Ventilation Control


  • General
  • Ventilation Control
  • stove house
  • interior fan
  • Series resistor
  • stove house
  • Heater radiator and power tap
  • Ventilation shafts

The climate in the car can be optimally adjusted with today's systems. When it is cold, the interior can be heated in various ways. This heat comes from the heat of the engine. A constant, pleasant temperature is maintained. At high outside temperatures, it is nice if a cool air temperature is blown into the interior. In cars without air conditioning, this is the pure outside air, but in cars with air conditioning this outside air is first cooled down considerably before it is blown into the interior. Too high a temperature causes loss of attention, slower reactions and fatigue.
The air conditioning also affects the humidity in the interior; this decreases. If the humidity is too high, you can get a stuffy and oppressive feeling, and if the humidity is too low, you can get a dry throat and dry eyes. The most pleasant climate is in a temperature between 20 and 23 degrees, with a humidity between 30 and 60% and of course with filtered air by means of a cabin filter.

Ventilation Control:
Due to variations in the outside temperature or in the speed of the car, the temperature in the interior of the car changes. In order to maintain the correct temperature, the stove and ventilation settings must be adjusted regularly with manual heating. Cars with automatic temperature control do not have this problem; it adjusts the ventilation speed and temperature itself. The control unit ensures that the set temperature is maintained. If, for example, 20 degrees Celsius is set and the window has been open for a while while it is cold outside, the interior temperature sensors will measure that the temperature in the interior has dropped. The temperature of the heating will rise (to eg 24 degrees) and the ventilation speed will increase. As soon as the interior has reached the temperature of 20 degrees Celsius again, the fan speed and outlet temperature are reduced again.

The sun sensor on the dashboard also influences the speed of the interior fan. The light intensity is measured by means of the ultraviolet radiation in the sunlight. In strong sunlight, the passenger compartment fan will blow a larger amount of cold air into the passenger compartment. The sun sensor can be recognized by the dot that is usually located in the center at the top of the dashboard. The picture shows a sun sensor.

Temperature regulation:
The temperature in the interior can be kept constant in two ways; namely through the use of:

  • Mixed air control: The cold and warm air are mixed together by means of stove valves in the stove housing. The cold air is the outside air temperature and the warm air is as warm as possible (heated to the maximum by the coolant). By opening the valve for the warm air a little further, a little more warm air is supplied to the outside air. More information about the stove house is described further down the page.
  • Fluid Control: Electronically controlling heater valves changes the coolant flow through the heater radiator. The outside air flows through the heater radiator. This warms the air. The air temperature therefore depends on the coolant temperature in the heater radiator. More information about the heater radiator is described further down the page.
  • Evaporator: The evaporator is part of the air conditioning system and is described on a separate page. By allowing the warm outside air to flow through the cold evaporator, this air is cooled.
    The interior fan will have to blow the air through the stove housing, radiator and / or evaporator to give the air the desired temperature and then let it flow into the interior.

Vehicles equipped with a separate climate system have an adapted heater housing, which means that the outlet temperatures from the left and right can differ.
The following image shows a separate climate control system in which the outlet temperature on the driver's side is 21 degrees Celsius and on the co-driver's side 23 degrees.

It is possible that the rear passengers also have one or two individual climate zone(s) with a rotary knob or display to set the temperature to two additional zones. In that case, the stove housing contains additional ducts for the mixed air control.

stove house:
The stove house is shown below. The interior fan is mounted under the heater radiator. The ventilation air enters from the side of the passenger compartment fan and is blown at the top through the heater radiator and the evaporator of the air conditioning. The stove housing is located in the middle under the dashboard and can in principle only be disassembled when the complete dashboard has been removed.

The image above shows a transparent stove housing containing a number of stepper motors. The stepper motors drive valves that regulate the airflow and temperature of the air. The figure below shows the airflow through a stove housing containing the valves operated by the stepper motors.

The interior fan draws in the outside air through the intake pipe. The end of this suction tube is usually located under the hood, behind the parafan. If the car is equipped with air conditioning, the passenger compartment fan blows the intake air through the evaporator. In the evaporator, moisture and heat are extracted from the outside air, causing dry and cooled air to enter the heater radiator. When the air conditioning is turned off, the air also flows through the evaporator, but will not undergo a change in temperature and moisture.

The temperature of the heater radiator influences the heating of the air; in the coolant circuit, taps provide an adapted flow rate; a lower coolant flow will cause less heating of the air. From the heater core, air arrives at at least three air control valves: one to the windshield, one to the vents in the dashboard and one to the footwell. The position of the valve determines how much air is blown to the respective outlets.

The operation of the air control valves, the temperature control valve, the air bypass valve and the recirculation valve can be manually operated. In that case there is a physical connection with a Bowden cable between the control slides or buttons on the dashboard and the valves. Nowadays we almost only see electronically controlled valves: a control unit controls the stepper motors.

An electronically controlled ventilation system often has more options than a manually operated system with cables:

  • multiple temperature zones: on the driver's side, the air control flaps and the temperature control flap can be operated separately from the co-driver's side. In this case, the valves are double. In luxury cars, up to four zones can be adjusted: the number of valves and air ducts in the same stove house is doubled;
  • a MAX position to operate the air conditioner at maximum capacity: in the MAX position, the air bypass valve opens and the temperature control valve closes: only cooled air enters the manifold housing with the air control valves. The recirculation valve will also close the supply of outside air and – the already cooled air from the interior – will draw in via the fan and cool it further;
  • automatic opening and closing of the recirculation valve when the air quality sensor harmful substances in the drawn-in outside air.

