit the current supply to the motor when the maximum travel of the flap or tap has been reached.
	Automatic climate control sensors	Most temperature sensors used in climate control systems are thermistors which are solid state electronic devices that change their electrical resistance in response to temperature changes.
	Evaporator temperature sensor	The evaporator temperature sensor is located in the air stream where the air leaves the evaporator and signals to the control unit the temperature of the air.
	Blower speed control	This controls the speed of the fan in the heating and air-conditioning system in the passenger cabin.
	Ventilation systems	A ventilation system is fitted to vehicles to make the environment in the cabin comfortable for its occupants.

HVAC procedures	Checking an A/C system	Some earlier systems have a sight glass somewhere in the high-pressure side of the system. This gives you some visual information about the state of the refrigerant.
	Checking a heating system	In some systems, a valve operated by a knob or lever inside the passenger compartment controls the flow of coolant through the heater core.
	Inspecting & adjusting an engine drive belt	A belt that has been soaked in oil will not grip properly on the pulleys and will slip. If the oil contamination is severe enough for this to happen, replace the belt.
	Replacing an engine drive belt	If you don't have a tension gauge, you can estimate the tension by pushing the belt inwards with your hand.