Single Stage (0H/1C) Setpoint Thermostat, 24 Volts, Snap Action, System Switch-Off, Cool, Vent, Fan Switch-Hi, Lo, Profile-Horizontal, Range 55-95° F., Temperature Scale 1,2,3,4,5, Terminals-Special R,W,Y,G Matched To 21D28-6. Replaces 1D51W-632 & 1F51-619 (MERCURY FREE).