Interior fan:
An interior fan is shown below. The interior fan is also called a "heater motor" or "supercharger". The blades are located in the middle of the interior fan, which ensure that the air is blown into the interior. The ventilation air is drawn in at the top of the engine and fed through the ducts on the side to the heater radiator. The heater radiator is mounted in the heater housing immediately after the interior fan.

The operation and the various control methods are discussed on the stove motor page.

Heater radiator and heater valve:
The heater radiator heats up the air that is blown into the interior. The heater radiator consists of two pipes (a supply and a discharge) that split into channels between which slats are placed. The slats provide a larger heat-exchanging surface.

The heater radiator works like the radiator at the front of the car as a heat exchanger. The cold air flowing through the fins is warmed by the coolant flowing through the channels along the fins. The heat from the coolant is transferred to the airflow. The heated air enters the interior of the car; this is the heater that is activated by the occupants. Since the warm air that the passenger compartment fan blows into the passenger compartment depends on the temperature of the coolant, it is logical that the heater is still cold immediately after starting the engine. The heater is not fully functional until the engine is at operating temperature.

The occupants can turn the heater up or down. By operating the stove, the opening angle of the stove valve changes. The heater valve controls the amount of coolant that flows through the heater radiator. The size of the coolant flow ultimately determines the air temperature.

The picture below shows a heater valve with tubes on both sides where the coolant hoses are slid together. In the center of these tubes is a rotating valve that blocks or lets the coolant flow through depending on the opening angle. The valve is operated by the lever, which can also be seen in this image. The lever can move a maximum of 90 degrees; in the extreme positions, the valve is fully open or closed. A Bowden cable to the heater control unit (mechanical) or an electric / stepper motor (electric) is connected to this lever. More on this later.

The control principles of the stove valve are described below:

Heater valve fully open:

  • Large coolant flow.
  • The coolant is not easily cooled by the airflow.
  • The material of the heater radiator remains very warm.
  • The air that is blown into the interior is therefore also warm.

Heater valve partially open or closed:

  • Little or no coolant flow.
  • As a result, the coolant cools more easily due to the airflow.
  • The material of the heater radiator cools down.
  • The air that is blown into the interior is lukewarm or cold, because the outside air temperature is not or hardly influenced.
stove valve

The images below show the components:

  • The principle of operation of the servomotor (left);
  • The stove valve and actuator in mounted condition (center);
  • The climate control ECU (right).

The relevant servomotor and ECU come from a Maserati Quattroporte from 2001. The servomotor is of the type DC electric motor with carbon brushes. This is done by means of a duty cycle controlled by an ECU. The electric motor drives both the output shaft and the runner that moves over the contact disc by means of several gear transmissions. The disc is supplied with a voltage of 5 volts and ground. Depending on the position of the runner, a signal is sent to the ECU that determines the position of the output shaft, and thus the heater valve. In the current state, the signal voltage is 4,5 volts. The moment the output shaft and the runner rotate a few degrees counterclockwise, the signal voltage drops to 4,4 volts or lower. In the extreme positions, the signal voltage will be between 0,5 and 4,5 volts.

Actuator_heater valve
DC Electric Motor With Mechanism
Actuator and heater valve
ECU climate control

The ECU controls the heater valve until the mechanical end stop is reached. Due to the low torque of the electric motor, the rotation is stopped by this end stop and the signal voltage of the wiper on the contact plate also remains constant. The ECU will terminate the control.

The electric motor is supplied with a power supply and ground. These are controlled by the ECU by means of a PWM control. The following scope image shows the control signals measured on the plus and ground connections of the electric motor during counterclockwise and clockwise rotation.

  • Turning counterclockwise: the ECU sends a block signal to the plus of the electric motor. The mass is constantly 0 volts;
  • Clockwise rotation: the polarity of the electric motor reverses.
DC motor control signals when turning left and right

It can happen that the shaft in a stove tap moves more heavily due to age. This mechanical resistance allows the ECU to "think" that the limit switch has been reached. The control is thus terminated. A new article will soon be published in which a fault was present in the control system with the above ECU and servomotor. After a diagnosis and repair of the printed circuit board, the system was working properly again. The symptoms, cause and solution will be explained with pictures.

In addition to this version with DC motor and PWM control, many stove valves and stove valves are operated by a stepper motor controlled.

The moment the heater is activated immediately in winter and the interior fan is set to position 4, the engine will also reach operating temperature less quickly. This is because the passing air cools the coolant again. That is not desirable, because we naturally want the engine to warm up as quickly as possible. It is therefore recommended that you only activate the heater after a few kilometers of driving.

With a parking heater or electric auxiliary heating the heater radiator and the cooling system of the combustion engine can be brought up to temperature more quickly.

Ventilation shafts:
The figure below shows the ventilation shafts and exits. Normally this is not visible, because the dashboard, center console and carpet are mounted over it. The stepper motors of the stove valves in the stove house regulate the airflow to the different directions (to the windscreen, to the left or right ventilation grilles, or to the footwells). There is a constant flow of ventilation to the rear compartment. The rear ventilation grilles in the center console can be closed mechanically.

Numbers 1, 2, 3: Air vent to the windscreen (eg for windscreen demisting / Defrost)

Numbers 4, 5: Demisting of both side windows of the front doors

Numbers 6, 8, 9, 11: Ventilation grilles for driver and passenger compartment

Numbers 7, 10: Air vents for driver and passenger compartment footwells

Numbers 12, 13, 16, 17: Rear passenger footwell vents

Numbers 14, 15: Ventilation grilles in the center console for the rear passengers

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