HVAC sensors and switches help automate the control of an HVAC system by responding to environmental changes and sending signals to the HVAC control system. Temperature and humidity sensors are installed at various points in the HVAC system and measure temperature, humidity, or both. Pneumatic temperature transmitters monitor air temperature and create a proportional pneumatic signal that is used to trigger heating or cooling cycles. Air quality sensors measure the factors that reduce air quality, such as carbon dioxide, volatile organic compounds (VOCs), and airborne particulates. Air pressure sensors and switches measure and respond to changes in air pressure and help regulate and optimize the HVAC system. Airflow switches detect the presence of airflow in a duct or plenum and change states when air starts or stops flowing. Economizers connect to HVAC systems and use an outdoor air intake when it would be more efficient than heating or cooling the return air.