Small automatic reset 120F OPEN ON RISE snap switch found in many Enviro models often referred to as the igniter switch. Switch is normally closed and opens when the temperature reaches approx. 120 degrees Fahrenheit (+/- 5 degrees) and resets when the temp drops to around 90F. Enviro part no. cross reference is EF-015 Case is made of CERAMIC on this model rather than the normal phenolic material.