Hi firmandha
sorry I totally dissagree with coolments answer. The return air temp should be warmer than space/room temp. depending on the load it will vary. Also depends on type of AC - FAV, VAV. For example on a VAV AHU with a warm day of say 28 deg. supply air may be 14, room temp 22.5 and return of 23 or 24 deg. Sometimes due to no suitable location of sensor RA Temp is used to control but the setpoint would be 23.5 instead of 22.5. Don't just take my word for it - go out and measure it in the field.