Johnson Controls W42AA-1 Humidistat

The Johnson Controls W42AA-1 Humidistat is a heavy-duty, 3-wire humidity controller designed for precision management of both humidifying and dehumidifying equipment. Engineered with a stable 150-strand human hair sensing element, this unit provides superior sensitivity and long-term accuracy compared to synthetic alternatives. It operates on line voltage (120/240 VAC) or low voltage circuits, utilizing a Single Pole Double Throw (SPDT) snap-acting switch to close the circuit on either a rise or fall in relative humidity. With an adjustable range of 10% to 90% RH and a fixed 4% differential, this mechanical humidistat is a reliable choice for exhaust fan control, fruit storage rooms, and commercial HVAC systems. The vertical mounting design of the sensing element minimizes dust accumulation, ensuring the PENN W42AA-1 maintains its responsiveness in demanding environments. A concealed dial lock screw allows for secure, tamper-proof setpoints once calibration is complete.

• Dual-Action Control: The SPDT contact configuration allows a single device to manage both humidifiers and dehumidifiers.
• High-Precision Sensing: Features a human hair element for maximum stability and fast response to humidity fluctuations.
• Universal Voltage Compatibility: Field-proven for both 120V/240V line voltage and 24V low voltage applications.
• Simplified Installation: Includes a separable mounting plate that fits standard vertical outlet boxes to reduce field labor.
• Industrial Durability: Equipped with a tension release device to prevent damage to the sensing strands during extreme humidity spikes.

Can I use this to control an exhaust fan based on high humidity?
Yes. By wiring to the "C" (Common) and "H" (High) terminals, the circuit will close when the humidity rises above your setpoint, activating the fan to exhaust moist air.

Should the cover be removed during installation?
No. It is recommended to keep the cover on during mounting and wiring to protect the sensitive human hair element from accidental physical damage or oils from your hands.

How do I maintain the sensing element?
If dust or grease builds up, the hair strands can be gently cleaned with a soft brush. In high-contamination environments, ensure the unit is mounted vertically to reduce debris settling on the strands.

Disclaimer: While we strive for 100% accuracy, product data and specifications are subject to change. Please contact our team to confirm compatibility or technical details before purchasing.