HVAC Air Delivery/Temperature Control Circuit Description

HVAC Air Delivery/Temperature Control Circuit Description

Air Delivery

Electric motors position the actuators that control the flow of air by the HVAC controller.

Most of the actuators have one motor and one position potentiometer. The HVAC recirculation/fresh air actuator does not have a potentiometer.

The position potentiometer indicates the relative position of the actuator to the HVAC controller. A voltage level indicates this position.

There are two drive circuits in the HVAC controller. The drive circuits can operate the motors in either direction. One of the drivers supplies approximately 7.5 volts to the motor. The voltage causes the motor to run. The polarity of this voltage affects the direction in which the motor will run.

The normal position of the HVAC recirculation/fresh air actuator allows outside air to enter the system. When RECIRC is selected, the motor moves to the recirc air position and restricts outside air. The controller will provide enough voltage to position the motor back to the outside air position when either of the following controls is selected:

* RECIRC
* DEFROST

The controller drives the motor for 22 seconds. Because there is no potentiometer on the HVAC recirculation/fresh air actuator, this action ensures that the motor attains the proper position.

Temperature Controls

The HVAC air temperature actuator positions the temperature door. The temperature setting of the HVAC controller determines the position of the temperature door.

The following actions occur when the controller is in the full COLD temperature setting:

* The temperature door is in the cold position.
* The HVAC air temperature actuator potentiometer signals about 4.0 volts to the HVAC controller.

The following actions occur when the controller is in the full HOT temperature setting:

* The temperature door is moved to the warm position.
* The potentiometer reads about 0.5 volt.

The position of the temperature door varies when the controller is in a mid-range setting. The temperature door is positioned to a point where the potentiometer output is the same as the preprogrammed value for the selected setting.

The operation of the HVAC mode door actuator is similar to the operation of the HVAC temperature actuator. The position of the HVAC mode door actuator depends on the mode that is selected.

For full A/C, the potentiometer output is about 0.2 volts.

For full DEFROST, the output is about 4.8 volts.

In the heat mode, the potentiometer output is about 2.5 volts.

If a motor stalls while changing the location of the actuator, the HVAC controller removes the voltage after 45 seconds. This action protects the motor. This action is repeated whenever the mode setting is changed.