Determination of return air temperature

An HVAC system includes a blower configured to provide a flow of air into a conditioned space. A temperature sensor measures a discharge air temperature of the flow of air provided to the conditioned space. A controller of the HVAC system determines that heating or cooling mode operation is not requested. Following this determination, the blower provides the flow of air at a predefined flow rate. The controller determines that the measured discharge air temperature satisfies predefined stability criteria associated with a change in the discharge air temperature during the first period of time being less than a threshold value. If the stability criteria are satisfied, a temperature offset is determined between the discharge air temperature and an indoor temperature. The temperature offset is stored in a lookup table such that the temperature offset is associated with the flow rate of air provided by the blower and the outdoor temperature.