humidistat

A humidistat is a control device that monitors relative humidity in an environment and operates humidification or dehumidification equipment to maintain a target humidity range. It serves as a humidity controller in homes, greenhouses, laboratories, and other spaces.

A humidistat uses a humidity sensor to measure RH. Sensor types include capacitive, resistive, and gypsum-based elements, deployed in electronic or mechanical formats. The sensor feeds a control circuit that compares the measured RH to a user-setpoint. Based on the result, the humidistat triggers a humidifier when RH is below the setpoint or a dehumidifier (or HVAC system) when RH is above it. Many models use a hysteresis or differential to prevent rapid cycling around the setpoint.

Humidistats come as standalone units, built into humidifiers and dehumidifiers, or integrated into broader climate-control systems. They range from mechanical, with bimetallic actuators, to electronic, with digital displays and programmable setpoints. Wire-in and wireless remote versions also exist.

Used to maintain comfortable and safe moisture levels, humidistats help prevent mold growth, condensation, wood warping, and static electricity in sensitive environments. They can improve energy efficiency by avoiding unnecessary humidification or dehumidification.

Placement should reflect ambient conditions and avoid proximity to heat sources, vents, or direct moisture, and allow free air flow around the sensor. Calibration may be required; sensors can drift over time and replacement may be necessary to maintain accuracy.