Randall - in my setup it holds within 1 degree easily. The use of a dimmer allows "fine tuning" so to speak such that the controller relay does not cycle on/off as much. Also, make sure to set the differential to 1 degree, I think the default is 3. Also, with resistive heaters like heat tape, pads and panels and lamps, you don't need the time delay (that may be just in cool mode).