HONEYWELL - HONEYWELL T-STAT HEAT/COOL MERCURY FREE - | 5-Year Warranty | The easy-to-see face has enlarged scale and raised numbers - -plus it click at every 2 F change | Mercury Free | Classic Styling | Premier Whit | Separate temperature setting and thermometer | The T87K heat only works with 2 or 3 wire heat only systems | The Round T87K and T87N thermostats provide electronic control of 24Vac heating and cooling systems with the classic twist to set dial.