At what temperature does the thermostat come on?

This varies depending on which unit you are using. Immersion heaters come on around 45 degrees Fahrenheit and go off at around 60 degrees. The disc thermostat is preset to come at 55 degrees (+ or – 5 degrees) Fahrenheit and go off at 65 degrees (+ or – 5 degrees). Larger steel units use a Fenwall thermostat, which is adjustable from 0 degrees Fahrenheit to 100 degrees.