W42AA Type Low or Line Voltage Humidistat

Johnson W42AA Type
Low or Line Voltage Humidistat

Introduction

The W42AA humidistat controls humidifying or dehumidifying equipment, or both. The humidistat is a 3-wire device with single-pole, double-throw, snap-acting contacts.

A typical W42 application is for operating an exhaust fan in electrically heated homes. The
humidistat automatically operates the exhaust fan to reduce the relative humidity when it is above normal. Other typical applications include humidity control in fruit storage rooms, curing rooms and hospitals where accuracy and reliability are required.

All series W42 humidistats are designed for use only as operating controls. Where an
operating control failure would result in personal injury and/or loss of property, it is the responsibility of the installer to add devices (safety, limit controls) or systems (alarm, supervisory systems) that protect against, or warn of, control failure.

General Description

The W42AA humidistat provides single pole, double-throw control for use on low or line voltage
applications. The humidistat’s terminals are marked for easy wiring.

The “C” terminal is common. The circuit between “C” and “L” terminals closes on a decrease in relative humidity. The circuit between “C” and “H” terminals closes on an increase in relative humidity. 

The humidistat’s sensing element consists of approximately 150 strands of carefully selected and processed human hair — proven to be the most sensitive and stable material known for humidity control.

An exclusive tension release device keeps the hair strands from being damaged by extreme humidity or mechanical strain. The sensing element is mounted vertically to minimize surface contamination by dust or foreign material. The humidistat mounts on a standard outlet box or directly to a flat surface where permitted by electrical codes.