Manual thermostat, Round™, mercury-free, provides electronic control of 24-volt (AC) heating and cooling systems with classic twist-to-set dial. Dimensions: 3-11/16-inch diameter x 1-3/4 inch deep. Classic styling. Premier White®. T87K heat-only works with 2- or 3-wire heat-only systems. Works with 1 heat/1 cool conventional systems and heat pumps with no auxiliary heat.