Manual thermostat, Round™, mercury-free, provides electronic control of 24-volt (AC) heating and cooling systems with classic twist-to-set dial. Dimensions: 4-inch diameter x 1-3/4-inch deep; 102-mm diameter x 45-mm deep. Classic styling; Premier White®. T87K heat only works with 2- or 3-wire heat-only systems. Includes 6-inch (152 mm) cover ring and switch position labels. Easy-to-see model with large raised markings and click at each 2-degree set point change.