Where should humidity sensors be placed in an AHU?

Return air humidity sensors should be located in the return air duct or the mixed air plenum (upstream of the cooling coil) to provide an accurate representation of the average space humidity without being affected by coil condensate carryover. Outdoor air humidity sensors require an aspirated shield or weatherproof enclosure and should be mounted on the north-facing side of the building whenever possible to avoid solar radiation errors. For supply air humidity monitoring (used in dew point control strategies), the sensor must be mounted downstream of the cooling coil and fan — at least 3–4 duct diameters from the coil discharge — to avoid saturated air or mist from the coil affecting readings. Capacitive RH sensors (the standard in BAS) require annual calibration verification against a reference standard, and sensor accuracy of ±2–3% RH is typical for the 20–80% RH range but degrades significantly in condensing environments without appropriate sensor protection.