Introduction

In vehicle powered units, temperature control is based on two values: The setting (Setpoint) of the controller and the evaporator return air temperature. The difference between these two temperatures will determine the mode of operation: cool, heat, or null.

Cool: When the temperature in the compartment is 3°F (2°C) higher than the setpoint, the unit runs in cool mode to reduce the evaporator return temperature to achieve the setpoint.

Heat: When the temperature in the compartment is 3°F (2°C) lower than the setpoint, the unit changes to heat mode to raise the evaporator return temperature to achieve the setpoint.

Null: Once the Setpoint Temperature has been reached, and the temperature remains at or within the temperature differential, (there is no demand for heat or cool), the unit stops operating and goes into the Null mode.

While in the Null mode, the unit is still monitoring the compartment temperature and will resume operation only if the temperature increases or decrease by 3°F (2°C) above or below the setpoint.

Defrost: After a period of time in cool mode, (time is setup during installation between 0 and 8 hours), the unit checks the coil temperature. If the temperature is cold enough to form ice, the unit runs in automatic Defrost Mode to eliminate ice that has accumulated in the evaporator coil. Defrost can also be initiated manually by selecting the defrost mode on the DSR controller. The unit will run in defrost until one of the two events occurs: 1) the coil temperature is back within range, or 2) the defrost termination timer has expired. (time is setup during installation).