Combined humidistat and electronic air flow responsive switch

In a control device adapted to be mounted on the return air duct in a forced air heating system for enabling operation of a humidifier in the presence of circulating air flow through the return air duct and preventing operation thereof in the absence of circulating air flow,

• a humidistat switch;
• a humidity sensing element exposed to the interior of the return air duct for controlling operation of said humidistat switch in response to relative humidity of air within said return air duct;
• a thermistor mounted in a restrictive passageway in communication with ambient air surrounding the device and with said air within said return air duct;
• and
• circuit means connected in circuit with said humidistat switch for effecting heating of said thermistor, for effecting energizing of said humidifier in response to cooling of said heated thermistor by ambient air flow through said passageway past said thermistor, said ambient air flow being caused by a pressure drop between said ambient air and said air in said return air duct when the circulating air flow exists, and for effecting de-energizing of said humidifier in the absence of said cooling when said circulating air flow does not exist.