The Aprilaire 8400 Series Non-Programmable Thermostats are attractive, simple to use and offer: Large LCD Screen with high contrast - for easy reading. Temperature Readout in Fahrenheit or Celsius Outdoor Temperature Reading for convenience and comparison (requires optional outdoor temperature sensor). Circulate Fan Mode means that the fan comes on for 10 minutes every 30 minutes for better indoor air quality plus reduces hot or cold spots within your home. Service Reminder tells you when you should change your air filter to protect your equipment and water panel to keep your humidifier running in optimum condition. It also can remind you to service your equipment or humidifier to insure the longevity of your investment