The Solatel THP-100 coordinates ventilation and CO2 (carbon dioxide). This is required when temperature and humidity are controlled by a vent or exhaust fan. Hot or humid air is blown out of the growing area and fresh air is then drawn in. This continues until both temperature and humidity fall below the dial settings. CO2 is only dispensed during the day when plants have light for photosynthesis. It is not dispensed when the fan is operating because this would waste CO2. The THP-100 provides all of these functions. When connecting CO2 equipment, a repeat cycle timer or CO2 concentration controller (not included) is plugged into the THP 100. The CO2 tank valve or CO2 generator then plugs into the timer or controller.