What temperature sensors should I specify for an AHU?

A typical air handling unit requires at minimum: outdoor air temperature sensor (NTC thermistor, -40 to 140°F range, with solar radiation shield), mixed air temperature sensor (averaging type spanning the full duct cross-section to detect stratification), supply air temperature sensor (after the cooling and heating coils, with a thermowell if in a wet airstream), and return air temperature sensor (in the return duct or plenum). For higher accuracy control, platinum RTDs (Pt1000, Class A with ±0.15°C accuracy) are preferred over thermistors because they maintain calibration over a wider temperature range without drift. Per ASHRAE Guideline 36, all temperature sensors should be verified to have a time constant of less than 30 seconds and the supply air sensor must be positioned at least 3–4 duct diameters downstream of the last coil to ensure adequate mixing.