The Duct Mounted Transmitter range from Flamefast is one of the most cost effective air quality transmitters available, with the ability to monitor Temperature, Relative Humidity, then either Carbon Dioxide (CO2) or Total Volatile Organic Compounds (TVOC).
 
With a wide range of output, including three 0-10V outputs, an on board relay, and the ability to select between Analogue or a Direct Thermistor temperature output, this versatile units is suitable for most general HVAC applications.