Mercury free builders pack. The Round thermostat is one of the most recognizable and popular temperature control units. It's reliable performance, classic styling, and convenience have made it an industry standard. Adjustable heat range from 40 deg. F. to 90 deg. F. Use with most residential gas, oil, steam, and gravity systems. Comfort control adjustable heat anticipator provides even control. Easy-to-use familiar design and twist-to-set interaction. Off white.