Do you know what your temperature differential is set at? If you press the "set" button 3 times it should show "Dif 1" in the upper Left corner of the screen. The number should be set to either 1 or 2. This is the amount of degrees above and below the set point temperature that the t-stat will operate. So if you set the set-point temp to 90 with a differential of 2 then the t-stat will come on when the temp is 88 and shutoff when the temp is 92. The only other things I can think of is make sure it is set to "F" for Farenheit and H1 for heating...