The 500 Series Thermostats are designed for years of quality performance. All models feature a hermetically sealed-in-glass mercury switch, and no subbase is required. Models are available for 24 volt heating and / or cooling, millivolt heating, and single or multistage heat pump applications. These thermostats are inexpensive and easy to install. This makes them great for apartments, housing authorities and low end construction. This new line of Robertshaw thermostats are well suited for any application ? residential, commercial or institutional.