The DoorKing 2600-584 Heater Kit is recommended for use in colder climates where temperatures routinely fall below 10°F (-12°C). The Heater Kit is compatible with DoorKing Telephone Entry System Models 1802, 1803, 1808, 1810, 1810 Access Plus, 1812, 1812 Plus, 1812 Access Plus, 1819, 1833, 1834, 1835, and 1837.