The Aprilaire 8120A Ventilation Controller is installed in the return duct of your furnace or heat pump. The Aprilaire 8120A Ventilation Controller features two settings that are adjustable: the cycle time and the ventilation time within cycle. Here's how this works: if you set the cycle time to 1 hour and the ventilation time knob to 20 minutes, you will get 20 minutes of ventilation every hour. It is important to understand that the ventilation requirement: 20 minutes per hour in this example may be satisfied by a heating, cooling, or fan call initiated by your thermostat, or from a fan call initiated by the Ventilation Controller itself. The Aprilaire 8120A Ventilation Controller monitors indoor relative humidity to help prevent high humidity conditions inside your home. The Aprilaire 8120A Ventilation Controller will not open the damper to the outside air if there is a chance of raising the humidity inside the house to high levels. The Aprilaire 8120A Ventilation Controller is designed to prevent indoor relative humidity levels from increasing beyond 60% due to ventilation. If the outdoor air temperature is below 0 degree F or above 100 degree F, the motorized normally-closed damper to the outside will not be opened. This feature can be disabled at installation if the installer chooses not to use outdoor air temperature as a parameter.